Create Generic object inside class

Question

I am making a base class from which other classes can be derived.

public class BaseClass<T> where T
{
    public BaseClass()
    {
        TClassObject = new T("SomeText"); // Error here
    }

    public T TClassObject { get; set; }
}

'T': cannot provide arguments when creating an instance of a variable type.

What I am missing here.

That works for no parameters constructor. My case is with a parameter, — fhnaseer, Apr 09 '13 at 13:10
Well, you don't read the post attentively enough. The post give you the exact procedure to resolve your problem. — Cyril Gandon, Apr 09 '13 at 13:12

score 5 · Accepted Answer · answered Apr 09 '13 at 12:59

5

From MSDN:

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

So it needs to be parameterless. You may want to look at Activator.CreateInstance

http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx

answered Apr 09 '13 at 12:59

Conrad Clark

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I have parameters to my generic object. Will Activator work for this? – fhnaseer Apr 09 '13 at 13:02
1

Yes. http://msdn.microsoft.com/en-us/library/wcxyzt4d.aspx. `Creates an instance of the specified type using the constructor that best matches the specified parameters.` Check it out. – Conrad Clark Apr 09 '13 at 13:02
What if I remove new()? – fhnaseer Apr 09 '13 at 13:07
No problem, since you won't be calling new() anyway. Just be careful not to hurt your code readability and maintenance by using `Activator.CreateInstance`. You could also accept a Func as a parameter for the BaseClass constructor, and write it like this: `BaseClass orangeBase = new BaseClass(()=> new Orange("tasty");` – Conrad Clark Apr 09 '13 at 13:08
var obj = Activator.CreateInstance(typeof(T), "SomeText"); Now this returns me object. How I cast this to T type? – fhnaseer Apr 09 '13 at 13:14
1

`(T) Activator.CreateInstance(typeof(T),"SomeText");` – Conrad Clark Apr 09 '13 at 13:25

score 2 · Answer 2 · edited May 23 '17 at 11:49

The where T : new() constraint states that T must have a parameterless constructor. Your code is calling into a constructor that takes a string parameter, and it isn't guaranteed that your T will have such a constructor.

It is not possible in C# to create a constraint on a specific constructor signature. If you need this functionality, you're better off using something like one of the answers in this thread.

Create Generic object inside class

2 Answers2