Say you want to implement a class that implements the IEnumerable interface in C#. Then you have two choices, either to implement the old-style non-generic IEnumerable interface or you can implements the generic IEnumerable<T> interface. Given those choices we of course want to implement the new generic version of the interface. Because otherwise lots of boxing and unboxing will happen when T is a value type, with the non-generic version you will not be able to conveniently use the Current property of your enumerator, and for general type-safety goodness.
Thus, you set out to implement the generic version. For example, let say we want to implement a class that enumerates all the integers staring from a given offset. First we might try this:
using System.Collections.Generic;
class Ints : IEnumerable<int> {
private readonly int offset;
public Ints(int o) { offset = o; }
public IEnumerator<int> GetEnumerator() {
int i = offset;
while( true ) yield return i++;
}
}
But then the compiler complains:
error CS0535: 'Ints' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'
Thank you for letting us know nice compiler. But we don’t want Ints to implement the non-generic interface. We want it to implement the generic interface.
Reading up on the documentation will reveal that the generic interface inherits from the non-generic interface. What a wonderful design. Thankfully we can make a general work-around for this design flaw in the library. Just add a non-generic method that calls the generic method:
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return GetEnumerator(); }
Once the author of my favorite C# book returns from vacation. I’ll complain to him that the section about IEnumerable didn’t make it clear that if you want to implement the generic interface you also have to implement the non-generic interface.
Update: It turns out that the errata to C# Precisely already mentions this curriousity.
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