build linq queries dynamically

Question

suppose I have a string list, like

list<string> cols = {"id", "name", "position"}.

This list is generated dynamically, and each one in this list represents a column name in a database table.

what I want to do is create a linq query dynamically which returns these columns only.

var q = from e in employ
        select new {
          id = id,
          name = name,
          position = position
};

How can I generate a query like that based on the input column lists?

Answer 1

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.