Can we restrict the type be a delegate

Question

We can restrict the type be the class or struct. Can we strict the type be a delegate?

You have just tried sub-types of Delegate? Although I would likely use the appropriate signature or Func<...>, etc, if possible. — , Nov 12 '10 at 02:27

score 9 · Accepted Answer · answered Nov 12 '10 at 03:04

9

A Delegate is a class, and you would normally be able to specify a non-sealed class as a constraint. However, the language specification specifically excludes System.Delegate as a valid constraint in section 10.1.5.

A class-type constraint must satisfy the following rules:

The type must be a class type.

The type must not be sealed.

The type must not be one of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.

The type must not be object. Because all types derive from object, such a constraint would have no effect if it were permitted.

At most one constraint for a given type parameter can be a class type.

answered Nov 12 '10 at 03:04

Anthony Pegram

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Is there any workaround? – user496949 Nov 12 '10 at 03:18
@user - Not a pretty or stable one. It would likely involve reflection, which removes compile-time safety. – Anthony Pegram Nov 12 '10 at 03:21
Amazing! Could you show an example of removing compile-time safety? – user496949 Nov 12 '10 at 03:27
@user, binarycoder's answer shows something you may try. Where it gets dicey is once you've figured out that the input is a delegate, where do you go from there? How many parameters does the delegate expect? Would you use the return value (if any)? I do not have such a code example. – Anthony Pegram Nov 12 '10 at 04:58

score 2 · Answer 2 · answered Nov 12 '10 at 03:27

As has already been pointed out, the C# specification does not allow a Delegate generic constraint. Nor are Delegate subclasses accepted by the compiler as a generic constraint. The best you can do is test and throw an exception. I will show this with a method, but if this is a generic class, the constructor would be a great place to do the check.

public void Foo<T>(T x)
{
    if (x == null)
        throw new ArgumentNullException("x");

    Delegate d = x as Delegate;
    if (d == null)
        throw new ArgumentException("Argument must be of Delegate type.", "x");

    // Use d here.
}

Can we restrict the type be a delegate

2 Answers2