Moq First() Last() and GetEnumerator() wierdness

Question

I am Moqing my Route Parts from a rps = new List... (3 Route Parts)

and Moqing GetEnumerator() for my Route as below

route.Setup(ro => ro.GetEnumerator()).Returns(rps.GetEnumerator());

but the Moq fails in the following code with "Sequence contains no elements" on the call to Last()

o.Route.Any(rp => rp.IsNonTowLocation && 
rp != o.Route.First() && 
rp != o.Route.Last())

Looking at First() Last() in the immediate windows I find the values change if I execute First() Last() multiple times. Its as if the MoveNext() gets called but not Reset() and the Enumerable it totally confused. Has anyone experienced this with Moq and found a solution?

Answer 1

You've setup your GetEnumerator so that it returns the same enumerator instance every time.

route.Setup(ro => ro.GetEnumerator()).Returns(rps.GetEnumerator());

This is equivalent to:

var enumerator = rps.GetEnumerator()
route.Setup(ro => ro.GetEnumerator()).Returns(enumerator);

If you want a new enumerator on every call, then you need to pass Returns a lambda expression:

route.Setup(ro => ro.GetEnumerator()).Returns(() => rps.GetEnumerator());

The lambda will get called every time GetEnumerator() is called - so First() and Last() should then work as expected.