ADO.NET Basics

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ADO.NET Fundamentals
Core ADO.NET Objects
Connection Handling & Lifecycle
SqlCommand & Command Execution
SqlDataReader — Connected Model
DataSet & DataAdapter — Disconnected Model
Parameters & SQL Injection Prevention
Stored Procedures in ADO.NET
Transactions in ADO.NET
Exceptions & Error Handling
Performance & Best Practices
ADO.NET vs EF vs Dapper

# 1. ADO.NET FUNDAMENTALS

Q. What is ADO.NET? What is its role in the .NET framework?

ADO.NET (ActiveX Data Objects .NET) is the data access technology in the .NET framework that provides a set of classes for connecting to data sources, executing commands, and managing data in a disconnected or connected manner.

Role in .NET framework:

Application Layer
      ↓
ADO.NET Layer (System.Data)
      ↓
.NET Data Provider (SqlClient, OracleClient, OleDb, ODBC)
      ↓
Database (SQL Server, Oracle, MySQL, etc.)

Key namespaces:

Namespace	Purpose
`System.Data`	Core objects: `DataSet`, `DataTable`, `DataRow`
`System.Data.SqlClient`	Legacy SQL Server provider (Framework)
`Microsoft.Data.SqlClient`	Modern SQL Server provider (recommended)
`System.Data.OleDb`	OLE DB provider (Access, Excel)
`System.Data.Odbc`	ODBC provider (generic)
`System.Data.Common`	Abstract base classes

Note: Microsoft.Data.SqlClient (NuGet package) is the current recommended SQL Server provider. It supports Always Encrypted, Active Directory authentication, and modern TLS. System.Data.SqlClient is legacy.

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Q. What is the difference between Connected and Disconnected architecture in ADO.NET?

Aspect	Connected	Disconnected
Connection state	Open throughout operation	Open briefly (fill/update only)
Key objects	`SqlConnection`, `SqlCommand`, `SqlDataReader`	`SqlDataAdapter`, `DataSet`, `DataTable`
Data location	Streamed from database	Cached in memory
Performance	Faster for large reads (streaming)	Better for offline/batch scenarios
Scalability	Holds connection resource	Releases connection quickly
Use case	Real-time reporting, large result sets	Web forms, grids, offline editing
Concurrency	Managed by DB	Manual (optimistic by default)

// Connected — connection stays open while reading
using var connection = new SqlConnection(connectionString);
connection.Open();
using var command = new SqlCommand("SELECT * FROM Products", connection);
using var reader  = command.ExecuteReader(); // connection must stay open
while (reader.Read())
    Console.WriteLine(reader["ProductName"]);
// connection closed here

// Disconnected — connection opened briefly to fill DataSet
using var connection = new SqlConnection(connectionString);
var adapter = new SqlDataAdapter("SELECT * FROM Products", connection);
var dataSet = new DataSet();
adapter.Fill(dataSet, "Products");  // connection opened/closed internally
// Work with dataSet.Tables["Products"] without any DB connection

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Q. What is the Provider Model in ADO.NET?

ADO.NET uses a provider model where each database vendor supplies a .NET data provider that implements standard interfaces. This allows code written against abstractions to work with any database.

// Abstract base classes / interfaces
System.Data.Common.DbConnection    // base of SqlConnection, OracleConnection
System.Data.Common.DbCommand       // base of SqlCommand, OracleCommand
System.Data.Common.DbDataReader    // base of SqlDataReader
System.Data.Common.DbDataAdapter   // base of SqlDataAdapter

// Provider-agnostic code using DbProviderFactory
using System.Data.Common;

DbProviderFactory factory = DbProviderFactories.GetFactory("Microsoft.Data.SqlClient");
using DbConnection conn   = factory.CreateConnection();
conn.ConnectionString     = connectionString;
conn.Open();

using DbCommand cmd = factory.CreateCommand();
cmd.Connection      = conn;
cmd.CommandText     = "SELECT COUNT(*) FROM Products";
int count           = (int)cmd.ExecuteScalar();

Common providers:

Provider	NuGet Package	Target DB
`Microsoft.Data.SqlClient`	`Microsoft.Data.SqlClient`	SQL Server / Azure SQL
`Npgsql`	`Npgsql`	PostgreSQL
`MySqlConnector`	`MySqlConnector`	MySQL / MariaDB
`Oracle.ManagedDataAccess`	`Oracle.ManagedDataAccess.Core`	Oracle DB
`Microsoft.Data.Sqlite`	`Microsoft.Data.Sqlite`	SQLite

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# 2. CORE ADO.NET OBJECTS

Q. What are the core ADO.NET objects and their purposes?

Object	Namespace	Purpose
`SqlConnection`	Microsoft.Data.SqlClient	Opens and manages a DB connection
`SqlCommand`	Microsoft.Data.SqlClient	Executes SQL text or stored procedures
`SqlDataReader`	Microsoft.Data.SqlClient	Forward-only, read-only, connected result set streaming
`SqlDataAdapter`	Microsoft.Data.SqlClient	Bridge between DB and `DataSet` (fill/update)
`SqlParameter`	Microsoft.Data.SqlClient	Parameterized query values, prevents SQL injection
`SqlTransaction`	Microsoft.Data.SqlClient	Wraps multiple commands in an atomic unit
`DataSet`	System.Data	In-memory relational cache (disconnected)
`DataTable`	System.Data	Single in-memory table with rows and columns
`DataRow`	System.Data	A single row in a `DataTable`
`DataColumn`	System.Data	Defines a column's schema in a `DataTable`
`DataRelation`	System.Data	Parent-child relationship between `DataTable`s

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Q. Which ADO.NET object keeps the connection open? Which works without a connection?

// SqlDataReader — REQUIRES open connection the entire time
using var connection = new SqlConnection(connectionString);
connection.Open();
using var command = new SqlCommand("SELECT Id, Name FROM Customers", connection);
using var reader  = command.ExecuteReader();

// Connection MUST stay open while iterating reader
while (reader.Read())
{
    int id    = reader.GetInt32(0);
    string name = reader.GetString(1);
    Console.WriteLine($"{id}: {name}");
}
// reader.Dispose() → safe to close connection now

// DataSet — works WITHOUT active connection
using var adapter = new SqlDataAdapter("SELECT Id, Name FROM Customers", connectionString);
var ds = new DataSet();
adapter.Fill(ds, "Customers"); // connection opened/closed internally

// From here — no DB connection required
foreach (DataRow row in ds.Tables["Customers"]!.Rows)
    Console.WriteLine($"{row["Id"]}: {row["Name"]}");

// Modify in memory
DataRow newRow = ds.Tables["Customers"]!.NewRow();
newRow["Name"] = "Alice";
ds.Tables["Customers"]!.Rows.Add(newRow);

// Push changes back (adapter handles connection internally)
var commandBuilder = new SqlCommandBuilder(adapter);
adapter.Update(ds, "Customers");

Rule: SqlDataReader holds connection open. DataSet/DataTable are fully disconnected in-memory structures.

