HOW TO: Make a Typed DataSet Return a Default Value Instead of DBNull by Using Visual Basic .NET (318039)
The information in this article applies to:
- Microsoft ADO.NET (included with the .NET Framework)
- Microsoft ADO.NET (included with the .NET Framework 1.1)
- Microsoft Visual Basic .NET (2002)
- Microsoft Visual Basic .NET (2003)
This article was previously published under Q318039 For a Microsoft Visual C# .NET version of this article, see 318048.
IN THIS TASKSUMMARY
This step-by-step article describes how to change the XML Schema file to make a typed DataSet return a default value instead of DBNULL.
Visual Studio .NET may return an exception if the value of a DataRow is DBNULL, so you must handle the exception properly for values that are DBNULL. If you modify the default value for DBNULL fields, you can retrieve a value you specify other than DBNULL.
back to the topRequirements
The following list outlines the recommended hardware, software, network infrastructure, and service packs that you need:
- Microsoft Windows XP, Windows 2000, or Windows NT 4.0 Service Pack 6a
- Microsoft Data Access Components (MDAC) 2.6 or later
- Microsoft Visual Studio .NET
This article assumes that you are familiar with the following topics:
- Visual Basic .NET syntax
- ADO.NET syntax
- Windows Forms
back to the topCreate the Project- Open Visual Studio .NET.
- Create a new Windows application in Visual Basic .NET. Form1 is added to the project by default.
- Place a Command button on Form1. Change the Name property of the button to btnTest.
- Use the Imports statement on the System and System.Data namespaces 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 - Use the Customers table in the Northwind database to create a typed DataSet.
- Paste the following code in the btnTest_Click event:
SqlDataAdapter1.Fill(DataSet11, "customers")
Dim crow As DataSet1.CustomersRow
For Each crow In DataSet11.Customers
If crow._Region = "-1" Then
Console.WriteLine("DBNULL")
End If
Next - Save your project.
back to the topTest the Project- Run the project and then click btnTest. The following error message appears.
An unhandled exception of type 'System.Data.StrongTypingException' occurred in nullvaluedbnull.exe
Additional information: Cannot get value because it is DBNull.
- Stop the program and open the DataSet1.xsd file from Solution Explorer.
- In the DataSet1.xsd, add the namespace codegen in the namespace declaration. Paste the following line of code for the codegen namespace immediately after the targetNamespace value:
xmlns:codegen="urn:schemas-microsoft-com:xml-msprop" - Modify the element Region to reflect the following code sample:
<xs:element name="Region" codegen:nullValue="-1" type="xs:string" minOccurs="0" /> - Save the DataSet1.xsd file. If you are prompted to reload the file, click Yes.
- Run the project and then click btnTest. The exception does not occur. Notice the DBNULLs that are printed in the output window.
back to the topPitfalls- The nullValue must match the type of the element; for example, use nullValue="0" to return 0 for null integer fields.
- The nullValue only retrieves data. You cannot use the nullValue to assign to a field.
back to the topREFERENCESUsing Annotations with a Typed DataSet
back to the top
Modification Type: | Major | Last Reviewed: | 9/4/2003 |
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Keywords: | kbHOWTOmaster KB318039 kbAudDeveloper |
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