InvalidCastException exception when you reference the value of a DataColumn that is NULL (310371)

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Steps to reproduce the behavior

1. Start Microsoft Visual Studio .NET.
2. Create a new Windows Application project in Visual Basic .NET. Form1 is added to the project by default.
3. Make sure that your project contains a reference to the System.Data namespace.
4. Place a Button control on Form1.
5. Change the Name property of the button to btnUntype and the Text property to Untype.
6. Use the Imports statement in the System namespace and the System.Data namespace, so that you are not required to qualify declarations in those namespaces later in your code. Add the following code to the "General Declarations" section of Form1:

      Imports System
      Imports System.Data
      Imports System.Data.SqlClient
   					

7. Add the following code in the btnUntype event:

           Dim str As String
           Dim ds As New DataSet()
           Dim r As DataRow
           Dim con As New SqlConnection("server=myserver;integrated security=sspi;database=northwind")
           Dim da As New SqlDataAdapter("select * from customers where customerid='ANTON'", con)
           da.Fill(ds, "customer")
           r = ds.Tables(0).Rows(0)
           'Uncomment to check for Null. 
           'If r.IsNull("fax") Then
           '   MessageBox.Show("Value is Null").
           'End If
           str = r("fax") 'Comment line to check for Null.
           MessageBox.Show(str) 'Comment line to check for Null.
           
   					

8. Modify the connection string (myConnString) as appropriate for your environment.
9. Save your project. On the Debug menu, click Start to run your project.
10. Click Untype. Notice the earlier-mentioned exception for an untyped DataSet.
11. Uncomment the IF statement, and then comment the lines below the IF statement to check for NULL.
12. Save your project. On the Debug menu, click Start, and then run your project. Notice that the message box displays "Value is Null".
13. Place another Button control on Form1.
14. Change the Name property of the button to btntype and the Text property to type.
15. Create a typed DataSet by using the customers table in the Northwind database. Rename the SQLDataAdapter to sqlda.
16. Add the following code to the btntype event:

            sqlda.Fill(DataSet11)
            Dim str As String
            Dim int As Integer
            Dim cust As DataSet1.CustomersRow
            cust = DataSet11.Customers.Rows(0)
            'Uncomment to catch null values.  
            'If cust.IsFaxNull Then
            '    MessageBox.Show("Value is Null").
            'End If
            str = cust.Fax() 
            MessageBox.Show(str)
            Comment line to check for Null.
    					

17. Run the application, and then click the type button. Notice the exception.
18. Uncomment the If...Then block of code, and then comment the last two lines of code.
19. Save the application, and then run the application and notice the message box.