LINQ 如何以列表形式返回分组依据？

Question

I have a datatable that is populated with 2 columns, DateTime and Data. I have been able to group the list using:

var dates = table.AsEnumerable().GroupBy(x => Convert.ToDateTime(x["DateTime"]).ToShortDateString());

The conversion is to drop the "time" portion of the datetime as I only want 1 date per day so I can iterate over it.

I can see during debugging that the LINQ works out as I intended, but I have almost no experience in LINQ and I do not know how to return these results in a way that is iterable.

Below that LINQ statement I have:

foreach (string date in dates) {
    string dummy2 = "";
}

with an error on foreach that says

Cannot convert type 'System.Linq.IGrouping<string, System.Data.DataRow>' to 'string'

The goal here is to return a list of just the unique dates which I can iterate over to perform additional processing/LINQ queries on the datatable

Answer 1

如果你想要一个唯一日期的列表，你可以做这样的事情。

var dates = table.Select(x => Convert.ToDateTime(x["DateTime"]).ToShortDateString()).Distinct();

Answer 2

If you just want the dates, then you don't need to use GroupBy , you can use Distinct . Also, you can get them as a DateTime object by just grabbing the Date property, which has a zeroed-out time component:

IEnumerable<DateTime> distinctDates = table.AsEnumerable()
    .Select(x => Convert.ToDateTime(x["DateTime"]).Date)
    .Distinct();

If you want the groups but are just trying to select the date strings from them, then you want to use the Key property of the groups, which is the property that was used to group the items:

IEnumerable<IGrouping<string, System.Data.DataRow>> dates = table.AsEnumerable()
    .GroupBy(x => Convert.ToDateTime(x["DateTime"]).ToShortDateString());

List<string> distinctDates = dates.Select(date => date.Key).ToList();

As a side note, unless you know the data type being returned by something, you might avoid using var , since that was hiding the fact that dates wasn't a collection of strings.

Answer 3

that's happening since as the error says you're trying to loop in a dictionary:

Cannot convert type 'System.Linq.IGrouping<string, System.Data.DataRow>' to 'string'

what you can do is add to your GroupBy statement a select portion:

var dates = table.AsEnumerable().GroupBy(x => Convert.ToDateTime(x["DateTime"]).ToShortDateString()).Select(x=> x.date);