The Linq query returns an IEnumerable of your class - and you can;t cast that to a single value any more than you can use an array of integers tin a simple arithmetic operation:
int[] ar = GetInts();
int x = ar + 6;
Instead, either return a single item using FirstOrDefult, or check the count and fetch the first member of the collection to pass to your method. Neither way will need casting as the Linq query specifically returns a collection of a named type.