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# 3. CONNECTION HANDLING & LIFECYCLE

Q. How do you create, open, and close a SqlConnection properly?

// Connection string formats
string connStr = "Server=localhost;Database=Northwind;Integrated Security=True;TrustServerCertificate=True;";

// SQL Server Authentication
string connStrSql = "Server=myserver;Database=mydb;User Id=sa;Password=P@ssw0rd;Encrypt=True;";

// Azure SQL
string connStrAzure = "Server=myserver.database.windows.net;Database=mydb;" +
                      "Authentication=Active Directory Default;Encrypt=True;";

// Approach 1: using statement (RECOMMENDED — guarantees Dispose/Close)
using (var connection = new SqlConnection(connStr))
{
    connection.Open();
    Console.WriteLine($"State: {connection.State}");        // Open
    Console.WriteLine($"Database: {connection.Database}");  // Northwind
    // ... execute commands
} // Dispose() called → connection returned to pool

// Approach 2: C# 8 using declaration
using var connection2 = new SqlConnection(connStr);
connection2.Open();
// ... execute commands
// Disposed at end of enclosing scope

// Approach 3: try/finally (explicit control)
var connection3 = new SqlConnection(connStr);
try
{
    connection3.Open();
    // ... execute commands
}
finally
{
    connection3.Close(); // or connection3.Dispose()
}

// Async open (preferred in ASP.NET Core)
using var connection4 = new SqlConnection(connStr);
await connection4.OpenAsync();

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Q. What is Connection Pooling in ADO.NET? How does it work?

Connection pooling reuses physical database connections instead of creating a new one for every request. ADO.NET enables pooling by default for SqlConnection.

// How pooling works internally:
// 1. First Open() → creates physical connection, adds to pool
// 2. Subsequent Open() with same connection string → reuses pooled connection
// 3. Close()/Dispose() → returns connection to pool (NOT physically closed)
// 4. Pool grows up to Max Pool Size (default 100)

// Connection string pool settings
string connStr =
    "Server=localhost;Database=mydb;Integrated Security=True;" +
    "Min Pool Size=5;" +         // pre-create 5 connections
    "Max Pool Size=100;" +       // maximum pool size
    "Connection Timeout=30;" +   // seconds to wait for available connection
    "Pooling=True;";             // default: true

// IMPORTANT: Always dispose connections to return them to pool
for (int i = 0; i < 10; i++)
{
    using var conn = new SqlConnection(connStr);
    await conn.OpenAsync(); // reuses pooled connection
    // ... query
} // Dispose returns to pool — NOT physically closed

// What happens if connection is NOT closed?
// → Pool exhausts → new requests throw SqlException:
//   "Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool."

// Clear pool manually (e.g., after connection string change)
SqlConnection.ClearPool(connection);         // clear specific pool
SqlConnection.ClearAllPools();               // clear all pools

// Check pool state via connection events
connection.StateChange += (sender, e) =>
    Console.WriteLine($"State changed: {e.OriginalState} → {e.CurrentState}");

Key rules:

Always use using or explicitly call Close()/Dispose()
Use the same connection string format to benefit from pool reuse
Never hold connections longer than needed

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Q. What happens if you do not close a SqlConnection?

// PROBLEM — connection not closed/disposed
public List<Product> GetProducts_BAD()
{
    var conn = new SqlConnection(connectionString); // connection opened
    conn.Open();
    var cmd    = new SqlCommand("SELECT * FROM Products", conn);
    var reader = cmd.ExecuteReader();
    var result = new List<Product>();
    while (reader.Read())
        result.Add(new Product { Id = reader.GetInt32(0), Name = reader.GetString(1) });
    return result;
    // conn, cmd, reader NEVER disposed → connection leak!
    // Pool drains → TimeoutException after max connections reached
}

// SOLUTION — using statements at every level
public async Task<List<Product>> GetProductsAsync(CancellationToken ct = default)
{
    var result = new List<Product>();

    await using var conn   = new SqlConnection(connectionString);
    await conn.OpenAsync(ct);

    await using var cmd    = new SqlCommand("SELECT Id, Name, Price FROM Products", conn);
    await using var reader = await cmd.ExecuteReaderAsync(ct);

    while (await reader.ReadAsync(ct))
    {
        result.Add(new Product
        {
            Id    = reader.GetInt32(0),
            Name  = reader.GetString(1),
            Price = reader.GetDecimal(2)
        });
    }
    return result;
}

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# 4. SQLCOMMAND & COMMAND EXECUTION

Q. What are the types of SqlCommand and execution methods?

CommandType:

using Microsoft.Data.SqlClient;

// 1. Text (default) — inline SQL
cmd.CommandType = CommandType.Text;
cmd.CommandText = "SELECT * FROM Products WHERE CategoryId = @CategoryId";

// 2. StoredProcedure — calls an SP
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "usp_GetProductsByCategory";

// 3. TableDirect — retrieves entire table (OleDb only, not SqlClient)
cmd.CommandType = CommandType.TableDirect;
cmd.CommandText = "Products";

Execution methods:

Method	Returns	Use For
`ExecuteReader()`	`SqlDataReader`	SELECT returning multiple rows
`ExecuteScalar()`	`object` (first col, row 1)	Aggregate: COUNT, SUM, single value
`ExecuteNonQuery()`	`int` (rows affected)	INSERT, UPDATE, DELETE, DDL
`ExecuteXmlReader()`	`XmlReader`	SELECT … FOR XML

using var conn = new SqlConnection(connectionString);
await conn.OpenAsync();

// ExecuteReader — multiple rows
using var cmd1   = new SqlCommand("SELECT Id, Name FROM Products", conn);
using var reader = await cmd1.ExecuteReaderAsync();
while (await reader.ReadAsync())
    Console.WriteLine($"{reader.GetInt32(0)}: {reader.GetString(1)}");

// ExecuteScalar — single value (first column of first row)
using var cmd2 = new SqlCommand("SELECT COUNT(*) FROM Products", conn);
int count      = (int)await cmd2.ExecuteScalarAsync(); // returns object, cast needed
Console.WriteLine($"Product count: {count}");

// ExecuteScalar for MAX
using var cmd3    = new SqlCommand("SELECT MAX(Price) FROM Products", conn);
object maxPrice   = await cmd3.ExecuteScalarAsync();
decimal price     = maxPrice == DBNull.Value ? 0m : (decimal)maxPrice;

// ExecuteNonQuery — INSERT/UPDATE/DELETE
using var cmd4    = new SqlCommand(
    "INSERT INTO Products (Name, Price) VALUES (@Name, @Price)", conn);
cmd4.Parameters.AddWithValue("@Name",  "Widget");
cmd4.Parameters.AddWithValue("@Price", 9.99m);
int rowsAffected  = await cmd4.ExecuteNonQueryAsync();
Console.WriteLine($"Rows inserted: {rowsAffected}"); // 1

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Q. What does `ExecuteScalar()` return? When do you use it?

ExecuteScalar() executes the query and returns the first column of the first row of the result set. Additional rows/columns are ignored.

