Question

When I enumerate over an IEnumerable twice Resharper complains about Possible multiple enumerations of IEnumerable. I know, in some case of DB-queries when you enumerate twice you get an exception.

I want to reproduce that behavior in tests. So, I basically want the following function to throw (because of multiple enumerations):

    private void MultipleEnumerations(IEnumerable<string> enumerable)
    {
        MessageBox.Show(enumerable.Count().ToString());
        MessageBox.Show(enumerable.Count().ToString());
    }

What should I pass to it? All the Lists, Collections etc. are ok with multiple enumerations.
Even this kind of IEnumerable doesn’t give an exception:

    private IEnumerable<string> GetIEnumerable()
    {
        yield return "a";
        yield return "b";
    }

Thanks.

Answer 1

You probably just want a custom class:

public class OneShotEnumerable<T> : IEnumerable<T>
{
    private readonly IEnumerable<T> _source;
    private bool _shouldThrow = false;

    public OneShotEnumerable(IEnumerable<T> source)
    {
        this._source = source;
    }

    public IEnumerator<T> GetEnumerator()
    {
        if (_shouldThrow) throw new InvalidOperationException();
        _shouldThrow = true;

        return _source.GetEnumerator();
    }

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