C# Create New T()

Hanshan picture Hanshan · Jun 30, 2011 · Viewed 155.9k times · Source

You can see what I'm trying (but failing) to do with the following code:

protected T GetObject()
{
    return new T();
}

Any help would be greatly appreciated.

EDIT:

The context was as follows. I was playing around with a custom controller class for all controllers to derive from, with standardised methods. So in context, I needed to create a new instance of the object of the controller type. So at time of writing, it was something like:

public class GenericController<T> : Controller
{
    ...

    protected T GetObject()
    {
        return (T)Activator.CreateInstance(ObjectType);
    }        

    public ActionResult Create()
    {
        var obj = GetObject()

        return View(obj);
    }

And so I decided reflection was easiest here. I agree that, certainly given the initial statement of the question, the most appropriate answer to mark as correct was the one using the new() constraint. I have fixed that up.

Answer

Alex Aza picture Alex Aza · Jun 30, 2011

Take a look at new Constraint

public class MyClass<T> where T : new()
{
    protected T GetObject()
    {
        return new T();
    }
}

T could be a class that does not have a default constructor: in this case new T() would be an invalid statement. The new() constraint says that T must have a default constructor, which makes new T() legal.

You can apply the same constraint to a generic method:

public static T GetObject<T>() where T : new()
{
    return new T();
}

If you need to pass parameters:

protected T GetObject(params object[] args)
{
    return (T)Activator.CreateInstance(typeof(T), args);
}