Question

Should I use using?

 Private Sub btntest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btntest.Click
        If sqlConnection.State = ConnectionState.Closed Then
            sqlConnection.Open()
        End If
        Dim query = "Select * from tablebusiness"

        Dim cmd = New MySqlCommand(query, sqlConnection)

        Dim data = cmd.ExecuteReader()

        Do While data.Read
        Loop

        Dim cmd1 = New MySqlCommand(query, sqlConnection)
    Dim data1 = cmd1.ExecuteReader //Error. Already have data reader 
                                  //Error There is already an open DataReader associated with this Connection which must be closed first.

    Dim check = 1

    'sqlConnection.Close()
End Sub

Answer 1

Although you have not let us know what the error is (which makes solving any problem much harder), I expect the issue is arising because you are trying to re-use the SqlConnection object for 2 different commands. Especially since you are not disposing your first command before initialising the second.

Firstly, use 2 different SqlConnection objects to manage the connection to the database. You are not putting any more overhead on the database or the code if you do this. Let the .NET framework connection pooling do its job – don’t try to do it yourself. You don’t need to do anything specific to enable connection pooling (although you can disable it by setting Pooling=false in your connection string).

Secondly use the using statement to correctly dispose your SqlConnection, SqlCommand, and SqlDataReader objects. e.g.

    Using connection As New SqlConnection(connectionString)
        connection.Open()
        Using Command As New SqlCommand(query, connection)
            Using reader As SqlDataReader = Command.ExecuteReader()
                While reader.Read()
                    'Do Stuff'
                End While
            End Using
        End Using
        connection.Close()
    End Using