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Delegates and Lambda Expressions in C# 3.0

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Mark Michaelis discusses the use of methods as a data type and their support for publish-subscribe patterns in this chapter.

PREVIOUS CHAPTERS DISCUSSED extensively how to create classes using many of the built-in C# language facilities for object-oriented development. The objects instantiated from classes encapsulate data and operations on data. As you create more and more classes, you see common patterns in the relationships between these classes.

One such pattern is to pass an object that describes a method that the receiver can invoke. The use of methods as a data type and their support for publish-subscribe patterns is the focus of this chapter. Both C# 2.0 and C# 3.0 introduced additional syntax for programming in this area. Although C# 3.0 supports the previous syntax completely, in many cases C# 3.0 will deprecate the use of the older-style syntax. However, I have placed the earlier syntax into Advanced Topic blocks, which you can largely ignore unless you require support for an earlier compiler.

Introducing Delegates

Veteran C and C++ programmers have long used method pointers as a means to pass executable steps as parameters to another method. C# achieves the same functionality using a delegate, which encapsulates methods as objects, enabling an indirect method call bound at runtime. Consider an example of where this is useful.

Defining the Scenario

Although not necessarily efficient, perhaps one of the simplest sort routines is a bubble sort. Listing 12.1 shows the BubbleSort() method.

Listing 12.1. BubbleSort() Method

static class SimpleSort1
{
  public static void BubbleSort(int[] items)
  {
      int i;
      int j;
      int temp;

      if(items==null)
      {
        return;
      }

      for (i = items.Length - 1; i >= 0; i--)
      {
          for (j = 1; j <= i; j++)
          {
              if (items[j - 1] > items[j])
              {
                  temp = items[j - 1];
                  items[j - 1] = items[j];
                  items[j] = temp;
              }
          }
      }
  }
  // ...
}

This method will sort an array of integers in ascending order. However, if you wanted to support the option to sort the integers in descending order, you would have essentially two options. You could duplicate the code and replace the greater-than operator with a less-than operator. Alternatively, you could pass in an additional parameter indicating how to perform the sort, as shown in Listing 12.2.

Listing 12.2. BubbleSort() Method, Ascending or Descending

class SimpleSort2
{
  public enum SortType
  {
      Ascending,
      Descending
  }

  public static void BubbleSort(int[] items, SortType sortOrder)
  {
      int i;
      int j;
      int temp;

      if(items==null)
      {
        return;
      }

      for (i = items.Length - 1; i >= 0; i--)
      {
          for (j = 1; j <= i; j++)
          {
              switch (sortOrder)
              {
                  case SortType.Ascending :
                      if (items[j - 1] > items[j])
                        {
                          temp = items[j - 1];
                          items[j - 1] = items[j];
                          items[j] = temp;
                      }

                      break;

                  case SortType.Descending :
                      if (items[j - 1] < items[j])
                        {
                          temp = items[j - 1];
                          items[j - 1] = items[j];
                          items[j] = temp;
                      }

                      break;
              }
          }
      }
  }
  // ...
}

However, this handles only two of the possible sort orders. If you wanted to sort them alphabetically, randomize the collection, or order them via some other criterion, it would not take long before the number of BubbleSort() methods and corresponding SortType values would become cumbersome.

Delegate Data Types

To increase the flexibility (and reduce code duplication), you can pass in the comparison method as a parameter to the BubbleSort() method. Moreover, in order to pass a method as a parameter, there needs to be a data type that can represent that method—in other words, a delegate. Listing 12.3 includes a modification to the BubbleSort() method that takes a delegate parameter. In this case, the delegate data type is ComparisonHandler.

Listing 12.3. BubbleSort() Method with Delegate Parameter

class DelegateSample
{
  // ...

  public static void BubbleSort(
      int[] items, ComparisonHandler comparisonMethod)
  {
      int i;
      int j;
      int temp;

      if(items==null)
      {
        return;
      }

      if(comparisonMethod == null)
      {
          throw new ArgumentNullException("comparisonMethod");
      }

      for (i = items.Length - 1; i >= 0; i--)
      {
          for (j = 1; j <= i; j++)
          {
              if (comparisonMethod(items[j - 1], items[j]))
              {
                temp = items[j - 1];
                items[j - 1] = items[j];
                items[j] = temp;
              }
          }
      }
  }
  // ...
}

ComparisonHandler is a data type that represents a method for comparing two integers. Within the BubbleSort() method you then use the instance of the ComparisonHandler, called comparisonMethod, inside the conditional expression. Since comparisonMethod represents a method, the syntax to invoke the method is identical to calling the method directly. In this case, comparisonMethod takes two integer parameters and returns a Boolean value that indicates whether the first integer is greater than the second one.

Perhaps more noteworthy than the particular algorithm, the ComparisonHandler delegate is strongly typed to return a bool and to accept only two integer parameters. Just as with any other method, the call to a delegate is strongly typed, and if the data types do not match up, then the C# compiler reports an error. Let us consider how the delegate works internally.

Delegate Internals

C# defines all delegates, including ComparisonHandler, as derived indirectly from System.Delegate, as shown in Figure 12.1.1

Figure 12.1

Figure 12.1 Delegate Types Object Model

The first property is of type System.Reflection.MethodInfo, which I cover in Chapter 17. MethodInfo describes the signature of a particular method, including its name, parameters, and return type. In addition to MethodInfo, a delegate also needs the instance of the object containing the method to invoke. This is the purpose of the second property, Target. In the case of a static method, Target corresponds to the type itself. The purpose of the MulticastDelegate class is the topic of the next chapter.

