What's with instantiating from a type parameter T not allowing constructor aguments?

Question

If I have classes:

class Useless
{
    private string _message ;
    public Useless(string message)
    {
        _message = message;
    }
}

class UselessFactory<T> where T : new()
{
    public Useless CreateUseless(string msg)
    {
        return new T(msg);
    }
}

Why can't I instantiate T with parameters, like I could do with the following?

return (Useless)Activator.CreateInstance(typeof(T), msg);

I'm pretty sure new() means that is must have a parameterless ctor (constrained ctor anti-pattern) — Johan Larsson, Apr 23 '13 at 07:53

Johan Larsson · Answer 1 · 2013-04-23T08:08:09.577

2

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor.

From the docs

Possible workaround (not the same thing)

If you make Message a public and mutable property you can do:

class UselessFactory<T> where T : UselessBase, new()
{
    public T CreateUseless(string msg)
    {
        return new T() { Message = msg };
    }
}

edited Apr 23 '13 at 08:08

answered Apr 23 '13 at 07:58

Johan Larsson

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This would not work, Message is not a know property of variable Type `T` – Habib Apr 23 '13 at 08:02
Tried it, it works. Note that I added a constraint to `T : UselessBase, new()` but I agree the example is not very good and probably not what OP wants. I also made message a public property. – Johan Larsson Apr 23 '13 at 08:07

score 1 · Answer 2 · edited May 23 '17 at 12:12

1

You could do the following:

return (T)Activator.CreateInstance(typeof(T), new object[] { msg });

I got it from this question: https://stackoverflow.com/a/731637/1522991

edited May 23 '17 at 12:12

Community

1
1

answered Apr 23 '13 at 08:06

Um, that's basically what I said I was doing in my question, but with one small correction, thanks. – ProfK Apr 23 '13 at 08:54

Martin Mulder · Accepted Answer · 2013-04-23T10:22:20.203

1

It is simply not supported at this moment.

There are work arounds:

Mark the type T with an interface or class (where T: MyClass) and set properties or call its methods to initialize the instance.
Your Activator.Activate (a slow dynamic method).
The fastest dynamic method is to once use reflection to find the correct ConstructorInfo and convert this into a strongly typed delegate (in your case: Func<string, T>). You can call this delegate which will construct an instance with the given parameter.

edited Apr 23 '13 at 10:22

answered Apr 23 '13 at 08:34

Martin Mulder

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Accepted for the `ConstructorInfo` option. I never knew this class existed, or, obviously, this application of it. – ProfK Apr 23 '13 at 08:55
Many developers do not know about it. If you need any help with it, let me know. – Martin Mulder Apr 23 '13 at 10:28

What's with instantiating from a type parameter T not allowing constructor aguments?

3 Answers3