How is the connection to databases established in Visual Basic, what ADO.NET classes are used to work with data, and how to properly implement closing the connection?

Answer.

In Visual Basic, the ADO.NET library is commonly used for interacting with databases. Key classes for connecting and accessing data:

• SqlConnection — to establish a connection to a SQL Server database.
• SqlCommand — to send SQL commands.
• SqlDataReader — to read data from the query result.
• SqlDataAdapter and DataSet — to work with data in memory and support a disconnected model.

Proper connection and closing the connection:

Imports System.Data.SqlClient

Using connection As New SqlConnection("connection_string_here")
    connection.Open()
    Using command As New SqlCommand("SELECT * FROM Users", connection)
        Using reader As SqlDataReader = command.ExecuteReader()
            While reader.Read()
                Console.WriteLine(reader("Username"))
            End While
        End Using ' Closes the reader
    End Using
End Using ' Automatically closes the connection

The practice of using the Using block ensures automatic closing of the connection, even if errors occur. This is critically important to prevent connection leaks.

Trick Question.

What is the difference between Dispose() and Close() for SqlConnection, and is it necessary to call both methods?

Answer: The Close() method closes the database connection, while Dispose() releases all resources associated with the connection object. Calling Dispose() includes calling Close(). Therefore, if Using is used, Dispose() is called automatically, and a separate call to Close() is not required.

connection.Close() ' Closes the connection, but the object still exists
connection.Dispose() ' Releases all resources (recommended to call through Using)

Examples of real mistakes due to ignorance of the nuances of the topic.

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Story

In a project, significant delays and errors in connecting to the database occurred: connections were opened manually but forgotten to close, leading to pool exhaustion. Solution: implementation of the Using block.

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Story

A developer mistakenly called only Close(), while the SqlConnection object remained "hanging" in memory. In the long term, this led to resource leaks and unpredictable application behavior.

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Story

Instead of working with SqlConnection, OleDbConnection was used to connect to SQL Server due to ignorance, which is less efficient and can lead to performance and compatibility issues. After switching to SqlConnection, performance improved by 30%.