Row_number over (Partition by xxx) in Linq?

Another idea is using a view, if possible.

Try this one liner:

var o = beatles
    .OrderBy( x => x.id )
    .GroupBy( x => x.inst )
    .Select( group => new { Group = group, Count = group.Count() } )
    .SelectMany( groupWithCount =>
        groupWithCount.Group.Select( b => b)
        .Zip(
            Enumerable.Range( 1, groupWithCount.Count ),
            ( j, i ) => new { j.inst, j.name, RowNumber = i }
        )
    );

foreach (var i in o)
{
    Console.WriteLine( "{0} {1} {2}", i.inst, i.name, i.RowNumber );
}

Output:

Guitar John 1
Guitar George 2
Guitar Paul 3
drums Ringo 1
drums Pete 2

B"H

I know this is old. But why isn't the solution simply?

var o = beatles.GroupBy(x => x.inst)
               .SelectMany(g =>
                   g.Select((j, i) => new { j.inst, j.name, rn = i + 1 })
               );