c# - 获取当前值和先前值

Question

在下面的 LINQ 查询中，我想添加一个属性来获取之前的 Department 以及当前的 Department。

下面的 LINQ 查询返回:

EffectiveDate EmployeeID  Department
11/4/2012     10          0000 
1/14/2013     10          9121 
2/2/2016      10          9123 

我怎样才能像这样在当前部门旁边显示上一个部门？

EffectiveDate EmployeeID  Department PreviousDepartment
11/4/2012     10          0000       null
1/14/2013     10          9121       0000
2/2/2016      10          9123       9121

这是当前查询

var users = from s in userTable
            where s.EmployeeID == "10"
            group new {s} by new { s.EmployeeID, s.Department} into g
            select new 
            {
                EffectiveDate = g.Max(m => m.s.EffectiveDate), 
                EmployeeID = g.Key.EmployeeID, 
                Department = g.Key.Department
                //PreviousDepartment = ???
            };

Answer 1

一种方法是将数据存储在内存中，然后像这样修改它:

var users = from s in userTable
            where s.EmployeeID == "10"
            group new {s} by new { s.EmployeeID, s.Department} into g
            select new MyClass
            {
                EffectiveDate = g.Max(m => m.s.EffectiveDate), 
                EmployeeID = g.Key.EmployeeID, 
                Department = g.Key.Department
                PreviousDepartment = null
            };

var result = users.ToList();

for(int i = 1; i < result.Count; i++)
{
    result[i].PreviousDepartment = result[i-1].Department;
}

请注意，代码正在生成 MyClass 的新实例而不是匿名类型，因为匿名类型属性是只读的。确保您使用正确的属性创建此类。