C# - Delegates

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C# delegates are similar to pointers to functions, in C or C++. A delegate is a reference type variable that holds the reference to a method. The reference can be changed at runtime.

Delegates are especially used for implementing events and the call-back methods. All delegates are implicitly derived from the System.Delegate class.

Declaring Delegates

Delegate declaration determines the methods that can be referenced by the delegate. A delegate can refer to a method, which has the same signature as that of the delegate.

For example, consider a delegate −

public delegate int MyDelegate (string s);

The preceding delegate can be used to reference any method that has a single string parameter and returns an int type variable.

Syntax for delegate declaration is −

delegate <return type> <delegate-name> <parameter list>

Instantiating Delegates

Once a delegate type is declared, a delegate object must be created with the new keyword and be associated with a particular method. When creating a delegate, the argument passed to the new expression is written similar to a method call, but without the arguments to the method. For example −

public delegate void printString(string s);
...
printString ps1 = new printString(WriteToScreen);
printString ps2 = new printString(WriteToFile);

Example of Delegate Implementation

Following example demonstrates declaration, instantiation, and use of a delegate that can be used to reference methods that take an integer parameter and returns an integer value.

using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {
   
   class TestDelegate {
      static int num = 10;
      
      public static int AddNum(int p) {
         num += p;
         return num;
      }
      public static int MultNum(int q) {
         num *= q;
         return num;
      }
      public static int getNum() {
         return num;
      }
      static void Main(string[] args) {
         //create delegate instances
         NumberChanger nc1 = new NumberChanger(AddNum);
         NumberChanger nc2 = new NumberChanger(MultNum);
         
         //calling the methods using the delegate objects
         nc1(25);
         Console.WriteLine("Value of Num: {0}", getNum());
         nc2(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Value of Num: 35
Value of Num: 175

Multicast Delegates

Delegate objects can be composed using the "+" operator. A composed delegate calls the two delegates it was composed from. Only delegates of the same type can be composed. The "-" operator can be used to remove a component delegate from a composed delegate.

Using this property of delegates you can create an invocation list of methods that will be called when a delegate is invoked. This is called multicasting of a delegate.

Example

The following example, demonstrates the multi-casting of a delegate through a program −

using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {
   class TestDelegate {
      static int num = 10;
      
      public static int AddNum(int p) {
         num += p;
         return num;
      }
      public static int MultNum(int q) {
         num *= q;
         return num;
      }
      public static int getNum() {
         return num;
      }
      static void Main(string[] args) {
         //create delegate instances
         NumberChanger nc;
         NumberChanger nc1 = new NumberChanger(AddNum);
         NumberChanger nc2 = new NumberChanger(MultNum);
         
         nc = nc1;
         nc += nc2;
         
         //calling multicast
         nc(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Value of Num: 75

Using Delegates to Reference Methods

Delegates can be used to encapsulate method references that allow methods to be passed as parameters and invoked dynamically.

Example

The following example demonstrates the use of delegate. The delegate printString can be used to reference method that takes a string as input and returns nothing.

We use this delegate to call two methods, the first prints the string to the console, and the second one prints it to a file −

using System;
using System.IO;

namespace DelegateAppl {
   class PrintString {
      static FileStream fs;
      static StreamWriter sw;
      
      // delegate declaration
      public delegate void printString(string s);

      // this method prints to the console
      public static void WriteToScreen(string str) {
         Console.WriteLine("The String is: {0}", str);
      }
      
      //this method prints to a file
      public static void WriteToFile(string s) {
         fs = new FileStream("c:\\message.txt",
         FileMode.Append, FileAccess.Write);
         sw = new StreamWriter(fs);
         sw.WriteLine(s);
         sw.Flush();
         sw.Close();
         fs.Close();
      }
      
      // this method takes the delegate as parameter and uses it to
      // call the methods as required
      public static void sendString(printString ps) {
         ps("Hello World");
      }
      
      static void Main(string[] args) {
         printString ps1 = new printString(WriteToScreen);
         printString ps2 = new printString(WriteToFile);
         sendString(ps1);
         sendString(ps2);
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

The String is: Hello World

Built-in Delegates

The following table shows the built-in generic delegates in C#:

Example

The following example demonstrates the use of built-in delegates Func and Predicate:

using System;

class Program {
    static void Main() {
        Func<int, int, int> subtract = (a, b) => a - b;
        Console.WriteLine($"Difference: {subtract(10, 4)}");

        Predicate<string> isShort = s => s.Length < 5;
        Console.WriteLine($"Is 'Hi' short? {isShort("Hi")}");
    }
}

When the above code is compiled and executed, it produces the following result −

Difference: 6
Is 'Hi' short? True