Question

Trying to tidy up scope and avoid possible multiple calls to RegisterWindowMessage.
Currently have a class used once with the following member

[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int RegisterWindowMessage(string lpString);

private int m_message = RegisterWindowMessage("MY_MSG"); 

As we only have one instance this seems ok, but think it would be more tidy to use. With my basic C# understanding this should call RegisterWindowMessage and assign the result to int and not allow it to change.

private const int message = RegisterWindowMessage("MY_MSG"); 

however attempting to do so leads to a

error CS0133: The expression being assigned to 'someclass.messageEvent' must be constant

so now I’m confused, does this mean the function was being assigned and called each time m_message was used previously, is there something else missing?

Answer 1

A const field has to be a compile-time constant. If you just want something which won’t change at execution time after initial assignment¹, make it readonly:

private static readonly int Message = RegisterWindowMessage("MY_MSG");

Note that I’ve made this static, which const is implicitly. This means RegisterWindowMessage will only be called once for this AppDomain, which is what I think you want.

EDIT: Hans is right, you should check the return value. You could either do that when you first use it, or when the type is initialized – usually it’s a bad idea for type initializers to throw exceptions, but you should see what the impact is.

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¹ Strictly speaking, a static readonly field can be assigned in the declaration or in the static constructor; an instance readonly field can be assigned in the declaration or in any instance constructor. It can be assigned multiple times, which is usually not useful, but can be just occasionally.