Why this simple generic code does not compile?

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Ympostor - 26 Jul 2007 15:44 GMT

public class TestGenericEnumerator<T> :
System.Collections.Generic.IEnumerable<T>
{
public System.Collections.Generic.IEnumerator<T> GetEnumerator()
{
return null;
}
}

I don't understand why this is returning:

Error 6 'TestGenericEnumerator<T>' does not implement interface member
'System.Collections.Generic.IEnumerable<T>.GetEnumerator()'.
'TestGenericEnumerator<T>.GetEnumerator()' is either static, not public,
or has the wrong return type.

Anyone? :(

Reply to this Message

Peter Morris - 26 Jul 2007 15:51 GMT

> public System.Collections.Generic.IEnumerator<T> GetEnumerator()

Try this

public System.Collections.Generic.IEnumerator<T> GetEnumerator<T>()

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Marc Gravell - 26 Jul 2007 16:08 GMT

> Try this
> public System.Collections.Generic.IEnumerator<T> GetEnumerator<T>()

The T in this method is now a different T to the class's template T
(which the compiler issues a CSO693 warning about) - but this means
that the above doesn't implement the interface, since you could use:

foreach(int i in new
TestGenericEnumerator<string>().GetEnumerator<int>()) {...}

which doesn't do the job of implementing IEnumerable<string>. Good
idea, though.

Marc

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Marc Gravell - 26 Jul 2007 15:51 GMT

Because IEnumerable<T> : IEnumerable, and you haven't implemented the
non-generic form; just add:

System.Collections.IEnumerator
System.Collections.IEnumerable.GetEnumerator() {
return GetEnumerator();
}

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Ympostor - 26 Jul 2007 16:45 GMT

Marc Gravell escribió:

> Because IEnumerable<T> : IEnumerable, and you haven't implemented the
> non-generic form; just add:
[quoted text clipped - 3 lines]
> return GetEnumerator();
> }

Thanks! Now it works, but, now I don't understand why I need this cast:

public class TestGenericEnumerator<T> :
System.Collections.Generic.IEnumerable<T>
{
System.Collections.Generic.IEnumerator<T>
System.Collections.Generic.IEnumerable<T>.GetEnumerator()
{
return null;
}

System.Collections.IEnumerator
System.Collections.IEnumerable.GetEnumerator()
{
return (

//why is this cast needed!?
(IEnumerable<T>)
//end cast

this).GetEnumerator();
}
}

Thanks in advance.

Reply to this Message

Marc Gravell - 26 Jul 2007 21:20 GMT

> why is this cast needed!?

Because you switched the generic version to an explicit
implementation, and so there is no this.GetEnumerator()

If you want to do this, consider having a single private
implementation that both the generic and non-generic versions invoke:

private IEnumerator<T> GetEnumerator() {
return null; // the actual implementation
}
IEnumerator<T> IEnumerable<T>.GetEnumerator() {
return GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator() {
return GetEnumerator();
}

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Why this simple generic code does not compile?

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