With dynamic type, we can try-call an arbitrary method m1(), which need not exist in any public interface. Behaviour — If the underlying type exposes m1, it is invoked. If unavailable you get a run time error, not a compile time error.
One usage idea — Instead of calling the standard ToString() on the unknown object, we want to try-call ToStringXml(), and pray the underlying type supports it. Traditionally, If we have a few types to add ToStringXml(), we would prefer compile-time type check by introducing an interface IXmlDumper. However, if you have 55 types that could pass into TryCall() and 33 of them exposes ToStringXml(), you may need to cast to IXmlDumper then call ToStringXml(). Worse, you would need to edit source code for the 33 types!
With dynamnic type, basically the compile-time type check is postponed to run time. Compile time type check would need m1() declared in the type C, like —
C c = new C(); c.m1();
But if we use dynamic, and type-check at run time, then we don’t need m1() exposed on any public interface —