Can LINQ order by a column index?

Question

I'm struggling to find LINQ orderby examples, where data is ordered by a column index. Is this possible to do?

Thank you

Answer 1

There's no such concept as a column in LINQ, only fields and properties. What you mean is probably to specify the index of a property in an anonymous type you create:

from p in persons
orderby 1
select new { p.FirstNam, p.LastName }

That's not possible, because a) you can't order by a projection property (output value in the anonymous type), and b) you can't order by index. To work around a), you can use the "into" keyword:

from p in persons
orderby p.FirstName
select new { First = p.FirstName, Last = p.LastName }
into tmp
   orderby tmp.First
   select tmp;

If you use a LINQ provider based on IQueryable (like LINQ to SQL, as opposed to LINQ to objects which is based on IEnumerable), you could parse the expression tree, find the expression based on some index, and add the corresponding OrderBy method call.

var query = from p in persons 
            select new { Name = p.FirstName + " " + p.LastName, ...}
query = AddOrderByPosition (query, 1);
foreach (var p in query) {...}

AddOrderByPosition would have to create a new expression tree that uses the original query expression as a source (a sub query, which is essentially what "into" creates too) and add a call to OrderBy at the end (no need for Select). You want to read the C# specs' chapter about transformation of query expressions first, and you should have some experience with reflection and expressions if you want to get this done in a sensible amount of time.

If you want that, I have some sample code I can provide.