Ternary operator in Linq

Question

var result = from payroll in currentMonthPayroll
    select new SalaryDifference
    {
        PreviousMonthSalary = previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault() == null ?
        default(decimal) : previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault().Salary,
    };

Is there a way not to repeat the previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault() logic to pick Salary value?

I tried:

previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId)
    .FirstOrDefault().Salary ?? default(decimal);

But, if previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault() is null, it throws exception.

Answer 1

You can use Enumerable.DefaultIfEmpty

previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId)
                    .DefaultIfEmpty(yourDefaultObject)
                    .First();

Where yourDefaultObject is an object with the default salary of default(decimal).

Answer 2

You can use the let keyword:

from payroll in currentMonthPayroll
let prevSalary = previousMonthPayroll.FirstOrDefault(t => t.EmployeeId == payroll.EmployeeId)
select new SalaryDifference
{
    PreviousMonthSalary = prevSalary != null ? prevSalary.Salary : default(decimal)
};

See MSDN