// Use cases for ExecuteScalar
using var conn = new SqlConnection(connectionString);
await conn.OpenAsync();

// 1. COUNT
int productCount = (int)(await new SqlCommand("SELECT COUNT(*) FROM Products", conn).ExecuteScalarAsync())!;

// 2. SUM
decimal? revenue = (decimal?)(await new SqlCommand("SELECT SUM(Amount) FROM Orders WHERE Year=2024", conn).ExecuteScalarAsync());

// 3. Get newly inserted ID (SCOPE_IDENTITY)
using var insertCmd = new SqlCommand(
    "INSERT INTO Products (Name, Price) VALUES (@Name, @Price); SELECT SCOPE_IDENTITY();", conn);
insertCmd.Parameters.AddWithValue("@Name", "Gadget");
insertCmd.Parameters.AddWithValue("@Price", 49.99m);
int newId = Convert.ToInt32(await insertCmd.ExecuteScalarAsync());
Console.WriteLine($"Inserted with ID: {newId}");

// 4. Check existence
using var existsCmd = new SqlCommand(
    "SELECT CASE WHEN EXISTS(SELECT 1 FROM Products WHERE Id=@Id) THEN 1 ELSE 0 END", conn);
existsCmd.Parameters.AddWithValue("@Id", 42);
bool exists = (int)(await existsCmd.ExecuteScalarAsync())! == 1;

// 5. Null handling — ExecuteScalar returns DBNull if no rows
using var nullCmd = new SqlCommand("SELECT MAX(Score) FROM Scores WHERE UserId = 999", conn);
object result = (await nullCmd.ExecuteScalarAsync())!;
int? maxScore = result == DBNull.Value ? null : (int?)result;

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# 5. SQLDATAREADER — CONNECTED MODEL

Q. What is SqlDataReader? What are its key characteristics?

SqlDataReader provides a forward-only, read-only, connected stream of rows from a SQL Server database. It is the fastest way to read data when you only need to iterate once.

using var conn   = new SqlConnection(connectionString);
await conn.OpenAsync();

using var cmd    = new SqlCommand(
    "SELECT Id, Name, Price, IsActive, CreatedAt FROM Products WHERE CategoryId = @CatId", conn);
cmd.Parameters.AddWithValue("@CatId", 5);

using var reader = await cmd.ExecuteReaderAsync(CommandBehavior.CloseConnection);
// CommandBehavior.CloseConnection → closes connection when reader is closed

// Check if any rows returned
Console.WriteLine($"Has rows: {reader.HasRows}");

while (await reader.ReadAsync())
{
    // Access by column index (fastest)
    int     id        = reader.GetInt32(0);

    // Access by column name (readable)
    string  name      = reader.GetString(reader.GetOrdinal("Name"));

    // Null-safe access
    decimal price     = reader.IsDBNull(2) ? 0m : reader.GetDecimal(2);
    bool    isActive  = reader.GetBoolean(3);
    DateTime created  = reader.GetDateTime(4);

    Console.WriteLine($"{id}: {name} — ${price} (Active: {isActive})");
}

// Multiple result sets (using NextResult)
using var multiCmd = new SqlCommand(
    "SELECT * FROM Products; SELECT * FROM Categories;", conn);
using var multiReader = await multiCmd.ExecuteReaderAsync();

do
{
    while (await multiReader.ReadAsync())
        Console.WriteLine(multiReader[0]);
} while (await multiReader.NextResultAsync()); // advance to next result set

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Q. What are the key properties and methods of SqlDataReader?

Member	Type / Returns	Description
`HasRows`	`bool`	`true` if the result set contains rows
`IsClosed`	`bool`	`true` if the reader is closed
`FieldCount`	`int`	Number of columns in the current row
`RecordsAffected`	`int`	Rows affected (for non-SELECT commands)
`Read()`	`bool`	Advances to next row; `false` when exhausted
`NextResult()`	`bool`	Advances to next result set
`GetName(int i)`	`string`	Column name by index
`GetOrdinal(name)`	`int`	Column index by name (cache this for performance)
`IsDBNull(int i)`	`bool`	Check for `NULL` before typed read
`GetString(i)`	`string`	Read string (throws if null)
`GetInt32(i)`	`int`	Read int (throws if null)
`GetDecimal(i)`	`decimal`	Read decimal (throws if null)

// Performance tip: cache ordinals outside the loop
using var reader = await cmd.ExecuteReaderAsync();

int idOrdinal    = reader.GetOrdinal("Id");
int nameOrdinal  = reader.GetOrdinal("Name");
int priceOrdinal = reader.GetOrdinal("Price");

while (await reader.ReadAsync())
{
    int     id    = reader.GetInt32(idOrdinal);
    string  name  = reader.GetString(nameOrdinal);
    decimal price = reader.IsDBNull(priceOrdinal) ? 0m : reader.GetDecimal(priceOrdinal);
}

// Can we update the DB using DataReader? NO — it is read-only
// To update, use SqlCommand.ExecuteNonQuery() or SqlDataAdapter.Update()

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# 6. DATASET & DATAADAPTER — DISCONNECTED MODEL

Q. What is a DataSet? What is a DataAdapter and how do they work together?

DataSet is an in-memory representation of a relational database — it can hold multiple DataTables, DataRelations, and constraints, all without any database connection.

SqlDataAdapter is the bridge — it fills a DataSet from the DB and pushes changes back.

// Fill DataSet with multiple tables
using var conn    = new SqlConnection(connectionString);
var adapter       = new SqlDataAdapter();

adapter.SelectCommand = new SqlCommand(
    "SELECT * FROM Orders; SELECT * FROM Customers;", conn);

var ds = new DataSet("Northwind");
adapter.Fill(ds); // opens/closes connection internally

// Rename tables for clarity
ds.Tables[0].TableName = "Orders";
ds.Tables[1].TableName = "Customers";

// Define relation (parent-child)
ds.Relations.Add("CustomerOrders",
    parentColumn: ds.Tables["Customers"]!.Columns["CustomerId"]!,
    childColumn:  ds.Tables["Orders"]!.Columns["CustomerId"]!);

// Traverse related rows
foreach (DataRow customer in ds.Tables["Customers"]!.Rows)
{
    Console.WriteLine($"Customer: {customer["CompanyName"]}");
    foreach (DataRow order in customer.GetChildRows("CustomerOrders"))
        Console.WriteLine($"  Order #{order["OrderId"]}: {order["OrderDate"]}");
}

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Q. How do you use DataAdapter to Fill and Update a DataSet?

using var conn    = new SqlConnection(connectionString);
var adapter       = new SqlDataAdapter("SELECT Id, Name, Price, Stock FROM Products", conn);

// SqlCommandBuilder auto-generates INSERT/UPDATE/DELETE commands
var builder       = new SqlCommandBuilder(adapter);
// OR set them manually:
adapter.InsertCommand = new SqlCommand(
    "INSERT INTO Products (Name, Price, Stock) VALUES (@Name, @Price, @Stock); SELECT SCOPE_IDENTITY();", conn);
adapter.InsertCommand.Parameters.Add("@Name",  SqlDbType.NVarChar, 100, "Name");
adapter.InsertCommand.Parameters.Add("@Price", SqlDbType.Decimal,    0, "Price");
adapter.InsertCommand.Parameters.Add("@Stock", SqlDbType.Int,        0, "Stock");
adapter.InsertCommand.UpdatedRowSource = UpdateRowSource.FirstReturnedRecord;

// Fill
var ds = new DataSet();
adapter.Fill(ds, "Products");

// Modify in-memory (no DB connection)
DataTable products = ds.Tables["Products"]!;

// INSERT — add new row
DataRow newRow = products.NewRow();
newRow["Name"]  = "New Widget";
newRow["Price"] = 19.99m;
newRow["Stock"] = 100;
products.Rows.Add(newRow);

// UPDATE — modify existing row
DataRow first = products.Rows[0];
first["Price"] = (decimal)first["Price"] * 1.1m; // 10% price increase

// DELETE — mark row for deletion
DataRow last = products.Rows[products.Rows.Count - 1];
last.Delete();

// Inspect row states
foreach (DataRow row in products.Rows)
    Console.WriteLine($"{row["Name"]}: RowState = {row.RowState}");
    // Added, Modified, Deleted, Unchanged

// Push ALL changes back to DB in one operation
int rowsAffected = adapter.Update(ds, "Products");
Console.WriteLine($"Rows affected: {rowsAffected}");

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Q. How do you work with DataTable directly?

