Readonly and const keywords

1. the const field can only be initialized in the declaration of this field.
The readonly field can be initialized in the Declaration or constructor. Therefore, the readonly field may have different values based on the constructors used.
2. the const field is the number of compiling times, while the readonly field can be used for running times.
3. const is static by default, and readonly must display the declaration if it is set to static.
4. For constants of the reference type, the possible values of const can only be string and null.
Readonly can be of any type