Question

Works

private void Add<H>(H toAdd, IList<dynamic> list)
{
     list.Add(toAdd);
}

Doesn’t work

private void Add<H>(IList<H> toAdd, IList<IList<dynamic>> list)
{
     list.Add(toAdd);
}

As you can imagine, the error is

The best overloaded method match for 'System.Collections.Generic.ICollection<System.Collections.Generic.IList<dynamic>>.Add(System.Collections.Generic.IList<dynamic>)' has some invalid arguments

If anyone knows why this is happening or even better, how to fix it, I am very curious. I assume it has to do with Generic Variance but the dynamic makes me less sure.

Thanks, Tom

Edit

   //doesn't work
   private void Add<H>(IList<H> toAdd, IList<IList<dynamic>> list) where H : object
   {
      list.Add(toAdd);
   }

   //works
   //this isn't good enough however because I only want to be able to
   //have one type of object in toAdd
   private void Add(IList<object> toAdd, IList<IList<dynamic>> list)
   {
      list.Add(toAdd);
   }

   //works
   private void Add<H>(IList<H> toAdd, IList<IList<dynamic>> list)
   {
      list.Add(toAdd.Cast<dynamic>().ToList());
   }

   private void Foo()
   {
      //works
      IList<dynamic> list1 = new List<object>();
      //works
      IList<object> list2 = new List<dynamic>();
      //works
      IList<IList<dynamic>> list4 = new List<IList<object>>();
      //works
      IList<IList<object>> list3 = new List<IList<dynamic>>();
   }

I added a couple more examples (not all surprising) just to illustrate

Answer 1

That’s because you cannot cast an IList<H> to an IList<dynamic>. Imagine what happens:

IList<H> myList = new List<H>(...);
IList<dynamic> myDynamicList = myList; // assuming this would compile
myDynamicList.Add(new Foo()); // boom

You can try to keep a list of IEnumerable<dynamic> instead of IList<dynamic>, since IEnumerable<out T> has a covariant type parameter.