Why can you not invoke extension methods directly?

Question

Can someone explain to me why in the following the 3rd invocation of DoSomething is invalid? ( Error message is \"The name \'DoSomething\' does not exist in the current cont

Answer 1

DoSomething requires an instance of A to do anything, and without a qualifier, the compiler can't see which DoSomething you need to invoke. It doesn't know to check in A_Ext for your method unless you qualify it with this.