Why do I (sometimes) have to reference assemblies referenced by the assembly I reference?

I have an assembly A that defines an interface with some overloads: ``` public interface ITransform { Point InverseTransform(Point point); Rect InverseTransform(Rect value); System.Drawing.Point InverseTransform(System.Drawing.Point point); } ``` ...and an assembly B that references A (the binary, not the project) and calls one of the overloads: ``` var transform = (other.Source.TransformToDisplay != null && other.Source.TransformToDisplay.Valid) ? other.Source.TransformToDisplay : null; if (transform != null) { e.Location = transform.InverseTransform(e.Location); } ``` To be precise, it calls the `System.Windows.Point` overload of the `InverseTransform` method, because that is the type of the property `Location` in `e`. But when I build B in the IDE I get: error CS0012: The type 'System.Drawing.Point' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. even though that's not even the overload I am calling. When I comment out the line where the overloaded method `InverseTransform` is called, it builds fine even though I'm still instantiating an object of type `ITransform`. Why? And is there a way to fix this without having to add a reference to `System.Drawing` everywhere?