The constant cannot be marked static

Question

I am trying to declare a PI constant like this:

public static const double PI = Math.PI;

but why am I getting this error?

T         


        

Answer 1

A constant is static by definition.

Answer 2

const implies static (you don't need an instance to reference the const value).

I want to also add this important point: When you link against (reference) an assembly with a public const, that value is copied into your assembly. So if the const value in the referenced assembly changes, your assembly will still have the originally compiled-in value.

If this behavior is not acceptable, then you should consider making the field a public static readonly field.

Lib.dll, provided as binary:

public class Foo {
    public const int HATS = 42;
    public static readonly int GLOVES = 33;
}

App.exe, references Lib.dll:

Foo.HATS    // This will always be 42 even if the value in Lib.dll changes,
            // unless App.exe is recompiled.

Foo.GLOVES  // This will always be the same as Foo.GLOVES in Lib.dll

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From MSDN:

Don’t create a constant to represent information that you expect to change at any time. For example, don’t use a constant field to store the price of a service, a product version number, or the brand name of a company. These values can change over time, and because compilers propagate constants, other code compiled with your libraries will have to be recompiled to see the changes.

From DotNetPerls:

DLLs. When you use a const field or declaration, the C# compiler actually embeds the const variable's value directly in the IL code. Therefore, it essentially erases the const as a separate entity.

Caution: If programs that depend on a const are not recompiled after the const value changes, they may break [because they'll continue to use the previous value].

Answer 3

You can't have static const. Try readonly instead of const or simply drop the "static" since "const" is implied static anyway.

Answer 4

Constants cannot be replaced in the code during compilation, not runtime, so there's no requirement for static vs instance definitions.

Answer 5

All constants declarations are implicitly static, and the C# specification states that the (redundant) inclusion of the static modifier is prohibited. I believe this is to avoid the confusion which could occur if a reader were to see two constants, one declared static and one not – they could easily assume that the difference in specification implied a difference in semantics. Having said that, there is no prohibition on redundantly specifying an access modifier which is also the default one, where there is a choice. For instance, a (concrete) method can be explicitly marked as private despite that being the default. The rule appears to be that where there is no choice (e.g. a method declaration in an interface) the redundant modifier is prohibited. Where there is a choice, it’s allowed.