Why do arrays in .net only implement IEnumerable and not IEnumerable<T>?
Arrays do implement IEnumerable<T>, but it is done as part of the special knowledge the CLI has for arrays. This works as if it were an explicit implementation (but isn't: it is done at runtime). Many tools will not show this implementation, this is described in the Remarks section of the Array class overview.
You could add a cast:
return ((IEnumerable<T>)_array).GetEnumerator();Note, older MSDN (pre docs.microsoft.com) coverage of this changed a few times with different .NET versions, check for the remarks section.
You can use generic method IEnumerable<T> OfType<T>() from System.Linq namespace, which extends IEnumerable interface. It will filter out all elements which type is different than T and return IEnumerable<T> collection. If you use (IEnumerable<T>)_array conversion operator, it might not be safe, because System.Array (and other nongeneric types) stores items of type System.Object.