Question

I’m calling stored procedure:

ObjectResult<ComplexType1> result = context.ListSomething(model.SelectedDatabase);
ViewBag.WebGrid = new WebGrid(source: result, rowsPerPage: int.MaxValue, canSort: false, canPage: false);

however when passing result as a source to WebGrid I receive:

System.InvalidOperationException: The result of a query cannot be
enumerated more than once.

I’m a bit supprised as ObjectResult implements IEnumerable (not IQueryable). What is the background behind that exception and why I have to call ToList() method:

var result = context.ListSomething(model.SelectedDatabase).ToList();
ViewBag.WebGrid = new WebGrid(source: result, rowsPerPage: int.MaxValue, canSort: false, canPage: false);

to make it work? Could you perhaps provide me with some documentation that explains it (for instance from Programming Entity Framework, 2nd Edition, as I’m reading it now, but I don’t remember anything that would explain that mechanism so far).

Thanks,
Pawel

Answer 1

It is because ObjectResult<T> internally accesses DataReader which allows enumerating result only once in forward only way so if you want to iterate the result again without calling ToList first you must execute stored procedure again and get new ObjectResult<T> instance.