Question

I have a class called Primes and this class implements GetEnumerator() without implementing IEnumerable interface.

public class Primes
{
    private long min;
    private long max;

    public Primes()
        : this(2, 100)
    {
    }

    public IEnumerator GetEnumerator()
    {...}

I don’t get it. Am I missing something?

Answer 1

Firstly, as others have said you can introduce your own methods without implementing interfaces anyway – you can write your own Dispose method without implementing IDisposable etc. For well-known interfaces I’d suggest this is almost a bad idea (as readers will have certain expectations) but it’s entirely valid.

More importantly though, the foreach statement in C# can work without IEnumerable being involved. The compiler effectively does compile-time duck typing on the names GetEnumerator(), Current and MoveNext(). This was primarily to allow strongly-typed (and non-boxing) iteration in C# 1, before generics. See section 8.8.4 of the C# 3 spec for more details.

However, it’s generally a bad idea to do this now if you do want to be able to easily iterate over the contents of an instance as a collection – and indeed I’d suggest implementing IEnumerable<T> instead of just IEnumerable.