Why C# does not support Multiple Inheritance ?

It supports only so called "weak form of multiple inheritance": each interface or class can have any number of types in its inheritance least, but the first type listed should be a class, and all other types can only be interfaces. In other words, this is multiple inheritance for interfaces; and each class can implement any number of interfaces but has exactly one base class (except the common root System.Object which does not have a base class).

Why? Multiple inheritance for classes is a very contradictory issue and causes inherent potential design problems. The absence of such inheritance also causes inherent potential design problems. This controversy is one of the inherent problems of OOP technology. This is not just the diamond problem; and this problem cannot be solved with explicit overriding. In fact, overriding is already explicit, so what? If you think there is a solution, you most likely misunderstood this problem. The problem is actually resolved in C++ by using the choice between virtual base and non-virtual. But this feature itself presents an inherent problem. I can explain why. Because this choice itself (between using virtual and non-virtual base) cannot be a matter of late binding. Once you have chosen some class to be a virtual base, it will remain virtual for all other derived classes you may decide to add later.

Unfortunately, it's impossible to explain it in a Quick Answer, but the problems are quite apparent; you will be able to feel them when you face serious OOD problems. Probably, if I had to cover all the aspects of such problems and multiple approached to overcome them (I mean, alternative decisions made in different programming languages, not application problems), I could write a whole book on the topic.

—SA