C#: Abstract classes need to implement interfaces?

Question

My test code in C#:

namespace DSnA
{
    public abstract class Test : IComparable
    {

    }
}

Results in the following compiler error:

error CS0535: 'DSnA.Test' does not implement interface member
'System.IComparable.CompareTo(object)'

Since the class Test is an abstract class, why does the compiler require it to implement the interface? Shouldn't this requirement only be compulsory for concrete classes?

Answer 1

In C#, a class that implements an interface is required to define all members of that interface. In the case of an abstract class, you simply define those members with the abstract keyword:

interface IFoo
{
    void Bar();
}

abstract class Foo : IFoo
{
    public abstract void Bar();
}

Or to put it another way: you don't have to "implement" it (which would be a terrible limitation on abstract classes); however, in C#, you do have to tell the compiler that you are deliberately passing the buck to concrete subclasses - and the above line of code shows how to do so.

_{The comments and downvotes complaining that this is not an answer to the question are missing the point. Someone coming to Stack Overflow, having received this compiler error, but having an abstract class in which it would be a mistake to supply an implementation, are stuck without a good solution - would have to write implementation methods that threw runtime exceptions, a horrendous work-around - until they have the above information. Whether it is good or bad that C# requires this explicitness is outside the scope of Stack Overflow, and not relevant to the question nor this answer.}