Action Comparison event Func Lambda List Find, Exists Predicate

Delegate

This C# keyword (delegate) is used to create method objects. With the Invoke() method, we can call a delegate as a method.

Notes, higher-order

We can pass the delegate as an argument to another method. The function is an object—a delegate is a higher-order procedure.

Example

This program declares a delegate type D that receives a string, and returns no value. In Main, we create an instance of type D.

Detail We specify the method body with a lambda expression. We can use lambda expressions with delegate types.

Then We call the delegate D method implementation with the argument "cat" using the Invoke method.

Tip This program uses the "delegate" keyword. But many programs use higher-order procedures with just lambda expressions.

using System;

class Program
{
    delegate void D(string value);

    static void Main()
    {
        // ... Specify delegate with lambda expression.
        D d = v => Console.WriteLine(v);
        // ... Invoke delegate.
        d.Invoke("cat");
    }
}
cat

Example 2

Next, we build upon the delegate concept. We declare a delegate type UppercaseDelegate. It receives a string, and returns a string.

Detail We create instances of this delegate Type. We need to declare a delegate type before creating instances of it.

Important When you construct instances of the delegate type, the target method must have the same return value and arguments.

Detail The second argument is of type UppercaseDelegate. When called, WriteOutput invokes the function argument.

Note The target method is specified as the sole argument. It is a method name, not a variable.

using System;

class Program
{
    delegate string UppercaseDelegate(string input);

    static string UppercaseFirst(string input)
    {
        char[] buffer = input.ToCharArray();
        buffer[0] = char.ToUpper(buffer[0]);
        return new string(buffer);
    }

    static string UppercaseLast(string input)
    {
        char[] buffer = input.ToCharArray();
        buffer[buffer.Length - 1] = char.ToUpper(buffer[buffer.Length - 1]);
        return new string(buffer);
    }

    static string UppercaseAll(string input)
    {
        return input.ToUpper();
    }

    static void WriteOutput(string input, UppercaseDelegate del)
    {
        Console.WriteLine("Your string before: {0}", input);
        Console.WriteLine("Your string after: {0}", del(input));
    }

    static void Main()
    {
        // Wrap the methods inside delegate instances and pass to the method.
        WriteOutput("perls", new UppercaseDelegate(UppercaseFirst));
        WriteOutput("perls", new UppercaseDelegate(UppercaseLast));
        WriteOutput("perls", new UppercaseDelegate(UppercaseAll));
    }
}Your string before: perls
Your string after: Perls
Your string before: perls
Your string after: perlS
Your string before: perls
Your string after: PERLS

Anonymous function

We can use the delegate keyword inline to specify a function argument. Methods like FindAll on List can receive this kind of syntax.

Info This is functionally equivalent to using a static method or lambda expression with the same return value and argument.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<int> values = new List<int>() { 1, 1, 1, 2, 3 };

        // This syntax enables you to embed multiple statements in an anonymous function.
        List<int> res = values.FindAll(delegate(int element)
        {
            if (element > 10)
            {
                throw new ArgumentException("element");
            }
            if (element == 8)
            {
                throw new ArgumentException("element");
            }
            return element > 1;
        });

        // Display results.
        foreach (int val in res)
        {
            Console.WriteLine(val);
        }
    }
}2
3

Lambda

The syntax for delegate functions can be complex. Lambda expressions provide a simple and terse way of specifying functions.

Tip Lambdas use the arrow syntax to separate formal parameters and the method body.

Info Whenever you see a method that receives a Predicate, Action, Func or Comparison, a lambda can be passed.

Events

Events allow you to specify that when external event occurs, such as a mouse click, a delegate method should always be executed.

But You can define custom events in any program. They are occasionally useful in complex programs.

A summary

Delegate syntax is at first confusing. Once you understand the instantiation model and type system, it offers immense benefits to certain programs.

With the type system, the language offers the ability to specify actions as data. Complicated (and possibly hard-to-understand) programs can be written.