// Create DataTable programmatically
var table = new DataTable("Employees");

// Define schema
table.Columns.Add("Id",         typeof(int))    .AutoIncrement = true;
table.Columns.Add("Name",       typeof(string));
table.Columns.Add("Department", typeof(string));
table.Columns.Add("Salary",     typeof(decimal));
table.Columns.Add("HireDate",   typeof(DateTime));

// Primary key
table.PrimaryKey = new[] { table.Columns["Id"]! };

// Add rows
table.Rows.Add(null, "Alice",   "Engineering", 95000m, DateTime.Parse("2020-01-15"));
table.Rows.Add(null, "Bob",     "Marketing",   75000m, DateTime.Parse("2021-06-01"));
table.Rows.Add(null, "Charlie", "Engineering", 85000m, DateTime.Parse("2019-03-20"));

// Query with LINQ
var engineers = table.AsEnumerable()
    .Where(r => r.Field<string>("Department") == "Engineering")
    .OrderByDescending(r => r.Field<decimal>("Salary"))
    .Select(r => new { Name = r.Field<string>("Name"), Salary = r.Field<decimal>("Salary") });

foreach (var e in engineers)
    Console.WriteLine($"{e.Name}: ${e.Salary:N0}");

// DataView for filtering and sorting (bound to UI controls)
var view       = new DataView(table);
view.RowFilter = "Department = 'Engineering'";
view.Sort      = "Salary DESC";

foreach (DataRowView row in view)
    Console.WriteLine(row["Name"]);

// Compute aggregate
object total = table.Compute("SUM(Salary)", "Department = 'Engineering'");
Console.WriteLine($"Engineering total salary: ${total:N0}");

// Copy / Clone
DataTable clone = table.Clone();  // schema only, no rows
DataTable copy  = table.Copy();   // schema + rows

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# 7. PARAMETERS & SQL INJECTION PREVENTION

Q. What is SQL Injection and how do you prevent it using SqlParameter?

SQL Injection occurs when user input is concatenated directly into a SQL string, allowing attackers to manipulate the query.

// VULNERABLE — SQL Injection
string userInput = "'; DROP TABLE Users; --";
string sql = $"SELECT * FROM Users WHERE Username = '{userInput}'";
// Executes: SELECT * FROM Users WHERE Username = ''; DROP TABLE Users; --'

// BAD — string concatenation
public async Task<User?> GetUserBad(string username)
{
    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();
    // DO NOT DO THIS:
    using var cmd = new SqlCommand(
        $"SELECT * FROM Users WHERE Username = '{username}'", conn);
    // ...
    return null;
}

// GOOD — parameterized query (ALWAYS use this)
public async Task<User?> GetUserSafe(string username)
{
    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    using var cmd = new SqlCommand(
        "SELECT Id, Username, Email FROM Users WHERE Username = @Username", conn);

    // Method 1: AddWithValue (convenient but infers type)
    cmd.Parameters.AddWithValue("@Username", username);

    // Method 2: Explicit type and size (RECOMMENDED — avoids implicit conversion)
    cmd.Parameters.Add("@Username", SqlDbType.NVarChar, 100).Value = username;

    // Method 3: SqlParameter object
    cmd.Parameters.Add(new SqlParameter
    {
        ParameterName = "@Username",
        SqlDbType     = SqlDbType.NVarChar,
        Size          = 100,
        Value         = username
    });

    using var reader = await cmd.ExecuteReaderAsync();
    if (await reader.ReadAsync())
        return new User { Id = reader.GetInt32(0), Username = reader.GetString(1) };

    return null;
}

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Q. What is the difference between `AddWithValue` and `Add` for parameters?

using var cmd = new SqlCommand("SELECT * FROM Orders WHERE CustomerId = @Id AND Status = @Status", conn);

// AddWithValue — infers SQL type from .NET type (may cause performance issues)
cmd.Parameters.AddWithValue("@Id",     customerId);  // infers Int32
cmd.Parameters.AddWithValue("@Status", "Pending");   // infers NVarChar(7) — bad! causes plan cache pollution

// Add with explicit type — RECOMMENDED
cmd.Parameters.Add("@Id",     SqlDbType.Int)             .Value = customerId;
cmd.Parameters.Add("@Status", SqlDbType.NVarChar, 50)    .Value = "Pending";

// Why explicit type matters:
// "Pending" via AddWithValue → NVarChar(7) — different plan for each string length
// "Pending" via Add NVarChar(50) → consistent, reused execution plan

// Handling NULL parameters
cmd.Parameters.Add("@Notes", SqlDbType.NVarChar, 500).Value =
    string.IsNullOrEmpty(notes) ? DBNull.Value : (object)notes;

// Alternative null handling
cmd.Parameters.Add("@Notes", SqlDbType.NVarChar, 500).Value = (object?)notes ?? DBNull.Value;

Rule: Use Add with explicit SqlDbType for production code. Reserve AddWithValue for quick scripts.

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# 8. STORED PROCEDURES IN ADO.NET

Q. How do you call a stored procedure using SqlCommand?

-- Sample stored procedure
CREATE PROCEDURE usp_GetOrdersByCustomer
    @CustomerId INT,
    @Status     NVARCHAR(50) = 'All'
AS
BEGIN
    SELECT OrderId, OrderDate, TotalAmount, Status
    FROM Orders
    WHERE CustomerId = @CustomerId
      AND (@Status = 'All' OR Status = @Status)
    ORDER BY OrderDate DESC;
END

public async Task<List<Order>> GetOrdersByCustomerAsync(int customerId, string status = "All")
{
    var orders = new List<Order>();

    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    using var cmd = new SqlCommand("usp_GetOrdersByCustomer", conn)
    {
        CommandType    = CommandType.StoredProcedure,
        CommandTimeout = 30 // seconds
    };

    cmd.Parameters.Add("@CustomerId", SqlDbType.Int)          .Value = customerId;
    cmd.Parameters.Add("@Status",     SqlDbType.NVarChar, 50) .Value = status;

    using var reader = await cmd.ExecuteReaderAsync();
    while (await reader.ReadAsync())
    {
        orders.Add(new Order
        {
            OrderId     = reader.GetInt32(0),
            OrderDate   = reader.GetDateTime(1),
            TotalAmount = reader.GetDecimal(2),
            Status      = reader.GetString(3)
        });
    }
    return orders;
}

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Q. How do you use OUTPUT parameters and return values with stored procedures?

