Question

I think there are people who may be able to answer this, this is a question out of curiosity:

The generic CreateInstance method from System.Activator, introduced in .NET v2 has no type constraints on the generic argument but does require a default constructor on the activated type, otherwise a MissingMethodException is thrown. To me it seems obvious that this method should have a type constraint like

Activator.CreateInstance<T>() where T : new() {
   ...
}

Just an omission or some anecdote lurking here?

Update

As pointed out, the compiler does not allow you to write

private T Create<T>() where T : struct, new()
error CS0451: The 'new()' constraint cannot be used with the 'struct' constraint

However, see comments a struct can be used as type argument to a generic method specifying a new() constraint. Under this circumstance the given answer seems the only valid reason to not constrain the method…

Thanks for looking over this!

Answer 1

I could be wrong, but the main benefit as I see it is that it allows you to do something like this:

// Simple illustration only, not claiming this is awesome code!
class Cache<T>
{
    private T _instance;

    public T Get()
    {
        if (_instance == null)
        {
            _instance = Create();
        }

        return _instance;
    }

    protected virtual T Create()
    {
        return Activator.CreateInstance<T>();
    }
}

Note that if Activator.CreateInstance<T> had a where T : new() constraint, then the Cache<T> class above would also need that constraint, which would be overly restrictive since Create is a virtual method and some derived class might want to use a different means of instantiation, such as calling a type’s internal constructor or using a static builder method.