C# 0 The Complete Reference

partial class XY {

  public int Y { get; set; }

}

The following file demonstrates the use of 

XY

:

// Demonstrate partial class definitions.

using System;

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class Test {

  static void Main() {

    XY xy = new XY(1, 2);

    Console.WriteLine(xy.X + "," + xy.Y);

  }

}

To use 

XY

, all files must be included in the compile. For example, assuming the 

XY

 files 

are called 

xy1.cs

,

 xy2.cs

, and 

xy3.cs

, and that the 

Test

 class is contained in a file called 

test.cs

,

then to compile 

Test

, use the following command line:

csc test.cs xy1.cs xy2.cs xy3.cs

One last point: It is legal to have partial generic classes. However, the type parameters 

of each partial declaration must match the other parts.

Partial Methods

As the preceding section described, you can use 

partial

 to create a partial type. Beginning 

with C# 3.0, there is a second use of 

partial

 that lets you create a 

partial method

 within a 

partial type. A partial method has its declaration in one part and its implementation in 

another part. Thus, in a partial class or structure, 

partial

 can be used to allow the declaration 

of a method to be separate from its implementation.

The key aspect of a partial method is that the implementation is not required! When the 

partial method is not implemented by another part of the class or structure, then all calls to 

the partial method are silently ignored. This makes it possible for a class to specify, but not 

require, optional functionality. If that functionality is not implemented, then it is simply 

ignored.

Here is an expanded version of the preceding program that creates a partial method 

called

Show( )

. It is called by another method called 

ShowXY( )

. (For convenience, all pieces 

of the partial class 

XY

 are shown in one file, but they could have been organized into 

separate files, as illustrated in the preceding section.)

// Demonstrate a partial method.

using System;

partial class XY {

  public XY(int a, int b) {

    X = a;

    Y = b;

  }

  // Declare a partial method.

  partial void Show();

}

partial class XY {

  public int X { get; set; }

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