Activate type with generic params array set to null

Question

Let's say I have this ctor:

public MyType<T>(int myInt, string myString, params T[] myObjects) { }

I could set the params array itself to null (rather than pass a null into the array):

MyType(1, "foo", (T[])null)

Let's say I want to do that dynamically. I tried this:

var args = new object[] { 1, "foo", null };
var myType = Activator.CreateInstance(typeof(MyType), BindingFlags.Instance | BindingFlags.NonPublic, null, args, null);

But that throws a MissingMethodException.

If I change the ctor's params T[] to params object[] then it works, but I prefer to keep the generic form.

How can I do this?

Answer 1

Activator.CreateInstance seems kind of a convience wrapper round Type.GetConstructor. The latter has the advantage that you can specify the ctor's signature explicitly, which will help you out in this case:

//usage:
var intItem = CreateGeneric<int> (1, "int_str", null);
var strItem = CreateGeneric<string> (2, "str_str", null);

public static MyType<T> CreateGeneric<T>(int myInt, string myString, T[] array)
{
    //specify c'tor signature and get ctor-info
    var types = new [] { typeof(int), typeof(string), typeof(T[])};
    var ctor = typeof(MyType<T>).GetConstructor( 
        BindingFlags.Instance | BindingFlags.NonPublic, null, types, null);
    
    //create instance
    var args = new object[] { myInt, myString, array };
    var instance = (MyType<T>)ctor.Invoke(args);
    
    return instance;
}

Full demo: https://dotnetfiddle.net/h3lpZc