-- SP with INPUT, OUTPUT parameters and RETURN value
CREATE PROCEDURE usp_CreateOrder
    @CustomerId   INT,
    @TotalAmount  DECIMAL(18,2),
    @OrderId      INT OUTPUT,       -- output parameter
    @OrderNumber  NVARCHAR(20) OUTPUT
AS
BEGIN
    INSERT INTO Orders (CustomerId, TotalAmount, OrderDate, Status)
    VALUES (@CustomerId, @TotalAmount, GETUTCDATE(), 'Pending');

    SET @OrderId     = SCOPE_IDENTITY();
    SET @OrderNumber = 'ORD-' + RIGHT('0000' + CAST(@OrderId AS VARCHAR), 4);

    RETURN 0; -- 0 = success, non-zero = error code
END

public async Task<(int OrderId, string OrderNumber, int ReturnCode)> CreateOrderAsync(
    int customerId, decimal totalAmount)
{
    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    using var cmd = new SqlCommand("usp_CreateOrder", conn)
    {
        CommandType = CommandType.StoredProcedure
    };

    // INPUT parameters
    cmd.Parameters.Add("@CustomerId",  SqlDbType.Int)         .Value = customerId;
    cmd.Parameters.Add("@TotalAmount", SqlDbType.Decimal)     .Value = totalAmount;

    // OUTPUT parameters
    var orderIdParam = cmd.Parameters.Add("@OrderId", SqlDbType.Int);
    orderIdParam.Direction = ParameterDirection.Output;

    var orderNumberParam = cmd.Parameters.Add("@OrderNumber", SqlDbType.NVarChar, 20);
    orderNumberParam.Direction = ParameterDirection.Output;

    // RETURN value parameter — always named "@RETURN_VALUE"
    var returnParam = cmd.Parameters.Add("@RETURN_VALUE", SqlDbType.Int);
    returnParam.Direction = ParameterDirection.ReturnValue;

    await cmd.ExecuteNonQueryAsync();

    // Read output parameters AFTER execution
    int    orderId     = (int)orderIdParam.Value;
    string orderNumber = (string)orderNumberParam.Value;
    int    returnCode  = (int)returnParam.Value;

    return (orderId, orderNumber, returnCode);
}

// Usage
var (orderId, orderNumber, code) = await CreateOrderAsync(customerId: 7, totalAmount: 149.99m);
Console.WriteLine($"Created order #{orderId} ({orderNumber}) — return code: {code}");

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Q. How do you pass a table-valued parameter to a stored procedure?

-- Create user-defined table type
CREATE TYPE OrderItemType AS TABLE
(
    ProductId INT,
    Quantity  INT,
    UnitPrice DECIMAL(18, 2)
);

-- Stored procedure accepting table-valued parameter
CREATE PROCEDURE usp_BulkInsertOrderItems
    @OrderId    INT,
    @Items      OrderItemType READONLY
AS
BEGIN
    INSERT INTO OrderItems (OrderId, ProductId, Quantity, UnitPrice)
    SELECT @OrderId, ProductId, Quantity, UnitPrice FROM @Items;
END

public async Task BulkInsertOrderItemsAsync(int orderId, List<OrderItem> items)
{
    // Build DataTable matching the TVP schema
    var tvp = new DataTable();
    tvp.Columns.Add("ProductId", typeof(int));
    tvp.Columns.Add("Quantity",  typeof(int));
    tvp.Columns.Add("UnitPrice", typeof(decimal));

    foreach (var item in items)
        tvp.Rows.Add(item.ProductId, item.Quantity, item.UnitPrice);

    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    using var cmd = new SqlCommand("usp_BulkInsertOrderItems", conn)
    {
        CommandType = CommandType.StoredProcedure
    };

    cmd.Parameters.Add("@OrderId", SqlDbType.Int).Value = orderId;

    // Table-Valued Parameter
    var tvpParam = cmd.Parameters.AddWithValue("@Items", tvp);
    tvpParam.SqlDbType  = SqlDbType.Structured;
    tvpParam.TypeName   = "dbo.OrderItemType"; // must match DB type name

    await cmd.ExecuteNonQueryAsync();
}

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# 9. TRANSACTIONS IN ADO.NET

Q. What is a SqlTransaction? How do you implement atomic operations?

A SqlTransaction groups multiple commands into a single atomic unit — either all succeed (commit) or all fail and are undone (rollback).

// Use case: Bank transfer — debit one account, credit another
public async Task TransferFundsAsync(int fromAccountId, int toAccountId, decimal amount)
{
    using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    using var transaction = conn.BeginTransaction(
        IsolationLevel.ReadCommitted); // specify isolation level

    try
    {
        // Command 1: Debit source account
        using var debitCmd = new SqlCommand(
            "UPDATE Accounts SET Balance = Balance - @Amount WHERE AccountId = @AccountId AND Balance >= @Amount",
            conn, transaction);
        debitCmd.Parameters.Add("@Amount",    SqlDbType.Decimal).Value = amount;
        debitCmd.Parameters.Add("@AccountId", SqlDbType.Int)   .Value = fromAccountId;

        int debitRows = await debitCmd.ExecuteNonQueryAsync();
        if (debitRows == 0)
            throw new InvalidOperationException("Insufficient funds or account not found");

        // Command 2: Credit destination account
        using var creditCmd = new SqlCommand(
            "UPDATE Accounts SET Balance = Balance + @Amount WHERE AccountId = @AccountId",
            conn, transaction);
        creditCmd.Parameters.Add("@Amount",    SqlDbType.Decimal).Value = amount;
        creditCmd.Parameters.Add("@AccountId", SqlDbType.Int)   .Value = toAccountId;

        int creditRows = await creditCmd.ExecuteNonQueryAsync();
        if (creditRows == 0)
            throw new InvalidOperationException("Destination account not found");

        // Command 3: Log the transfer
        using var logCmd = new SqlCommand(
            "INSERT INTO TransferLog (FromAccount, ToAccount, Amount, TransferDate) VALUES (@From, @To, @Amt, GETUTCDATE())",
            conn, transaction);
        logCmd.Parameters.Add("@From", SqlDbType.Int)    .Value = fromAccountId;
        logCmd.Parameters.Add("@To",   SqlDbType.Int)    .Value = toAccountId;
        logCmd.Parameters.Add("@Amt",  SqlDbType.Decimal).Value = amount;
        await logCmd.ExecuteNonQueryAsync();

        // All commands succeeded — commit
        await transaction.CommitAsync();
        Console.WriteLine($"Transfer of ${amount} completed successfully");
    }
    catch
    {
        // Any failure — rollback ALL changes
        await transaction.RollbackAsync();
        Console.WriteLine("Transfer failed — all changes rolled back");
        throw; // re-throw to propagate
    }
}

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Q. What are Transaction Isolation Levels in ADO.NET?

