Force IEnumerable<T> to evaluate without calling .ToArray() or .ToList()

Is there a way to force an IEnumerable<T> collection to contain results without calling .ToArray() or .ToList(); ?

Yes, but it is perhaps not what you want:

IEnumerable<T> source = …;
IEnumerable<T> cached = new List<T>(source);

The thing is, IEnumerable<T> is not a concrete type. It is an interface (contract) representing an item sequence. There can be any concrete type "hiding behind" this interface; some might only represent a query, others actually hold the queried items in memory.

If you want to force-evaluate your sequence so that the result is actually stored in physical memory, you need to make sure that the concrete type behind IEnumerable<T> is a in-memory collection that holds the results of the evaluation. The above code example does just that.

You can use a foreach loop:

foreach (var item in fooBars) { }

Note that this evaluates all items in fooBars, but throws away the result immediately. Next time you run the same foreach loop or .ToArray(), .ToList(), the enumerable will be evaluated once again.