Constant is we doesn't have change value. Read-only same as constant but we should be constructor value change at run time.

Const has fixed value You have intialise same time

Readonly fields are delayed initalized constants. However they have one more thing different is that; When we declare a field as const it is treated as a static field. where as the Readonly fields are treated as normal class variables.const keyword used ,when u want's value constant at compile time but in case of readonly ,value constant at run timeForm the use point of view if we want a field that can have differnet values between differnet objects of same class, however the value of the field should not change for the life span of object; We should choose the Read Only fields rather than constants.Since the constants have the same value accross all the objects of the same class; they are treated as static.