// Isolation levels control what a transaction can "see" from concurrent transactions
var levels = new Dictionary<IsolationLevel, string>
{
    [IsolationLevel.ReadUncommitted] = "Fastest — can read dirty (uncommitted) data",
    [IsolationLevel.ReadCommitted]   = "Default — reads only committed data, non-repeatable reads possible",
    [IsolationLevel.RepeatableRead]  = "Same row always returns same data within transaction",
    [IsolationLevel.Serializable]    = "Strictest — fully isolated, no phantom reads",
    [IsolationLevel.Snapshot]        = "Uses row versioning — high concurrency without read locks"
};

// Concurrency problems by isolation level:
// Dirty Read    | ReadUncommitted | possible
// NonRepeatable | ReadUncommitted, ReadCommitted | possible
// Phantom Read  | ReadUncommitted, ReadCommitted, RepeatableRead | possible

using var conn = new SqlConnection(connectionString);
await conn.OpenAsync();

// Set isolation level via BeginTransaction
using var tx1 = conn.BeginTransaction(IsolationLevel.Serializable);

// Or set at session level via command
using var setLevel = new SqlCommand("SET TRANSACTION ISOLATION LEVEL SNAPSHOT", conn);
await setLevel.ExecuteNonQueryAsync();
using var tx2 = conn.BeginTransaction();

// Savepoints — partial rollback within a transaction
using var tx = conn.BeginTransaction();
try
{
    using var cmd1 = new SqlCommand("INSERT INTO Audit (Action) VALUES ('Step1')", conn, tx);
    await cmd1.ExecuteNonQueryAsync();

    tx.Save("SavePoint1"); // create savepoint

    using var cmd2 = new SqlCommand("UPDATE Inventory SET Stock = -1 WHERE Id = 1", conn, tx);
    await cmd2.ExecuteNonQueryAsync();

    // Partial rollback to savepoint (cmd1 still committed)
    tx.Rollback("SavePoint1");

    await tx.CommitAsync(); // commits only cmd1
}
catch
{
    await tx.RollbackAsync();
    throw;
}

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# 10. EXCEPTIONS & ERROR HANDLING

Q. How do you handle database errors in ADO.NET? What is SqlException?

SqlException is thrown by Microsoft.Data.SqlClient when SQL Server returns a warning or error. It inherits from DbException.

public async Task<int> SafeInsertAsync(Product product)
{
    using var conn = new SqlConnection(connectionString);

    try
    {
        await conn.OpenAsync();

        using var cmd = new SqlCommand(
            "INSERT INTO Products (Name, Price) VALUES (@Name, @Price); SELECT SCOPE_IDENTITY();", conn);
        cmd.Parameters.Add("@Name",  SqlDbType.NVarChar, 100).Value = product.Name;
        cmd.Parameters.Add("@Price", SqlDbType.Decimal)       .Value = product.Price;

        return Convert.ToInt32(await cmd.ExecuteScalarAsync());
    }
    catch (SqlException ex)
    {
        // SqlException-specific information
        Console.WriteLine($"SQL Error Number: {ex.Number}");    // SQL Server error code
        Console.WriteLine($"Severity:         {ex.Class}");     // 0-10: info, 11-16: user, 17+: system
        Console.WriteLine($"State:            {ex.State}");
        Console.WriteLine($"Server:           {ex.Server}");
        Console.WriteLine($"Procedure:        {ex.Procedure}"); // SP name if applicable
        Console.WriteLine($"Line:             {ex.LineNumber}");
        Console.WriteLine($"Message:          {ex.Message}");

        // Common SQL error numbers
        // 2    — cannot connect (timeout)
        // 208  — invalid object name
        // 547  — foreign key violation
        // 2601 — unique index violation
        // 2627 — primary key violation

        switch (ex.Number)
        {
            case 2627: // PK violation
            case 2601: // unique index violation
                throw new DuplicateKeyException($"Product '{product.Name}' already exists", ex);
            case 547:  // FK violation
                throw new ReferentialIntegrityException("Referenced record not found", ex);
            default:
                throw new DataAccessException("Database error during insert", ex);
        }
    }
    catch (InvalidOperationException ex) when (ex.Message.Contains("connection"))
    {
        throw new DataAccessException("Connection error", ex);
    }
    // Note: using statement guarantees conn.Dispose() even on exception
}

// Iterate multiple errors in SqlException
catch (SqlException ex)
{
    foreach (SqlError error in ex.Errors)
    {
        Console.WriteLine($"  Error {error.Number} (Severity {error.Class}): {error.Message}");
    }
    throw;
}

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Q. Where should you close a connection when an exception occurs?

// WRONG — connection not closed if exception occurs before finally
void BadPattern()
{
    var conn = new SqlConnection(connectionString);
    conn.Open();
    // exception here → conn never closed → connection leak
    var cmd = new SqlCommand("SELECT 1/0", conn); // division by zero
    cmd.ExecuteNonQuery();
    conn.Close(); // never reached!
}

// CORRECT — using statement (equivalent to try/finally)
async Task GoodPattern()
{
    // Connection is ALWAYS closed, even on exception
    await using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    await using var cmd = new SqlCommand("SELECT Id FROM Products WHERE Id = @Id", conn);
    cmd.Parameters.Add("@Id", SqlDbType.Int).Value = 1;

    var result = await cmd.ExecuteScalarAsync();
} // conn.Dispose() → Close() → returned to pool

// CORRECT — explicit try/finally (old-style)
async Task ExplicitPattern()
{
    SqlConnection? conn = null;
    try
    {
        conn = new SqlConnection(connectionString);
        await conn.OpenAsync();
        // ... commands
    }
    catch (SqlException ex)
    {
        Console.WriteLine($"DB Error: {ex.Message}");
        throw;
    }
    finally
    {
        conn?.Dispose(); // ALWAYS runs
    }
}

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# 11. PERFORMANCE & BEST PRACTICES

Q. What are the best practices for high-performance ADO.NET code?

1. Use SqlDataReader over DataSet for read performance

// FAST — streaming, forward-only
public async Task<List<ProductDto>> GetProductsFastAsync()
{
    var result = new List<ProductDto>();

    await using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();

    await using var cmd = new SqlCommand(
        "SELECT Id, Name, Price FROM Products WHERE IsActive = 1", conn)
    {
        CommandTimeout = 30
    };

    await using var reader = await cmd.ExecuteReaderAsync(
        CommandBehavior.SequentialAccess); // streaming for BLOBs

    // Cache ordinals outside the loop
    int idOrd    = reader.GetOrdinal("Id");
    int nameOrd  = reader.GetOrdinal("Name");
    int priceOrd = reader.GetOrdinal("Price");

    while (await reader.ReadAsync())
    {
        result.Add(new ProductDto(
            reader.GetInt32(idOrd),
            reader.GetString(nameOrd),
            reader.GetDecimal(priceOrd)));
    }
    return result;
}

2. Explicit SqlDbType instead of AddWithValue

// BAD — implicit type inference, plan cache pollution
cmd.Parameters.AddWithValue("@Name", "Widget");

// GOOD — explicit type, consistent execution plans
cmd.Parameters.Add("@Name", SqlDbType.NVarChar, 100).Value = "Widget";

3. Async all the way

// Use async versions throughout
await conn.OpenAsync(ct);
await cmd.ExecuteNonQueryAsync(ct);
await cmd.ExecuteReaderAsync(ct);
await cmd.ExecuteScalarAsync(ct);
await reader.ReadAsync(ct);

4. Batch inserts with table-valued parameters or SqlBulkCopy

// SqlBulkCopy — fastest way to insert large volumes
using var bulk = new SqlBulkCopy(connectionString)
{
    DestinationTableName = "Products",
    BatchSize            = 1000,
    BulkCopyTimeout      = 300
};

bulk.ColumnMappings.Add("Name",  "Name");
bulk.ColumnMappings.Add("Price", "Price");
bulk.ColumnMappings.Add("Stock", "Stock");

await bulk.WriteToServerAsync(productsDataTable);
// Or from IDataReader:
await bulk.WriteToServerAsync(reader);

5. Connection pooling — always dispose

// Each using block returns connection to pool
for (int i = 0; i < 100; i++)
{
    await using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync(); // reuses pooled connection
    // ... single unit of work
} // returned to pool → not physically closed

6. Optimize SELECT — return only needed columns

// BAD
var cmd = new SqlCommand("SELECT * FROM Products", conn);

// GOOD — only what you need
var cmd = new SqlCommand("SELECT Id, Name, Price FROM Products WHERE IsActive = 1", conn);

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Q. What is `CommandBehavior` and when do you use it?

