linq where clause and count result in null exception

Question

The code below works unless p.School.SchoolName turns out to be null, in which case it results in a NullReferenceException.

if (ExistingUsers.Where(p => p.StudentID == item.StaffID &&
                        p.School.SchoolName == item.SchoolID).Count() > 0)
{
    // Do stuff.
}

ExistingUsers is a list of users:

public List<User> ExistingUsers;

Here is the relevant portion of the stacktrace:

System.NullReferenceException: Object reference not set to an instance of an object.

at System.Linq.Enumerable.WhereListIterator1.MoveNext()
at System.Linq.Enumerable.Count[TSource](IEnumerable1 source)

How should I handle this where clause?

Thanks very much in advance.

Answer 1

I suspect p.School is null, not SchoolName. Simply add a null check before accessing SchoolName. Also, use Any() to check if there are any results instead of Count() > 0 unless you're really in need of the count. This performs better since not all items are iterated if any exist.

var result = ExistingUsers.Where(p => p.StudentID == item.StaffID
                            && p.School != null
                            && p.School.SchoolName == item.SchoolID)
                         .Any();

if (result) { /* do something */ }

Answer 2

For all database nullable columns, we should either add null check or do simple comparision a == b instead of a.ToLower() == b.ToLower() or similar string operations.
My observation as below:
As they get iterated through Enumerable of LINQ Query for comparision against with input string/value, any null value (of database column) and operations on it would raise exception, but Enumerable becomes NULL, though query is not null.