Answer:   Constants
Evaluated at compile time
Support only value type variables
They are used when the value is not changing at compile time
Cannot be initialized at the time of declaration or in a constructor

Read-only Variables
Evaluated at run-time
They can hold the reference type variables
Used when the actual value is unknown before the run-time
Can be initialized at the time of declaration or in a constructor

Answer:   1) const fields has to be initialized while declaration only, while readonly fields can be initialized at declaration or in the constructor.
2) const variables can declared in methods ,while readonly fields cannot be declared in methods.
3) const fields cannot be used with static modifier, while readonly fields can be used with static modifier.
4) A const field is a compile-time constant, the readonly field can be used for run time constants.