How to specify types not allowed in a .NET Generics constraint?

Question

Is it possible to specify a constraint on a generic class that disallows certain types? I don't know if it is possible and if it is, I am not sure what the syntax would be. Something like:

public class Blah<T> where : !string {
}

I can't seem to find any notation that would allow such a constraint.

Answer 1

The closest you can get is a run-time constraint.

Edit: Originally I put the run-time check in the constructor call. That's actually not optimal, as it incurs overhead on every instantiation; I believe it would be much more sensible to put the check in the static constructor, which will be invoked once per type used as the T parameter for your Blah<T> type:

public class Blah<T> {
    static Blah() {
        // This code will only run ONCE per T, rather than every time
        // you call new Blah<T>() even for valid non-string type Ts
        if (typeof(T) == typeof(string)) {
            throw new NotSupportedException("The 'string' type argument is not supported.");
        }
    }
}

Obviously not ideal, but if you put this constraint in place and document the fact that string is not a supported type argument (e.g., via XML comments), you should get somewhere near the effectiveness of a compile-time constraint.

Answer 2

No, you can't directly specify "negated" type constraints.