c# - 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() 逻辑来选择 Salary 值？

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() 为空，它会抛出异常。

Answer 1

您可以使用 Enumerable.DefaultIfEmpty

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

其中 yourDefaultObject 是一个默认薪水为 default(decimal) 的对象。