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Anonymous Functions (C# Programming Guide)

An anonymous function is an "inline" statement or expression that can be used wherever a delegate type is expected. You can use it to initialize a named delegate or pass it instead of a named delegate type as a method parameter.

There are two kinds of anonymous functions, which are discussed individually in the following topics:

In C# 1.0, you created an instance of a delegate by explicitly initializing it with a method that was defined elsewhere in the code. C# 2.0 introduced the concept of anonymous methods as a way to write unnamed inline statement blocks that can be executed in a delegate invocation. C# 3.0 introduced lambda expressions, which are similar in concept to anonymous methods but more expressive and concise. These two features are known collectively as anonymous functions. In general, applications that target version 3.5 and later of the .NET Framework should use lambda expressions.

The following example demonstrates the evolution of delegate creation from C# 1.0 to C# 3.0:

class Test
{
    delegate void TestDelegate(string s);
    static void M(string s)
    {
        Console.WriteLine(s);
    }

    static void Main(string[] args)
    {
        // Original delegate syntax required  
        // initialization with a named method.
        TestDelegate testDelA = new TestDelegate(M);

        // C# 2.0: A delegate can be initialized with 
        // inline code, called an "anonymous method." This
        // method takes a string as an input parameter.
        TestDelegate testDelB = delegate(string s) { Console.WriteLine(s); };

        // C# 3.0. A delegate can be initialized with 
        // a lambda expression. The lambda also takes a string 
        // as an input parameter (x). The type of x is inferred by the compiler.
        TestDelegate testDelC = (x) => { Console.WriteLine(x); };

        // Invoke the delegates.
        testDelA("Hello. My name is M and I write lines.");
        testDelB("That's nothing. I'm anonymous and ");
        testDelC("I'm a famous author.");

        // Keep console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
    Hello. My name is M and I write lines.
    That's nothing. I'm anonymous and
    I'm a famous author.
    Press any key to exit.
 */

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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