build linq queries dynamically

Question

suppose I have a string list, like

list cols = {\"id\", \"name\", \"position\"}.

This list is generated dynamically, and each

Answer 1

You can use Expression Trees to build dynamic Linq queries. Here is an example: http://msdn.microsoft.com/en-us/library/bb882637.aspx

Answer 2

As Chocoboboy said, System.Linq.Dynamic would help. Unfortunately, this is not included in the .NET framework, but you can download it from the Scott Guthrie's blog. In your case, you need to call Select(string selector) (with the column list hard-coded or sourced from a list). Optionally, my example includes a dynamic Where clause (Where("salary >= 50")):

List<string> cols = new List<string>(new [] { "id", "name", "position" });
var employ = new[] { new { id = 1, name = "A", position = "Manager", salary = 100 },
    new { id = 2, name = "B", position = "Dev", salary = 50 },
    new { id = 3, name = "C", position = "Secretary", salary = 25 }
};
string colString = "new (id as id, name as name, position as position)";
//string colString = "new ( " + (from i in cols select i + " as " + i).Aggregate((r, i) => r + ", " + i) + ")";
var q = employ.AsQueryable().Where("salary >= 50").Select(colString);
foreach (dynamic e in q)
    Console.WriteLine(string.Format("{0}, {1}, {2}", e.id, e.name, e.position));

However, this approach somehow defeats the purpose of LINQ strongly-typed queries, so I would use it with caution.