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 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)
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 };
}
TA, not TP