Dynamic Linq select first in groups after GroupBy

Question

I think I've read every post on this subject but still can't seem to get this to work. I want to duplicate the following GroupBy with dynamic Linq. I've tried this Dynamic GroupBy but can't seem to get the First().

var result = mydata
    .GroupBy(g => g.ID)
    .Select(z => z.First())
    .Select("new (ID, Field1, Field2)");

The GroupBy needs to be dynamic. I need the first row of the grouped field. So..

ID:1, Field1:W, Field2:L 
ID:1, Field1:A, Field2:B 
ID:2, Field1:A, Field2:B 
ID:2, Field1:C, Field2:D

Should end up being:

ID:1, Field1:W, Field2:L 
ID:2, Field1:A, Field2:B 
OR
ID:1, Field1:A, Field2:B 
ID:2, Field1:C, Field2:D

Yes lol..I need the GroupBy part to be dynamic so I can use a string instead of strong typing "ID". — user3478586, May 29 '15 at 18:45
btw, I tried the link you provided. But can not get it to yield the result created by the query I said I need to duplicate. — user3478586, May 29 '15 at 18:50

user3478586 · Accepted Answer · 2015-05-29T23:25:05.427

I solved it by adding to DynamicQueryable

public static object First(this IQueryable source)
        {
            if (source == null) throw new ArgumentNullException("source");
            return source.Provider.Execute(
                Expression.Call(
                    typeof(Queryable), "First",
                    new Type[] { source.ElementType },
                    source.Expression));
        }

Above not required..but added anyway for consistency.

And adding to IEnumerableSignatures

void First()

Query becomes:

var result = mydata
    .GroupBy("ID", "it")
    .Select("it.First()")
    .Select("new (ID, Field1, Field2)");

jdweng · Answer 2 · 2015-05-29T21:13:12.057

-2

What about this

            var mydata = new[] {
                new { ID = 1, Field1 = "W", Field2 = "L"},
                new { ID = 1, Field1 = "A", Field2 = "B"}, 
                new { ID = 2, Field1 = "A", Field2 = "B"}, 
                new { ID = 2, Field1 = "C", Field2 = "D"}
            }.ToList();


            var result = mydata.GroupBy(x => x.ID)
                .Select(x => x.ToList()).ToList();

            var output = result[0][0];

            var output1 = result[0];
            var output2 = result[1];

edited May 29 '15 at 21:13

answered May 29 '15 at 20:56

jdweng

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This question is specifically about [tag:dynamic-linq], which allows to [build string based query expressions](http://stackoverflow.com/tags/dynamic-linq/info), not about standard expression(delegate)-based LINQ. – Eugene Podskal May 29 '15 at 21:10
I created sample data for testing. Should work on dynamic expressions. – jdweng May 29 '15 at 21:18
This code is not dynamic-linq. Dynamic LINQ deals with strings. Not with expressions or delegates. – Eugene Podskal May 29 '15 at 21:19
I disagree. Using ToList() makes it a dynamic expression. I think the real issue is using ToList() in the GroupBy is the solution, Then var output = result.FirstOrDefault() gives correct results. – jdweng May 30 '15 at 05:42
You can take a look at [another answer](http://stackoverflow.com/a/30540462/3745022) that really uses [tag:dynamic-linq]. Have you read the [tag info](http://stackoverflow.com/tags/dynamic-linq/info)? No offense, but what you propose does not use the technology required by OP. – Eugene Podskal May 30 '15 at 07:32

Dynamic Linq select first in groups after GroupBy

2 Answers2