Handle Null Values in Linq Query (C#)

 Avoid a null reference exception as shown in the following example:

var query1 =
    from c in categories
    where c != null 
    join p in products on c.ID equals
        (p == null ? null : p.CategoryID)
    select new { Category = c.Name, Name = p.Name };

The where clause filters out all null elements in the categories sequence. This technique is independent of the null check in the join clause. The conditional expression with null in this example works because Products.CategoryID is of type int? which is shorthand for Nullable

In a join clause, if only one of the comparison keys is a nullable value type, you can cast the other to a nullable type in the query expression. In the following example, assume that EmployeeID is a column that contains values of type int?:

void TestMethod(Northwind db)
{
    var query =
        from o in db.Orders
        join e in db.Employees
            on o.EmployeeID equals (int?)e.EmployeeID
        select new { o.OrderID, e.FirstName };
}

Use of ternary operator as in the below example and the MobileNo = "N/A" for the null values:
Solution 1
var user = from u in Users
join uc in UserClients on u.Id equals uc.UserId
into myuserwithclient
from m in myuserwithclient.DefaultIfEmpty()
select new {
u.Id,
FirstName = u.FirstName,
LastName = u.LastName,
UserId = m.UserId,
MobileNo = (m.MobileNo == null) ? "N/A" : m.MobileNo
};

Solution 2
var user = from u in Users
join uc in UserClients on u.Id equals uc.UserId
into myuserwithclient
from m in myuserwithclient.DefaultIfEmpty()
select new {
u.Id,
FirstName = u.FirstName,
LastName = u.LastName,
UserId = m.UserId,
MobileNo = m.MobileNo ?? "N/A" 
};



Reference: http://msdn.microsoft.com/en-us/library/bb882535.aspx