C# Delegate

Overview

• Delegate defines a function signature
• Delegate is a place holder for a future function
• Delegate is an object
• Need to be instantiated before being used
• Can be passed around like an object

Define a simple delegate

• // Define a delegate for fxn signature
  public delegate 
    void SwapItems(
    ref string str1, ref string str2);
   
  // Example class
  public class MySwapClass {
    // Define a conforming method 
    public void MySwapFxn(
      ref string str1, ref string str2){
      // Implementation
    }
   
    // Use delegate as parameter
    public void SortItems(
      string[] items, SwapItems fxn){
      fxn(items[0], items[1]);
    }
   
    // Run
    public static void Main()
    {
      MySwapClass msc = new MySwapClass();
      // Construct a delegate object
      SwapItems myswapfxn = new SwapItems(
        msc.MySwapFxn);
      // Items to sort
      string[] items = {"one", "two"};
      // Pass delegate object
      msc.SortItems(items, myswapfxn);
    }
  }

Define a multicast delegate

• // Define a multicase delegate
  // Need to be a void return type
  public delegate void MyDelegate();
   
  // Class using delegate
  public class DelegateConsumer
  {
    // Delegate object
    public MyDelegate md;
   
    // Constructor
    public DelegateConsumer()
    {
    }
   
    // Invoke delegate
    public void InvokeDelegate()
    {
      if (md != null)
      {
        md();
      }
    }
  }
   
  // Handlers
  public class MyDelegateHandlers
  {
    public static void Handler1()
    {
      Console.WriteLine(
        "Invoking Handler1...");
    }
   
    public static void Handler2()
    {
      Console.WriteLine(
        "Invoking Handler2...");
    }
  }
   
  // Test run
  public class TestMulticastDelegate
  {
    public static void Main()
    {
      DelegateConsumer dc = 
        new DelegateConsumer();
      MyDelegate d1 = new MyDelegate(
        MyDelegateHandlers.Handler1);
      MyDelegate d2 = new MyDelegate(
        MyDelegateHandlers.Handler2);
      dc.md += d1;
      dc.md += d2;
      dc.InvokeDelegate();
    }
  }