Linq 中的三元运算符

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,
    };

有没有办法不重复previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault()选择Salary值的逻辑？

我试过：

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

但是，如果previousMonthPayroll.Where(t => t.EmployeeId == payroll.EmployeeId).FirstOrDefault()为 null，则会抛出异常。

Answer 1

您可以使用Enumerable.DefaultIfEmpty

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

其中yourDefaultObject是默认工资为 的对象default(decimal)。

Answer 2

正如所有答案所证明的那样，有多种方法可以做到这一点，我首选的方法是：

var result = from payroll in currentMonthPayroll
             select new SalaryDifference
             {
                 PreviousMonthSalary = previousMonthPayroll
                     .Where(t => t.EmployeeId == payroll.EmployeeId)
                     .Select(prev => prev.Salary)
                     .FirstOrDefault()
             };