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How to group by several generic linq expressions

I am trying to use Linq expressions to build a query, and I am trying to group by multiple columns. Let's say I have a basic collection:

IEnumerable<Row> collection = new Row[]
{
    new Row() { Col1 = "a", Col2="x" },
    new Row() { Col1 = "a", Col2="x" },
    new Row() { Col1 = "a", Col2="y" },
};

I know that you can group them using lambda expressions:

foreach (var grp in collection.GroupBy(item => new { item.Col1, item.Col2 }))
{
    Debug.Write("Grouping by " + grp.Key.Col1 + " and " + grp.Key.Col2 + ": ");
    Debug.WriteLine(grp.Count() + " rows");
}

This is correct, as you can see:

Grouping by a and x: 2 rows
Grouping by a and y: 1 rows

But now, let's say, I get a set of selectors for grouping against, which is passed to me as a parameter in my method and that the type of the object is common:

void doLinq<T>(params Expression<Func<T,object>>[] selectors)
{
    // linq stuff
}

Whoever calls the method will look like this:

doLinq<Row>(entity=>entity.Col1, entity=>entity.Col2);

How would I build a group-by expression?

foreach (var grp in collection.GroupBy(
      item => new { 
          // selectors??
      }))
{
    // grp.Key. ??
}

Edit

I updated above to hopefully clarify why I need a set of selectors.

Edit # 2

Created an entity type in doLinq generic.

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4 answers

. :

  • ( inelegantly []), . , .
  • IEquality ( IEqualityComparer).

foreach (System.Linq.IGrouping<object[], T> g in collection.GroupBy(
    new Func<T, object[]>(
        item => selectors.Select(sel => sel.Compile().Invoke(item)).ToArray()
    ),
    new ColumnComparer()
)
{ ... }


public class ColumnComparer : IEqualityComparer<object[]>
{
    public bool Equals(object[] x, object[] y)
    {
        return Enumerable.SequenceEqual(x, y);
    }

    public int GetHashCode(object[] obj)
    {
        return (string.Join("", obj.ToArray())).GetHashCode();
    }
}

Linq Linq MySql. Linq, , - ...

0

, , linq-to-sql - , . .

, :

  • linq

. .

http://msdn.microsoft.com/en-us/library/bb882637.aspx

, :)

:

 void doLinq(params string[] selectors) // checking two expressions for equality is messy, so I used strings
     foreach (var grp in collection.GroupBy(
          item => new { 
              Col1 = (selectors.Contains("Col1") ? item.Col1 : String.Empty),
              Col2 = (selectors.Contains("Col2") ? item.Col2 : String.Empty)
              // need to add a line for each column :(
          }))
     {
          string[] grouping = (new string[]{grp.Key.Col1, grp.Key.Col2 /*, ...*/ }).Where(s=>!s.IsNullOrEmpty()).ToArray();
          Debug.Write("Grouping by " + String.Join(" and ", grouping)+ ": ");
          Debug.WriteLine(grp.Count() + " rows");
     }
 }
+1

linq-to-sql, , GroupBy? , , keySelector. , Sql Server CE, Sql Server Express, , , :

using System;
using System.Linq;
using System.Collections.Generic;
using System.Data.Linq;
using System.Linq.Expressions;

namespace ConsoleApplication1 {
    class Props {
        public List<object> list = new List<object>();
        public override bool Equals(object obj) {
            return Enumerable.SequenceEqual(list, (obj as Props).list);
        }
        public override int GetHashCode() {
            return list.Select(o => o.GetHashCode()).Aggregate((i1, i2) => i1 ^ i2);
        }
    }
    class Program {
        static void Main(string[] args) {
            Lol db = new Lol(@"Data Source=.\SQLExpress;Initial Catalog=Lol;Integrated Security=true");
            db.Log = Console.Out;
            doLinq(db.Test, row => row.Col1, row => row.Col2);
            Console.ReadLine();
        }
        static void doLinq<T>(Table<T> table, params Func<T, object>[] selectors) where T : class {
            Func<T, Props> selector = item => {
                var props = new Props();
                foreach (var sel in selectors) props.list.Add(sel(item));
                return props;
            };
            foreach (var grp in table.GroupBy(selector)) {
                Console.Write("Grouping by " + string.Join(", ", grp.Key.list) + ": ");
                Console.WriteLine(grp.Count() + " rows");
            }
        }
    }
}

Lol "" . :

SELECT [t0].[Col1], [t0].[Col2]
FROM [dbo].[Test] AS [t0]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

Grouping by a, x: 2 rows
Grouping by a, y: 1 rows

, , linq-to-sql , sql groupBy, , .

Edit: minor additions to complete, and the connection string now assumes Sql Server Express.

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