Chapter 12 - Delegates and Lambda Expressions

C# achieves the same functionality using a delegate, which encapsulates methods as objects, enabling an indirect method call bound at runtime.

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
{
// ...
{
int i;
int j;
int temp;
if(items==null)
{
return;
}
public static void BubbleSort(int[] items, ComparisonHandler comparisonMethod)

if(comparisonMethod == null)
{
throw new ArgumentNullException("comparisonMethod");
}
for (i = items.Length - 1; i >= 0; i--)
{
for (j = 1; j <= i; 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

if (comparisonMethod(items[j - 1], items[j]))

1. The C# standard doesn't specify the delegate implementation's class hierarchy. .NET's implementation, however, does derive indirectly from System.Delegate.



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 Comparison- Handler 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)
        {
            // ...
        }
        // ...
    }

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)
{
// ...
}
{
return first > second;
}
static void Main()
{
int[] items = new int[100];
public static bool GreaterThan(int first, int second)
{
return first > second;
}
public static bool GreaterThan(int first, int second)

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