Readonly and Constant in C#

Constant: Constant fields are define at the time of declaration and once they are defined can't be changed.

NOTE:

1. This can't be declared static.
2. Local scope 
3. This is implicitly static variable.
4. This must to initialised

Readonly: Readonly can be initialised either at the time of declaration or within the constructor of the same class and change the value in the same class constructor.