Troubles implementing IEnumerable

Since IEnumerable<T> implements IEnumerable you need to implement this interface as well in your class which has the non-generic version of the GetEnumerator method. To avoid conflicts you could implement it explicitly:

IEnumerator IEnumerable.GetEnumerator()
{
    // call the generic version of the method
    return this.GetEnumerator();
}

public IEnumerator<T> GetEnumerator()
{
    for (int i = 0; i < Count; i++)
        yield return _array[i];
}