Sam Gentile's Blog

This is part of a series.  C# 4 allows you to declare optional parameters in methods, constructors, and indexers. A parameter is optional if it specifies a default value in its declaration:

     void Foo(int x = 42) { Console.WriteLine(x); }

Optional parameters may be omitted when calling the method:

      Foo();      // 42

The default argument of 42 is actually passed to the optional parameter x - the compiler bakes the value 42 into the compiled code at the calling side. The previous call to Foo is semantically identical to:

      Foo(42);

because the compiler simply substitutes the default value of an optional parameter whenever it is used. The default value of an optional parameter must be specified by a constant expression. Optional parameters cannot be marked with ref or out.

Mandatory parameters must occur before optional parameters in both the method declaration and the method call. In the following example, the explicit value of 1 is passed to x and the default value of 0 is passed to y:

     void Foo(int x = 0, int y = 0) {Console.WriteLine(x + "," + y);  }

     void Test()

     {

          Foo(1);             // 1, 0

     }