Section 2.8. Split a Class into Multiple Files

2.8. Split a Class into Multiple Files

If you've cracked open a .NET 2.0 Windows Forms class, you'll have noticed that all the automatically generated code is missing! To understand where it's gone, you need to learn about a new feature called partial classes, which allow you to split classes into several pieces.

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Note: Have your classes grown too large to manage in one file? With the new Partial keyword, you can split a class into separate files.

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2.8.1. How do I do that?

Using the new Partial keyword, you can split a single class into as many pieces as you want. You simply define the same class in more than one place. Here's an example that defines a class named SampleClass in two pieces:

Partial Public Class SampleClass     Public Sub MethodA( )         Console.WriteLine("Method A called.")     End Sub End Class      Partial Public Class SampleClass     Public Sub MethodB( )         Console.WriteLine("Method B called.")     End Sub End Class

In this example, the two declarations are in the same file, one after the other. However, there's no reason that you can't put the two SampleClass pieces in different source code files in the same project. (The only restrictions are that you can't define the two pieces in separate assemblies or in separate namespaces.)

When you build the application containing the previous code, Visual Studio will track down each piece of SampleClass and assemble it into a complete, compiled class with two methods, MethodA( ) and MethodB( ). You can use both methods, as shown here:

Dim Obj As New SampleClass( ) Obj.MethodA( ) Obj.MethodB( )

Partial classes don't offer you much help in solving programming problems, but they can be useful in breaking up extremely large, unwieldy classes. Of course, the existence of large classes in your application could be a sign that you haven't properly factored your problem, in which case you should really break your class down into separate, not partial, classes. One of the key roles of partial classes in .NET is to hide the designer code that is automatically generated by Visual Studio, whose visibility in previous versions has been a source of annoyance to some VB programmers.

For example, when you build a .NET Windows form in Visual Basic 2005, your event handling code is placed in the source code file for the form, but the designer code that creates and configures each control and connects its event handlers is nowhere to be seen. In order to see this code, you need to select Project Show All Files from the Visual Studio menu. When you do, the file that contains the missing half of the class appears in the Solution Explorer as a separate file. Given a form named Form1, you'll actually wind up with a Form1.vb file that contains your code and a Form1.Designer.vb file that contains the automatically generated part.

2.8.2. What about...

...using the Partial keyword with structures? That works, but you can't create partial interfaces, enumerations, or any other .NET programming construct.

2.8.3. Where can I learn more?

To get more details on partial classes, refer to the index entry "Partial keyword" in the MSDN Help.