Abstract Method in Non Abstract Class

First, I think that what you’re asking doesn’t logically make sense. If you have an abstract method, it basically means that the method is unfinished (as @ChrisSinclair pointed out). But that also means the whole class is unfinished, so it also has to be abstract.

Or another way to put it: if you had an abstract method on a class that wasn’t abstract, that would mean you had a method that cannot be called. But that means the method is not useful, you could remove it and it would all work the same.

Now, I’ll try to be more concrete by using an example: imagine the following code:

Animal[] zoo = new Animal[] { new Monkey(), new Fish(), new Animal() };

foreach (Animal animal in zoo)
    animal.MakeSound();

Here, Animal is the non-abstract base class (which is why I can put it directly into the array), Monkey and Fish are derived from Animal and MakeSound() is the abstract method. What should this code do? You didn’t state that clearly, but I can imagine few options:

1. You can’t call MakeSound() on a variable typed as Animal, you can call it only using a variable typed as one of the derived classes, so this is a compile error.

   This is not a good solution, because the whole point of abstract is to be able to treat instances of derived classes as the base class, and still get behaviour that’s specific to the derived class. If you want this, just put a normal (no abstract, virtual or override) method into each derived class and don’t do anything with the base class.

2. You can’t call MakeSound() on an object whose runtime type is actually Animal, so this is a runtime error (an exception).

   This is also not a good solution. C# is a statically typed language and so it tries to catch errors like “you can’t call this method” at compile time (with obvious exceptions like reflection and dynamic), so making this into a runtime error wouldn’t fit with the rest of the language. Besides, you can do this easily by creating a virtual method in the base class that throws an exception.

To sum up, you want something that doesn’t make much sense, and smells of bad design (a base class that behaves differently than its derived classes) and can be worked around quite easily. These are all signs of a feature that should not be implemented.