Linq 从多个 where 条件中获取计数

Question

I want something really simple, i want from a list of records to find the ones of a specific date (records can have the same date more than once) and with a specific value,

and count how many of those values exist. So far i'm using this but im not sure if it is the best approach

Here is Records

    public class Records
    {
        public DateTime Date { get; set; }
        public int Record { get; set; }

    }

and here is the code

 List<Records> records = GetRecords();
 var distinctDates = records.Select(x => x.Date).Distinct().ToList();


 for (int i = 0; i < distinctDates.Count(); i++)
 { 
     statistics.Rows.Add(new object[] { distinctDates[i].ToString("MM/dd/yyyy HH:mm"),
     records.Select(x => x).Where(d => d.Date == distinctDates[i]).Count(x => x.Record == -1).ToString()});
 }

I want to know the sum of the appearance of a specific Date and a specific value in Record So if the database has these values:

11/11/1980, 3

11/11/1980, 3

12/11/1980, 2

I want to get as a result 2 when i will seek for the count of the conditions 11/11/1980 and 3

Answer 1

var result = records.GroupBy(x => x.Date)
                    .Select(x => new { Date = x.Key, Count = x.Count() });

is what you want

result will now have a list of dates and count.

if you want a specific value then like this:

var result = records.Where(x => x.Record == -1).
                    .GroupBy(x => x.Date)
                    .Select(x => new { Date = x.Key, Count = x.Count() });

To group by the date and not the date time use the date property of the object in the GroupBy statement

                    .GroupBy(x => x.Date.Date)

Answer 2

What you want is a couting of all dates. GroupBy() allows you to distinctly select all dates. Then you can Count() all grouped records with some conditions as you want.

records.GroupBy(x => x.Date).Select(x => new
{
    Date = x.Key,
    Count = x.Count(y => y.Record == -1)
}) // This should give you all distinct dates with count

Answer 3

So given a DateTime date and an int recordValue, you want to count all Records from your recordCollection that have a property Date equal to date and a property Record equal to RecordValue.

How about this:

Datetime date = new DateTime(1980, 11, 11);
int recordValue = 3;
var result = recordCollection

    // keep only the records with date and recordValue:
    .Where(record => record.Date == date && record.Record == recordValue)

    // and Count them:
    .Count();