Anonymous method  is a method which is introduced in C# from 3.0 and higher version. This  method uses delegate keyword to pass a block of code. By using this methods, we can  reduce the coding overhead in instantiating delegates because we do not have to create a separate method. Creating anonymous methods is  a way to pass a code block as a delegate parameter. 

An anonymous method is an inline method . It does not have a name.

Syntax for Anonymous method is given below:

// Create a delegate.
delegate void Del(int x);

// Instantiate the delegate using an anonymous method.
Del d = delegate(int y) { /* ..... */ };

Example of Anonymous method:

using System;

namespace Anonymous_Methods
{
    class Anonymous
    {
        delegate int Del(int l, int b); //Anonymous method is created using delegate keyword.
        static void Main(string[] args)
        {
         
            new Anonymous().Subtraction();     
            new Anonymous().Multiplication();
            new Anonymous().Addition();


        }  
        public void Subtraction()
        {
            
            Del sub = delegate (int length, int breadth)
            {
                return length - breadth;
            };
            Console.WriteLine("Subtraction is:");
            Console.WriteLine(sub.Invoke(10, 5).ToString()); 
        }
        public void Addition()
        {
           
            Del add = delegate (int length, int breadth)
            {
                return length + breadth;
            };
            Console.WriteLine("Addition is:");
            Console.WriteLine(add.Invoke(7, 4).ToString());
        }
        public void Multiplication()
        {
           
            Del A = delegate (int length, int breadth)
            {
                return length * breadth;
            };
            Console.WriteLine("Area of rectangle is:");
            Console.WriteLine(A.Invoke(5, 6).ToString());
        }
    }
}

Output of the above program is shown below.

Also read : What is TempData? How to store object in TempData?