From your code I am guessing that you are trying to filter and modify your code at the same time, which usually is not possible to do out of the box.
If I understood you snippet correctly you want to return a "ModelNumber" property if "Model" is not empty and it is contained it currently iterated element.
Here is a possible solution:
var filteredItems =
string.IsNullOrWhiteSpace(Model)
? ItemList
: ItemList.Where(x => x.ModelNumber.ToLowerInvariant().Contains(Model.ToLowerInvariant())).ToList();
Hope this helps, good luck and next time please try to be more specific, or show us more code.
Cheers!