Instantiating a generic type with an overloaded constructor

Question

class MyClass<T> where T: new()
{
    public Test()
    {
        var t = new T();
        var t = new T("blah"); // <- How to do this?
    }
}

I know that's not possible as written, and as far as I understand it's just simply not allowed (although I would love nothing more than to be wrong about that). The first solution that comes to mind is to use an object initializer, but let's suppose that's a last resort because it would conflict with other oop goals in this case.

My next inclination would be to use reflection, but before I do that I'd like to know if I am overlooking any easier way to do this or perhaps a different design pattern that would work better?

Answer 1

You can't do it directly. You'll need to use Activator.CreateInstance, but doing so has all the drawbacks of reflection, meaning that you miss out on compile-time checking etc.

public Test()
{
    var t = (T)Activator.CreateInstance(typeof(T), (object)"blah");
}

Answer 2

You can force user to give you the way to construct the object from string, like this:

internal class MyClass<T>
    where T : new()
{
    public void Test(Func<string, T> func)
    {
        var t = func("blah");
    }
}

Function can be part of contruction MyClass object, so you won't have to pass it each time you call methods that have to create T. Actually with that aproach new() might not be needed.