Advanced C# - Instance Method Targets

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When a delegate instance is assigned to an instance method, the delegate instance must maintain a reference not only to the method, but also to the instance of that method. The System.Delegate class’s Targetproperty represents this instance (and will be null for a delegate referencing a static method). For example:

  public delegate void ProgressReporter (int percentComplete);

  class Test
  {
    static void Main() {new Test();}
    Test ()
    {
      ProgressReporter p = InstanceProgress;
      p(99);                         // 99
      Console.WriteLine (p.Target == this);                               // True
      Console.WriteLine (p.Method);  // Void InstanceProgress(Int32)
    }

    void InstanceProgress (int percentComplete)
    {
      Console.WriteLine(percentComplete);
    }
  }

Generic Delegate Types

A delegate type may contain generic type parameters. For example:

  public delegate T Transformer<T> (T arg);

With this definition, we can write a generalizedTransformutility method that works on any type:

  public class Util
  {
    public static void Transform<T> (T[] values, Transformer<T> t)
    {
      for (int i = 0; i < values.Length; i++)
        values[i] = t(values[i]);
    }
  }  

  class Test
  {
    static void Main()
    {
      int[] values = new int[] {1, 2, 3};
      Util.Transform(values, Square);     //
dynamically hook in Square
      foreach (int i in values)
        Console.Write (i + " ");         // 1 4 9
    }

    static int Square (int x) { return x * x; }
  }

Next: Delegates Versus Interfaces >>
 

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