c# delegate

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C# version 1.0 delegate

// Declare delegate type PerformCalculatorpublic delegate int PerformCalculator(int x, int y);public class SampleClass{    private int Add(int x, int y) {        return x + y;    }    private int Minus(int x, int y)    {        return x - y;    }    public int DoCalculate(string type, int x, int y)    {        // Create PerformCalculator instance        PerformCalculator add = new PerformCalculator(Add);        PerformCalculator minus = new PerformCalculator(Minus);        if (type == "add")            // Execute delegate            return add(x, y);        else            return minus(x, y);    }}

C# version 2.0 introduced anonymous method, creating anonymous methods is essentially a way to pass a code block as a delegate paramater.

    // Declare delegate type PerformCalculator    public delegate int PerformCalculator(int x, int y);    public class Delegate    {public int DoCalculate(string type, int x, int y)        {            // Create PerformCalculator instance            PerformCalculator add = delegate(int dx, int dy) { return dx + dy; };            PerformCalculator minus = delegate(int dx, int dy) { return dx - dy; };            if (type == "add")                // Execute delegate                return add(x, y);            else                return minus(x, y);        }    }

C# 3.0 introduced lambda expressions, which are similar in concept to anonymous methods but more expressive and concise. 

    // Declare delegate type PerformCalculator    public delegate int PerformCalculator(int x, int y);    public class Delegate    {        public int DoCalculate(string type, int x, int y)        {            // Create PerformCalculator instance            PerformCalculator add = (dx, dy) => dx + dy;            PerformCalculator minus = (dx, dy) => dx - dy;            if (type == "add")                // Execute delegate                return add(x, y);            else                return minus(x, y);        }    }

Delegates are the basis for Events.

The delegate must be instantiated with a method or lamda expression that has a compatiable return type and input parameter.

c# delegate