What is the difference between Const and Static in C#?

Question

I am eager to know the difference between a const variable and a static variable.

As far as I know a const is also static and can not be accessed on instance variabl

Answer 1

One subtle but crucial difference is that consts are evaluated at compile time, whereas statics are evaluated at run time. This has an important impact on versioning. For example, suppose you write:

public const int MaxValue = 100;

You compile and ship your assembly (Assembly A). Then someone else writes an assembly (Assembly B) which references MaxValue. In this case, the value 100 is compiled into their assembly as well as yours.

If you had written this:

public static readonly int MaxValue = 100;

then the reference in their assembly would remain just that, a reference. When someone ran Assembly B, the value 100 would be loaded from your assembly, Assembly A.

This can, for example, affect simple patching scenarios. If you issue an updated Assembly A where MaxValues is declared as 200, and the user copies that version over the previous version (but does not recompile Assembly B), then in the first scenario Assembly B will continue to operate as if MaxValues were 100, because that's the const value that was compiled into Assembly B. In the second scenario, Assembly B will pick up the new value because it loads the non-const static variable at runtime.