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DbConnection, DbCommand and DbException

Once you have created a DbProviderFactory and a DbConnection, you can then work with commands and data readers to retrieve data from the data source.

This example takes a DbConnection object as an argument. A DbCommand is created to select data from the Categories table by setting the CommandText to a SQL SELECT statement. The code assumes that the Categories table exists at the data source. The connection is opened and the data is retrieved using a DbDataReader.

' Takes a DbConnection and creates a DbCommand to retrieve data 
' from the Categories table by executing a DbDataReader.  
Private Shared Sub DbCommandSelect(ByVal connection As DbConnection)

    Dim queryString As String = _
       "SELECT CategoryID, CategoryName FROM Categories" 

    ' Check for valid DbConnection. 
    If Not connection Is Nothing Then 
        Using connection
            Try 
                ' Create the command. 
                Dim command As DbCommand = connection.CreateCommand()
                command.CommandText = queryString
                command.CommandType = CommandType.Text

                ' Open the connection.
                connection.Open()

                ' Retrieve the data. 
                Dim reader As DbDataReader = command.ExecuteReader()
                Do While reader.Read()
                    Console.WriteLine("{0}. {1}", reader(0), reader(1))
                Loop 

            Catch ex As Exception
                Console.WriteLine("Exception.Message: {0}", ex.Message)
            End Try 
        End Using 
    Else
        Console.WriteLine("Failed: DbConnection is Nothing.")
    End If 
End Sub

This example takes a DbConnection object as an argument. If the DbConnection is valid, the connection is opened and a DbCommand is created and executed. The CommandText is set to a SQL INSERT statement that performs an insert to the Categories table in the Northwind database. The code assumes that the Northwind database exists at the data source, and that the SQL syntax used in the INSERT statement is valid for the specified provider. Errors occurring at the data source are handled by the DbException code block, and all other exceptions are handled in the Exception block.

' Takes a DbConnection and executes an INSERT statement. 
' Assumes SQL INSERT syntax is supported by provider. 
Private Shared Sub ExecuteDbCommand(ByVal connection As DbConnection)

    ' Check for valid DbConnection object. 
    If Not connection Is Nothing Then 
        Using connection
            Try 
                ' Open the connection.
                connection.Open()

                ' Create and execute the DbCommand. 
                Dim command As DbCommand = connection.CreateCommand()
                command.CommandText = _
                  "INSERT INTO Categories (CategoryName) VALUES ('Low Carb')" 
                Dim rows As Integer = command.ExecuteNonQuery()

                ' Display number of rows inserted.
                Console.WriteLine("Inserted {0} rows.", rows)

            ' Handle data errors. 
            Catch exDb As DbException
                Console.WriteLine("DbException.GetType: {0}", exDb.GetType())
                Console.WriteLine("DbException.Source: {0}", exDb.Source)
                Console.WriteLine("DbException.ErrorCode: {0}", exDb.ErrorCode)
                Console.WriteLine("DbException.Message: {0}", exDb.Message)

            ' Handle all other exceptions. 
            Catch ex As Exception
                Console.WriteLine("Exception.Message: {0}", ex.Message)
            End Try 
        End Using 
    Else
        Console.WriteLine("Failed: DbConnection is Nothing.")
    End If 
End Sub

The DbException class is the base class for all exceptions thrown on behalf of a data source. You can use it in your exception handling code to handle exceptions thrown by different providers without having to reference a specific exception class. The following code fragment demonstrates how to use DbException to display error information returned by the data source using GetType, Source, ErrorCode, and Message properties. The output will display the type of error, the source indicating the provider name, an error code, and the message associated with the error.

    Try
        ' Do work here.
    Catch ex As DbException
        ' Display information about the exception.
        Console.WriteLine("GetType: {0}", ex.GetType())
        Console.WriteLine("Source: {0}", ex.Source)
        Console.WriteLine("ErrorCode: {0}", ex.ErrorCode)
        Console.WriteLine("Message: {0}", ex.Message)
    Finally
        ' Perform cleanup here.
    End Try
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