Linq按日期(月)分组

Question

我想知道如何按月分组一些数据并求和字段。例如，我有一个MyObjects(DateTimeField, AmountField)的列表。我想按DateTimeField分组，但不是整个日期，只是按月，然后将每个月的AmountField相加。

我不知道为什么objectgrouped是空的?

    IEnumerable<MyObject> objectList= GetMyObject();
    var ObjectGrouped = objectList.GroupBy(l => l.ObjectDate.GetValueOrDefault().Month)
                          .Select(lg =>
                                new
                                {
                                    CurrentMonth = lg.Key,
                                    Total = lg.Sum(w => w.ObjectAmount)
                                });
ObjectDate      ObjectAmount
1/1/2013              3.3
3/1/2013              6.9
13/2/2013            5
3/4/2013              3.6
13/4/2013            15.2

Answer 1

我想你说的"month"是指"month and year":

忽略空值:

var query = myList.Where(i => i.DateTimeField.HasValue)
                  .GroupBy(i => i.DateTimeField.Value.Month)
                  .Select(g => new {
                            Month=g.Key,
                            Total=g.Sum(i=>i.AmountField) 
                         });

将空值放在单独的桶中("0"月):

var query = myList.GroupBy(i => i.DateTimeField.HasValue ? i.DateTimeField.Value.Month : 0)
                  .Select(g => new {
                            Month=g.Key,
                            Total=g.Sum(i=>i.AmountField) 
                         });

Answer 2

试试这个:

List<DateTime> list = new List<DateTime>();
var grouppedResult = list.GroupBy(x => x.Month);

Answer 3

var groups = list.GroupBy(l => l.DateTimeField.Year * 12 + l.DateTimeField.Month)
                .Select(g => new {
                                Month=g.First().DateTimeField,
                                Sum=g.Select(a=>a.AmountField).Sum() 
                             })
                .ToList();