Difference between dynamic and System.Object

Question

What is the difference between a variable declared as dynamic and a variable declared as System.Object? Running the following function would seem to indicate that both variables get cast to the correct type dynamically:

void ObjectTest()
{
    System.Object MyTestVar = "test";
    dynamic MyTestVar2 = "Testing 123";

    Console.WriteLine("{0}", MyTestVar.GetType());
    Console.WriteLine("{0}", MyTestVar2.GetType());

    MyTestVar = 123;
    MyTestVar2 = 321;

    Console.WriteLine("{0}", MyTestVar.GetType());
    Console.WriteLine("{0}", MyTestVar2.GetType());
}

Answer 1

The difference is that MyTestVar2.ToUpper() compiles and works, without any explicit casting.

object is a normal type.
dynamic is a basically a placeholder type that causes the compiler to emit dynamic late-bound calls.

GetType() is a normal function defined by the object class that operates on the instance that you call it on.
GetType() is completely unaffected by the declared type of a variable that refers to the object you call it on. (except for nullables)

Answer 2

You should probably start with this excellent MSDN article. The differences can be summed up quite succinctly:

At compile time, an element that is typed as dynamic is assumed to support any operation.

System.Object only has a handful of operations that it supports - ToString(), Equals(), etc.