I have an abstract class called Grouping. I have a subclass called GroupingNNA.

public class GroupingNNA : Grouping {
  // blah blah blah
}

I have a List that contains items of type GroupingNNA, but is actually declared to contain items of type Grouping.

List<Grouping> lstGroupings = new List<Grouping>();
lstGroupings.Add(
  new GroupingNNA { fName = "Joe" });
lstGroupings.Add(
  new GroupingNNA { fName = "Jane" });

The Problem: The following LINQ query blows up on me because of the fact that lstGroupings is declared as List< Grouping> and fName is a property of GroupingNNA, not Grouping.

var results = from g in lstGroupings
              where r.fName == "Jane"
              select r;

Oh, and this is a compiler error, not a runtime error. Thanks in advance for any help on this one!

More Info: Here is the actual method that won't compile. The OfType() fixed the LINQ query, but the compiler doesn't like the fact that I'm trying to return the anonymous type as a List< Grouping>.

private List<Grouping> ApplyFilterSens(List<Grouping> lstGroupings, string fSens) {

  // This works now! Thanks @Lasse
  var filtered = from r in lstGroupings.OfType<GroupingNNA>()
                 where r.QASensitivity == fSens
                 select r;

  if (filtered != null) {
    **// Compiler doesn't like this now**
    return filtered.ToList<Grouping>();
  }
  else
    return new List<Grouping>();
  }