Part 98 - Anonymous methods in c#

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In this video we will discuss, what anonymous methods are with an example.

What is an anonymous method?
In simple terms, anonymous method is a method without a name.



Let's understand how a method can exist without a name

using System;

using System.Collections.Generic;

class Program

{

    public static void Main()

    {

        List<Employee> listEmployees = new List<Employee>()

        {

            new Employee{ ID = 101, Name = "Mark"},

            new Employee{ ID = 102, Name = "John"},

            new Employee{ ID = 103, Name = "Mary"},

        };

        // Step 2: Create an instance of Predicate<Employee>

        // delegate and pass the method name as an argument

        // to the delegate constructor

        Predicate<Employee> predicateEmployee =

            new Predicate<Employee>(FindEmployee);

        // Step 3: Now pass the delegate instance as

        // the argument for Find() method

        Employee employee =

            listEmployees.Find(x => predicateEmployee(x));

        Console.WriteLine("ID = {0}, Name {1}",

            employee.ID, employee.Name);

        // Anonymous method is being passed as an argument to

        // the Find() method. This anonymous method replaces

        // the need for Step 1, 2 and 3

        employee = listEmployees.Find(delegate(Employee x)

                                  { return x.ID == 102; });

        Console.WriteLine("ID = {0}, Name {1}",

            employee.ID, employee.Name);

    }

    // Step 1: Create a method whose signature matches

    // with the signature of Predicate<Employee> delegate

    private static bool FindEmployee(Employee Emp)

    {

        return Emp.ID == 102;

    }

    public class Employee

    {

        public int ID { get; set; }

        public string Name { get; set; }

    }

}

In this example, Find() method expects a delegate to be passed as the argument. If you want to look at the signature of the delegate, right click on Find() method and select "Go To Definition" from the context menu. At this point you should see the following method.
public T Find(Predicate<T> match);

Right click on Predicate<T> and select  "Go To Definition"

Now you should see the signature of the Predicate delegate.
public delegate bool Predicate<in T>(T obj);

Notice that the delegate returns bool and expects an object of Type <T>. In our case T is Employee. 

So, to the Find() method we need to pass an instance of Predicate<Employee> delegate as an argument. Delegates are function pointers. This means when we create an instance of a delegate, we pass the name of the method as an argument to the delegate constructor. 

Step 1: Create a method whose signature matches with the signature of Predicate<Employee> delegate

private static bool FindEmployee(Employee Emp)

{

    return Emp.ID == 102;

}

Step 2: Create an instance of Predicate<Employee> delegate and pass the method name as an argument to the delegate constructor

Predicate<Employee> predicateEmployee =

    new Predicate<Employee>(FindEmployee);

Step 3: Now pass the delegate instance as the argument for Find() method

Employee employee =

    listEmployees.Find(x => predicateEmployee(x));

Anonymous methods were introduced in C# 2 and they eliminate the need for Step 1, 2 & 3, that is they provide us a way of creating delegate instances without having to write a separate method.

Now let us see, how to pass anonymous method as an argument to Find() method. 
employee = listEmployees.Find(delegate(Employee x) { return x.ID == 102; });

Subscribing for an event handler is another example

private void Form1_Load(object sender, EventArgs e)

{

    Button Button1 = new Button();

    Button1.Text = "Click Me";

    Button1.Click += new EventHandler(Button1_Click);

    this.Controls.Add(Button1);

}

void Button1_Click(object sender, EventArgs e)

{

    MessageBox.Show("Button Clicked");

}

The code above can be replaced with the following code

private void Form1_Load(object sender, EventArgs e)

{

    Button Button1 = new Button();

    Button1.Text = "Click Me";

    Button1.Click += delegate(object obj, EventArgs eventArgs)

    {

        MessageBox.Show("Button Clicked");

    };

    this.Controls.Add(Button1);

}

With anonymous Methods delegate parameters are optional. This means the below code

Button1.Click += delegate(object obj, EventArgs eventArgs)

{

    MessageBox.Show("Button Clicked");

};

can be rewritten as shown below

Button1.Click += delegate

{

    MessageBox.Show("Button Clicked");

};