// CommandBehavior flags control reader behavior
using var reader = await cmd.ExecuteReaderAsync(
    CommandBehavior.CloseConnection |    // Close connection when reader is closed
    CommandBehavior.SingleRow       |    // Hint: only one row expected (optimization)
    CommandBehavior.SequentialAccess     // Columns must be read left-to-right (BLOB streaming)
);

// CloseConnection — useful in repository methods that return a reader
public static SqlDataReader GetReaderWithAutoClose(string sql, SqlConnection conn)
{
    conn.Open();
    var cmd = new SqlCommand(sql, conn);
    return cmd.ExecuteReader(CommandBehavior.CloseConnection);
    // Caller reads then disposes reader → connection auto-closed
}

// SequentialAccess — for reading large binary/text columns
using var reader = await cmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess);
while (await reader.ReadAsync())
{
    // Must read columns left-to-right when SequentialAccess is used
    long bytesRead;
    var buffer = new byte[4096];
    using var ms = new MemoryStream();
    long dataIndex = 0;
    while ((bytesRead = reader.GetBytes(0, dataIndex, buffer, 0, buffer.Length)) > 0)
    {
        ms.Write(buffer, 0, (int)bytesRead);
        dataIndex += bytesRead;
    }
}

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# 12. ADO.NET vs EF vs DAPPER

Q. What is the difference between ADO.NET, Entity Framework Core, and Dapper?

Aspect	ADO.NET (raw)	Dapper	Entity Framework Core
Abstraction level	Low (manual everything)	Thin micro-ORM	High (full ORM)
Performance	Fastest	Near ADO.NET	Slower (query translation, tracking)
SQL control	Full control	Full control	Limited (can use raw SQL)
Mapping	Manual (`reader.GetString`)	Automatic (reflection)	Automatic (+ conventions)
Migrations	Manual	Manual	Built-in
Learning curve	Low	Low	High
Code amount	High (verbose)	Low	Lowest
Best for	Max performance, reporting	APIs, microservices	CRUD-heavy enterprise apps

// Same query: "Get customer by ID" — three approaches

// 1. ADO.NET — full control, most verbose
public async Task<Customer?> GetByIdAdoNet(int id)
{
    await using var conn = new SqlConnection(connectionString);
    await conn.OpenAsync();
    await using var cmd  = new SqlCommand(
        "SELECT Id, Name, Email, CreatedAt FROM Customers WHERE Id = @Id", conn);
    cmd.Parameters.Add("@Id", SqlDbType.Int).Value = id;
    await using var reader = await cmd.ExecuteReaderAsync();
    if (!await reader.ReadAsync()) return null;
    return new Customer
    {
        Id        = reader.GetInt32(0),
        Name      = reader.GetString(1),
        Email     = reader.GetString(2),
        CreatedAt = reader.GetDateTime(3)
    };
}

// 2. Dapper — concise, still uses raw SQL
// Install: dotnet add package Dapper
public async Task<Customer?> GetByIdDapper(int id)
{
    await using var conn = new SqlConnection(connectionString);
    return await conn.QuerySingleOrDefaultAsync<Customer>(
        "SELECT Id, Name, Email, CreatedAt FROM Customers WHERE Id = @Id",
        new { Id = id });
}

// 3. Entity Framework Core — no SQL, LINQ-based
public async Task<Customer?> GetByIdEF(int id)
    => await _context.Customers
        .AsNoTracking()
        .FirstOrDefaultAsync(c => c.Id == id);

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Q. When should you choose ADO.NET over EF Core or Dapper?

// Choose ADO.NET when:

// 1. MAXIMUM PERFORMANCE required — bulk operations, ETL, reporting
public async Task BulkLoadProductsAsync(IEnumerable<Product> products)
{
    using var bulk = new SqlBulkCopy(connectionString);
    bulk.DestinationTableName = "Products";

    var table = new DataTable();
    table.Columns.Add("Name",  typeof(string));
    table.Columns.Add("Price", typeof(decimal));

    foreach (var p in products)
        table.Rows.Add(p.Name, p.Price);

    await bulk.WriteToServerAsync(table);
    // Inserts 100,000 rows in seconds — impossible to match with EF Core
}

// 2. Complex stored procedures with OUTPUT params
var (orderId, number, code) = await CreateOrderAsync(customerId, amount);

// 3. Fine-grained connection/transaction control
// 4. Targeting a DB not well-supported by EF Core
// 5. Minimal dependency — no ORM overhead in microservices

// Choose EF Core when:
// - Rapid CRUD development
// - Code-first migrations needed
// - Team prefers LINQ over SQL
// - Domain model / DDD focus

// Choose Dapper when:
// - Performance close to ADO.NET needed
// - Full SQL control but automatic mapping
// - Existing stored procedures / complex SQL
// - Lightweight microservice

// Hybrid approach (common in enterprise)
public class ProductRepository
{
    // Reads — Dapper (concise, fast)
    public Task<IEnumerable<Product>> GetAllAsync()
        => _conn.QueryAsync<Product>("SELECT * FROM Products WHERE IsActive = 1");

    // Writes — EF Core (change tracking, validation)
    public async Task<int> CreateAsync(Product product)
    {
        _context.Products.Add(product);
        await _context.SaveChangesAsync();
        return product.Id;
    }

    // Bulk operations — ADO.NET
    public Task BulkImportAsync(DataTable data)
    {
        using var bulk = new SqlBulkCopy(_connectionString);
        bulk.DestinationTableName = "Products";
        return bulk.WriteToServerAsync(data);
    }
}

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Q. What is the complete CRUD example using ADO.NET?

public class ProductRepository
{
    private readonly string _connectionString;

    public ProductRepository(string connectionString)
    {
        _connectionString = connectionString;
    }

