Error message when you use CommandBuilder: "An unhandled exception of type 'System.NullReferenceException' occurred"

This article refers to the following Microsoft .NET Framework Class Library namespaces:
  • System.Data.OleDb
  • System.Data.SqlClient

Symptoms

If you use the CommandBuilder object to explicitly get commands for the DataAdapter object as follows:
da.InsertCommand = cb.GetInsertCommand
and then run the following Visual Basic .NET code
cb.DataAdapter = Nothing
or the following Visual C# .NET code
cb.DataAdapter = null;
the commands that you add to the DataAdapter are deleted, and you receive the following error message:
An unhandled exception of type 'System.NullReferenceException' occurred in
app_name.exe

Additional information: Object reference not set to an instance of an object.

Cause

CommandBuilder deletes the commands that it generates when it is disassociated from a DataAdapter. CommandBuilder and DataAdapter are linked; when they are unlinked or disassociated, the commands are nulled. This problem does not affect commands that you build from the beginning (from scratch).

Resolution

Use one of the following methods to resolve this problem:
  • Do not disassociate the CommandBuilder from the DataSet.
  • Build the commands yourself, either in code or through Visual Data Tools.

Status

This behavior is by design.

More Information

Steps to Reproduce the Behavior

  1. Create a new Visual Basic .NET Windows Application project. Form1 is added to the project by default.
  2. Add a Button control to Form1.
  3. Switch to Code view, and add the following code to the top of the Code window:
    Imports System.Data.OleDb
    Imports System.Data.SqlClient
  4. Add the following code to the Click event of the Button:
            Dim con As New SqlConnection("server=myserver;uid=sa;pwd=mypassword;" & _
    "database=northwind")
    Dim da As New SqlDataAdapter("Select * From Customers", con)
    Dim cb As New SqlCommandBuilder(da)
    Dim cmdInsert As New SqlCommand( _
    "Insert Into Customer (CustomerID) Value ('AAAAA')", con)
    da.InsertCommand = cmdInsert
    da.UpdateCommand = cb.GetUpdateCommand
    da.DeleteCommand = cb.GetDeleteCommand
    Debug.WriteLine(da.InsertCommand.CommandText)
    Debug.WriteLine(da.DeleteCommand.CommandText)
    cb.DataAdapter = Nothing ' Comment out this line to avoid the error.
    cb = Nothing
    Debug.WriteLine(da.InsertCommand.CommandText)
    Debug.WriteLine(da.DeleteCommand.CommandText) 'Error occurs here.
  5. Modify your connection string as appropriate for your environment.
  6. Press the F5 key to compile and to run the application.
Propriedades

ID do Artigo: 310367 - Última Revisão: 23 de mar de 2009 - Revisão: 1

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