Constans and Readonly Keywords

Constants:

        Constants are values which are known at compile time and do not change. (To create a constant value that is initialized at runtime, use the readonly keyword.) Constants are declared as a field, using the const keyword before the type of the field.

• A constant variable should have value at design time. 
• All the constant variables are static ie they are shared across all the instances of the class. You dont have to add the keyword "static".
• Constants are copied to all the dlls where is refereeing the parent class ie even if you change the the original constant value and recompile the parent dll , the other dll will still use the old value. The size of the executable goes up since all these constants are copied to the respective dlls
• Constant variables can be called with Class name only. 

 Readonly:

The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

• We cannot modify the readonly values outside the constructor or variable initializer. 

I think the following example program helps you to find the differences between Constant and Readonly

class MyClass         {             public int x;             public int y;             public const  int c1=100;             public readonly int c2;//c2=some value  is also acceptable..             public MyClass(int p1, int p2)             {                 x = p1;                 y = p2;                // c1 = p1 + p2; Generates error because we can't modify the constant variables...                 c2 = p1 + p2;                 c2 = c2 + 100; // some changes to readonly for better understanding...                 //c1= 10; then error occurs...             }              public void modify()             {                 //c2 = c2 + 200;    Error :: A readonly field cannot be assigned to (except in a constructor or a variable initializer)                    Console.WriteLine("c2 value after trying to modify:"+c2 );             }         }         class program         {             public static void Main()             {                 MyClass obj = new MyClass(11, 22);                 obj.x = 155;  //acceptable bcz normal variable...                 //obj.c2 = 55; generates an error this is the use of readonly...After creating the instance we cannot modify the readonly variables...                 Console.WriteLine("x = {0}, y = {1}", obj.x, obj.y);                 Console.WriteLine("c1 = {0}, c2 = {1}", MyClass .c1, obj.c2);//calling const c1 with its class name...and Readonly with Object Name.                 Console.ReadLine();             }         } // Constant variables can be initialized at the time of declaration only whereas readonly initialized At declaration or any where. // Constant variables can be called with class name only...but readonly can be  called with object name only... Output:             x = 155, y = 22           c1 = 100, c2 = 133           c2 value after trying to modify :133