How to create an instance with generics?

Question

I am trying to do something simple: creating an instance with generics. I get an error saying that I cannot create an instance because I do not have the new constraint. However, I do have it on my return statement! Any thoughts?

public IAction CreateAction<TA, TP>(ActionParamBase param)
            where TA : IAction
            where TP : ActionParamBase
        {
            Ensure.That(param).Is<TP>();

            return new TA { Param = param as TP };
        }

You need to do this: where TA : class, new() Check: http://stackoverflow.com/questions/4737970/what-does-where-t-class-new-mean — Francisco Goldenstein, Jul 30 '15 at 14:24
TA is a generic interface.. you can't create an instance of interfaces — ro-E, Jul 30 '15 at 14:24
ok guys thanks for your help ! it worked! => where TA : IAction, new() — , Jul 30 '15 at 14:25

score 5 · Answer 1 · answered Jul 30 '15 at 14:24

You have to specify the new() constraint on the TA type parameter in order to be able to call the constructor:

public IAction CreateAction<TA, TP>(ActionParamBase param)
        where TA : IAction
                 , new()
        where TP : ActionParamBase
    {
        ...
    }

Only then it knows (and forces) that TA has a parameterless constructor.

(Just a small note: it only works for parameterless constructors, not for constructors having arguments. In this case you are fine since you use initializers)

score 3 · Accepted Answer · edited May 23 '17 at 10:24

3

What it means is having new in the constraint of your TA part. See this SO post for reference, then see code below: Passing arguments to C# generic new() of templated type

public IAction CreateAction<TA, TP>(ActionParamBase param)
        where TA : IAction, new()
        where TP : ActionParamBase
    {
        Ensure.That(param).Is<TP>();

        return new TA { Param = param as TP };
    }

edited May 23 '17 at 10:24

Community

1
1

answered Jul 30 '15 at 14:24

GEEF

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it's complaining about `TA`, not `TP` – Igor Pashchuk Jul 30 '15 at 14:27
Yea sorry I typod that when I initially typed it ;) – GEEF Jul 30 '15 at 14:29

How to create an instance with generics?

2 Answers2