Declaring a delegate type

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The following syntax creates a delegate type with name NumberInOutDelegate, representing a method which takes an int and returns an int.

public delegate int NumberInOutDelegate(int input);

This can be used as follows:

public static class Program
    static void Main()
        NumberInOutDelegate square = MathDelegates.Square;
        int answer1 = square(4); 
        Console.WriteLine(answer1); // Will output 16

        NumberInOutDelegate cube = MathDelegates.Cube;
        int answer2 = cube(4);
        Console.WriteLine(answer2); // Will output 64            

public static class MathDelegates
    static int Square (int x)
        return x*x;

    static int Cube (int x)
        return x*x*x;

The example delegate instance is executed in the same way as the Square method. A delegate instance literally acts as a delegate for the caller: the caller invokes the delegate, and then the delegate calls the target method. This indirection decouples the caller from the target method.

You can declare a generic delegate type, and in that case you may specify that the type is covariant (out) or contravariant (in) in some of the type arguments. For example:

public delegate TTo Converter<in TFrom, out TTo>(TFrom input);

Like other generic types, generic delegate types can have constraints, such as where TFrom : struct, IConvertible where TTo : new().

Avoid co- and contravariance for delegate types that are meant to be used for multicast delegates, such as event handler types. This is because concatenation (\+) can fail if the run-time type is different from the compile-time type because of the variance. For example, avoid:

public delegate void EventHandler<in TEventArgs>(object sender, TEventArgs e);

Instead, use an invariant generic type:

public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);

Also supported are delegates where some parameters are modified by ref or out, as in:

public delegate bool TryParser<T>(string input, out T result);

(sample use TryParser<decimal> example = decimal.TryParse;), or delegates where the last parameter has the params modifier. Delegate types can have optional parameters (supply default values). Delegate types can use pointer types like int* or char* in their signatures or return types (use unsafe keyword). A delegate type and its parameters can carry custom attributes.

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