Dynamic LINQ - Struggling with GROUP BY / SELECT Syntax

问题

I have a repeating set of data that I'm retrieving from multiple SQL tables (long story but they all have different names despite having the same data) into a .NET DataTable:-

Point_Date  ->  Point_Value0  ->  Point_Value1  ->  Point_Value2  ->  Point_ValueX  
24/11/2014 16:18:07  ->  15.1  ->  NULL  ->  NULL  
24/11/2014 16:19:07  ->  15.2  ->  NULL  ->  NULL  
24/11/2014 16:20:07  ->  15.3  ->  NULL  ->  NULL  
24/11/2014 16:18:07  ->  NULL  ->  16.1  ->  NULL  
24/11/2014 16:19:07  ->  NULL  ->  16.2  ->  NULL  
24/11/2014 16:20:07  ->  NULL  ->  16.3  ->  NULL  
24/11/2014 16:18:07  ->  NULL  ->  NULL  ->  17.1  
24/11/2014 16:19:07  ->  NULL  ->  NULL  ->  17.2  
24/11/2014 16:20:07  ->  NULL  ->  NULL  ->  17.3

I want to group the data on the date/time field using LINQ so that I end up with records like:-

24/11/2014 16:18:07 -> 15.1 -> 16.1 -> 17.1

My problem is that I don't know how many sets of data there will be (there are three in the example but there could be any number) so I need to use dynamic LINQ.

I'm ok for the LINQ query for a fixed number of fields:-

var dtReport = (from row in dtPoints.AsEnumerable()
                group row by row.Field<DateTime>("Point_Date")
                        into t
                        select new
                        {
                        TempDate = t.Key,
                        Value1 = (double?) t.Sum(r => r.Field<double?>("Point_Value0") ?? 0),
                        Value2 = (double?)t.Sum(r => r.Field<double?>("Point_Value1") ?? 0),
                        Value3 = (double?)t.Sum(r => r.Field<double?>("Point_Value2") ?? 0)
                        });

But I'm having a real struggle making it dynamic using System.Linq.Dynamic, the following gives me an error:-

        var myRpt2 = dtPoints.AsEnumerable()
            .AsQueryable()
            .GroupBy("new ( it[\"Point_Date\"] as GrpByCol1)", "it")
            .Select("new (it.key as TempDate, it.sum(\"Point_Value0\") as SumValue)");

the error being:-

System.Linq.Dynamic.ParseException {"No applicable aggregate method 'sum' exists"}

I just can't figure out how to refer to the 'Point_Value' fields once I've done the GroupBy - there will be multiple 'sum(Point_ValueX)' fields depending on the number of sets of data but I can't even get it working for a single field at the moment !

Many Thanks,

David.

回答1:

I don't think you can get it done that way. It seems that dynamic linq is not equipped to parse an expression containing an indexer.

You can however use a combination of dynamic LINQ and regular LINQ:

var myRpt2 = (
        dtPoints.AsEnumerable()
        .AsQueryable()
        .GroupBy("new ( it[\"Point_Date\"] as GrpByCol1)", "it")
        as IEnumerable<IGrouping<DynamicClass,DataRow>>
    )
    .Select (r => new { ((dynamic)r.Key).GrpByCol1, 
                        Sum = r.Sum(x => x.Field<decimal>("Point_Value0"))});

The main twist is the cast of the GroupBy result to IEnumerable<IGrouping<DynamicClass,DataRow>>.

来源：https://stackoverflow.com/questions/27110445/dynamic-linq-struggling-with-group-by-select-syntax

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linq