Defining a Delegate Type

You saw how to define a method that uses a delegate, and you learned how to invoke a call to the delegate simply by treating the delegate variable as a method. However, you have yet to learn how to declare a delegate data type. For example, you have not learned how to define ComparisonHandler such that it requires two integer parameters and returns a bool.

Although all delegate data types derive indirectly from System.Delegate, the C# compiler does not allow you to define a class that derives directly or indirectly (via System.MulticastDelegate) from System.Delegate. Listing 12.4, therefore, is not valid.

Listing 12.4. System.Delegate Cannot Explicitly Be a Base Class

// ERROR: 'ComparisonHandler' cannot
// inherit from special class 'System.Delegate'
public class ComparisonHandler: System.Delegate
{
  // ...
}

In its place, C# uses the delegate keyword. This keyword causes the compiler to generate a class similar to the one shown in Listing 12.4. Listing 12.5 shows the syntax for declaring a delegate data type.

Listing 12.5. Declaring a Delegate Data Type

public delegate bool ComparisonHandler (
  int first, int second);

In other words, the delegate keyword is shorthand for declaring a reference type derived ultimately from System.Delegate. In fact, if the delegate declaration appeared within another class, then the delegate type, ComparisonHandler, would be a nested type (see Listing 12.6).

Listing 12.6. Declaring a Nested Delegate Data Type

class DelegateSample
{
  public delegate bool ComparisonHandler (
    int first, int second);
}

In this case, the data type would be DelegateSample.ComparisonHandler because it is defined as a nested type within DelegateSample.

Instantiating a Delegate

In this final step of implementing the BubbleSort() method with a delegate, you will learn how to call the method and pass a delegate instance—specifically, an instance of type ComparisonHandler. To instantiate a delegate, you need a method that corresponds to the signature of the delegate type itself. In the case of ComparisonHandler, that method takes two integers and returns a bool. The name of the method is not significant. Listing 12.7 shows the code for a greater-than method.

Listing 12.7. Declaring a ComparisonHandler-Compatible Method

public delegate bool ComparisonHandler (
  int first, int second);
____________________________________________________________

____________________________________________________________
class DelegateSample
{


  public static void BubbleSort(
    int[] items, ComparisonHandler comparisonMethod)
  {
      // ...
  }

  public static bool GreaterThan(int first, int second)
  {
    return first > second;
  }
  // ...
}

With this method defined, you can call BubbleSort() and pass the delegate instance that contains this method. Beginning with C# 2.0, you simply specify the name of the delegate method (see Listing 12.8).

Listing 12.8. Passing a Delegate Instance as a Parameter in C# 2.0

public delegate bool ComparisonHandler (
  int first, int second);
_________________________________________________________

_________________________________________________________
class DelegateSample
{
  public static void BubbleSort(
      int[] items, ComparisonHandler comparisonMethod)
  {
       // ...
  }

  public static bool GreaterThan(int first, int second)
  {
      return first > second;
  }

  static void Main()
  {
     int[] items = new int[100];
     Random random = new Random();
     for (int i = 0; i < items.Length; i++)
     {
         items[i] = random.Next(int.MinValue, int.MaxValue);
     }

      BubbleSort(items, GreaterThan);
      
      for (int i = 0; i < items.Length; i++)
      {
          Console.WriteLine(items[i]);
      }
  }

}

Note that the ComparisonHandler delegate is a reference type, but you do not necessarily use new to instantiate it. The facility to pass the name rather than explicit instantiation is delegate inference, a new syntax beginning with C# 2.0. With this syntax, the compiler uses the method name to look up the method signature and verify that it matches the method's parameter type.

The approach of passing the delegate to specify the sort order is significantly more flexible than the approach listed at the beginning of this chapter. With the delegate approach, you can change the sort order to be alphabetical simply by adding an alternative delegate to convert integers to strings as part of the comparison. Listing 12.10 shows a full listing that demonstrates alphabetical sorting, and Output 12.1 shows the results.

Listing 12.10. Using a Different ComparisonHandler-Compatible Method

using System;
class DelegateSample
{

  public delegate bool ComparisonHandler(int first, int second);

  public static void BubbleSort(
      int[] items, ComparisonHandler comparisonMethod)
  {
      int i;
      int j;
      int temp;

      for (i = items.Length - 1; i >= 0; i--)
      {
          for (j = 1; j <= i; j++)
          {
              if (comparisonMethod(items[j - 1], items[j]))
              {
                  temp = items[j - 1];
                  items[j - 1] = items[j];
                  items[j] = temp;
              }
          }
      }
  }

  public static bool GreaterThan(int first, int second)
  {
      return first > second;
  }

  public static bool AlphabeticalGreaterThan(
      int first, int second)
  {
      int comparison;
      comparison = (first.ToString().CompareTo(
          second.ToString()));

      return comparison > 0;
  }
  
  static void Main(string[] args)
  {

      int i;
      int[] items = new int[5];

      for (i=0; i<items.Length; i++)
      {
          Console.Write("Enter an integer: ");
          items[i] = int.Parse(Console.ReadLine());
      }

        BubbleSort(items, AlphabeticalGreaterThan);

      
      for (i = 0; i < items.Length; i++)
      {
          Console.WriteLine(items[i]);
      }
  }
}

Output 12.1:

Enter an integer: 1
Enter an integer: 12
Enter an integer: 13
Enter an integer: 5
Enter an integer: 4
1
12
13
4
5

The alphabetic order is different from the numeric order. Note how simple it was to add this additional sort mechanism, however, compared to the process used at the beginning of the chapter.

The only changes to create the alphabetical sort order were the addition of the AlphabeticalGreaterThan method and then passing that method into the call to BubbleSort().

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