    // CREATE
    public async Task<int> CreateAsync(Product product, CancellationToken ct = default)
    {
        await using var conn = new SqlConnection(_connectionString);
        await conn.OpenAsync(ct);

        await using var cmd = new SqlCommand(
            @"INSERT INTO Products (Name, Price, Stock, CategoryId, CreatedAt)
              VALUES (@Name, @Price, @Stock, @CategoryId, GETUTCDATE());
              SELECT SCOPE_IDENTITY();", conn);

        cmd.Parameters.Add("@Name",       SqlDbType.NVarChar, 200).Value = product.Name;
        cmd.Parameters.Add("@Price",      SqlDbType.Decimal)       .Value = product.Price;
        cmd.Parameters.Add("@Stock",      SqlDbType.Int)            .Value = product.Stock;
        cmd.Parameters.Add("@CategoryId", SqlDbType.Int)            .Value = (object?)product.CategoryId ?? DBNull.Value;

        return Convert.ToInt32(await cmd.ExecuteScalarAsync(ct));
    }

    // READ ALL
    public async Task<List<Product>> GetAllAsync(CancellationToken ct = default)
    {
        var products = new List<Product>();

        await using var conn = new SqlConnection(_connectionString);
        await conn.OpenAsync(ct);

        await using var cmd    = new SqlCommand(
            "SELECT Id, Name, Price, Stock, CategoryId FROM Products WHERE IsDeleted = 0 ORDER BY Name", conn);
        await using var reader = await cmd.ExecuteReaderAsync(ct);

        int idOrd    = reader.GetOrdinal("Id");
        int nameOrd  = reader.GetOrdinal("Name");
        int priceOrd = reader.GetOrdinal("Price");
        int stockOrd = reader.GetOrdinal("Stock");
        int catOrd   = reader.GetOrdinal("CategoryId");

        while (await reader.ReadAsync(ct))
        {
            products.Add(new Product
            {
                Id         = reader.GetInt32(idOrd),
                Name       = reader.GetString(nameOrd),
                Price      = reader.GetDecimal(priceOrd),
                Stock      = reader.GetInt32(stockOrd),
                CategoryId = reader.IsDBNull(catOrd) ? null : reader.GetInt32(catOrd)
            });
        }
        return products;
    }

    // READ BY ID
    public async Task<Product?> GetByIdAsync(int id, CancellationToken ct = default)
    {
        await using var conn = new SqlConnection(_connectionString);
        await conn.OpenAsync(ct);

        await using var cmd = new SqlCommand(
            "SELECT Id, Name, Price, Stock, CategoryId FROM Products WHERE Id = @Id AND IsDeleted = 0", conn);
        cmd.Parameters.Add("@Id", SqlDbType.Int).Value = id;

        await using var reader = await cmd.ExecuteReaderAsync(CommandBehavior.SingleRow, ct);
        if (!await reader.ReadAsync(ct)) return null;

        return new Product
        {
            Id         = reader.GetInt32(0),
            Name       = reader.GetString(1),
            Price      = reader.GetDecimal(2),
            Stock      = reader.GetInt32(3),
            CategoryId = reader.IsDBNull(4) ? null : reader.GetInt32(4)
        };
    }

    // UPDATE
    public async Task<bool> UpdateAsync(Product product, CancellationToken ct = default)
    {
        await using var conn = new SqlConnection(_connectionString);
        await conn.OpenAsync(ct);

        await using var cmd = new SqlCommand(
            @"UPDATE Products
              SET Name = @Name, Price = @Price, Stock = @Stock, CategoryId = @CategoryId, UpdatedAt = GETUTCDATE()
              WHERE Id = @Id AND IsDeleted = 0", conn);

        cmd.Parameters.Add("@Id",         SqlDbType.Int)             .Value = product.Id;
        cmd.Parameters.Add("@Name",       SqlDbType.NVarChar, 200)   .Value = product.Name;
        cmd.Parameters.Add("@Price",      SqlDbType.Decimal)          .Value = product.Price;
        cmd.Parameters.Add("@Stock",      SqlDbType.Int)              .Value = product.Stock;
        cmd.Parameters.Add("@CategoryId", SqlDbType.Int)              .Value = (object?)product.CategoryId ?? DBNull.Value;

        int rows = await cmd.ExecuteNonQueryAsync(ct);
        return rows > 0;
    }

    // DELETE (soft delete)
    public async Task<bool> DeleteAsync(int id, CancellationToken ct = default)
    {
        await using var conn = new SqlConnection(_connectionString);
        await conn.OpenAsync(ct);

        await using var cmd = new SqlCommand(
            "UPDATE Products SET IsDeleted = 1, DeletedAt = GETUTCDATE() WHERE Id = @Id AND IsDeleted = 0", conn);
        cmd.Parameters.Add("@Id", SqlDbType.Int).Value = id;

        return await cmd.ExecuteNonQueryAsync(ct) > 0;
    }
}

public class Product
{
    public int      Id         { get; set; }
    public string   Name       { get; set; } = string.Empty;
    public decimal  Price      { get; set; }
    public int      Stock      { get; set; }
    public int?     CategoryId { get; set; }
}

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ADO.NET Basics

Table of Contents

# 1. ADO.NET FUNDAMENTALS

Q. What is ADO.NET? What is its role in the .NET framework?

Q. What is the difference between Connected and Disconnected architecture in ADO.NET?

Q. What is the Provider Model in ADO.NET?

# 2. CORE ADO.NET OBJECTS

Q. What are the core ADO.NET objects and their purposes?

Q. Which ADO.NET object keeps the connection open? Which works without a connection?

# 3. CONNECTION HANDLING & LIFECYCLE

Q. How do you create, open, and close a SqlConnection properly?

Q. What is Connection Pooling in ADO.NET? How does it work?

Q. What happens if you do not close a SqlConnection?

# 4. SQLCOMMAND & COMMAND EXECUTION

Q. What are the types of SqlCommand and execution methods?

Q. What does ExecuteScalar() return? When do you use it?

# 5. SQLDATAREADER — CONNECTED MODEL

Q. What is SqlDataReader? What are its key characteristics?

Q. What are the key properties and methods of SqlDataReader?

# 6. DATASET & DATAADAPTER — DISCONNECTED MODEL

Q. What is a DataSet? What is a DataAdapter and how do they work together?

Q. How do you use DataAdapter to Fill and Update a DataSet?

Q. How do you work with DataTable directly?

# 7. PARAMETERS & SQL INJECTION PREVENTION

Q. What is SQL Injection and how do you prevent it using SqlParameter?

Q. What is the difference between AddWithValue and Add for parameters?

# 8. STORED PROCEDURES IN ADO.NET

Q. How do you call a stored procedure using SqlCommand?

Q. How do you use OUTPUT parameters and return values with stored procedures?

Q. How do you pass a table-valued parameter to a stored procedure?

# 9. TRANSACTIONS IN ADO.NET

Q. What is a SqlTransaction? How do you implement atomic operations?

Q. What are Transaction Isolation Levels in ADO.NET?

# 10. EXCEPTIONS & ERROR HANDLING

Q. How do you handle database errors in ADO.NET? What is SqlException?

Q. Where should you close a connection when an exception occurs?

# 11. PERFORMANCE & BEST PRACTICES

Q. What are the best practices for high-performance ADO.NET code?

Q. What is CommandBehavior and when do you use it?

# 12. ADO.NET vs EF vs DAPPER

Q. What is the difference between ADO.NET, Entity Framework Core, and Dapper?

Q. When should you choose ADO.NET over EF Core or Dapper?

Q. What is the complete CRUD example using ADO.NET?

Q. What does `ExecuteScalar()` return? When do you use it?

Q. What is the difference between `AddWithValue` and `Add` for parameters?

Q. What is `CommandBehavior` and when do you use it?