Question

I faced with a sample code in Java and it brought me a question.

Java sample code is:

...
public interface CLibrary extends Library {
    CLibrary INSTANCE = (CLibrary) Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"), CLibrary.class);
    void printf(String format, Object... args);
}

public static void main(String[] args) throws IOException {
    CLibrary.INSTANCE.printf("Hello, World\n");
}

But in C# we cannot write like that:

public interface IMyInterface {
    static readonly int staticInt = 5;                          // compile error
    static readonly SomeClass staticInstance = new SomeClass(); // compile error
}

What is the difference between these two languages/frameworks?

What design policy permit java to have const fields in an interface or what prevents .NET from having that?

Answer 1

The use of interfaces to hold constants is usually frowned on these days in Java too. (I’d say that storing non-compile-time-constant fields like your example is even more frowned upon.)

Fundamentally, it’s at odd with the idea of an interface: a contract that the implementation will uphold. The implementation isn’t going to provide the field, so what’s it doing there?

I suspect the C# team decided that it was sufficiently at odds with the concept behind interfaces to not include it in the language. I don’t know whether it’s just a C# restriction or a CLR restriction too. For example, I know that the CLR allows interfaces to declare nested types, but C# doesn’t currently allow this.