Question

I have a very simple generic constructor method:

public T Instance<T, TT>(TT parms) where T : class
{
    return (T)Activator.CreateInstance(typeof(T), new[] { parms });
}

When I call the method like:

Instance<MyClass, string>("SomeStringValue").Customers.Where(x => x.Id == Id).Select(p => blah..blah...blah;

I get a ‘System.MissingMethodException: Constructor on type ‘MyClass’ not found.
at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture…’

I have tried adding bindingflags etc. but without any luck.

I am basically trying to instantiate an object with arguments.
I can without any problems explicitly declare the object:

var myObj = new MyClass("SomeStringValue");

But I need to make use of my generic constructor.

Can anyone clarify what I am missing?

EDIT
A complete program (simplified).

public abstract class A 
{
    public T Instance<T, TT>(TT parms) where T : class
    {
       return (T)Activator.CreateInstance(typeof(T), new[] { parms });
    }
}

public class B 
{
    public B(string someValue)
    {
        var myValue = someValue;
    }
}

public class C
{
  public void DoStuff()
  {
     var x = Instance<B, string>("SomeStringValue");
  }
}

Is what I am trying to do.

Answer 1

The Activator<T>.CreateInstance() overload you’re trying to call expects a second argument of type Object[]. The new array you are creating is a TT[]. If TT were constrained to a class type, then a TT[] might satisfy the Object[] parameter. An unconstrained generic TT[], however, cannot be used as an Object[]. Once the compiler determines that the TT[] cannot be passed as an Object[], it then (because of the params specification on the second parameter of the overload), it checks whether the second parameter you’re passing qualifies as an Object. Because all arrays derive from Object, it will. The compiler will thus create a single-element array of type Object which holds the TT[] that you were trying to pass in. Since there is no constructor that expects an Object[], the call will fail.

If you want to prevent this problem, create a new Object[] containing the appropriate parameters, instead of creating a new TT[]. That should solve your problem.