4 May 2013

Why C# or VB.NET doesn't support multiple inheritance

Why C# or VB.NET doesn't support multiple inheritance

1) First reason is ambiguity around Diamond problem, consider a class A has foo() method and then B and C derived from A and has there own foo() implementation and now class D derive from B and C using multiple inheritance and if we refer just foo() compiler will not be able to decide which foo() it should invoke. This is also called Diamond problem because structure on this inheritance scenario is similar to 4 edge diamond, see below

           A foo()
           / \
          /   \
   foo() B     C foo()
          \   /
           \ /

In my opinion even if we remove the top head of diamond class A and allow multiple inheritances we will see this problem of ambiguity.

Some times if you give this reason to interviewer he asks if C++ can support multiple inheritance than why not c# oR vb.net.In that case I would try to explain him the second reason which I have given below that its not because of technical difficulty but more to maintainable and clearer design was driving factor though this can only be confirmed by any of java designer and we can just speculate. Wikipedia link has some good explanation on how different language address problem arises due to diamond problem while using multiple inheritances.

2) Second and more convincing reason to me is that multiple inheritances does complicate the design and creates problem during casting, constructor chaining etc and given that there are not many scenario on which you need multiple inheritance its wise decision to omit it for the sake of simplicity. Also c# and avoids this ambiguity by supporting single inheritance with interfaces. Since interface only have method declaration and doesn't provide any implementation there will only be just one implementation of specific method hence there would not be any ambiguity.

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