Question

If the original collection has objects with property prop:

prop = "a";
prop = "a";
prop = "b";
prop = "b";
prop = "c";

and I’m grouping by prop, I need the output:

List<string>{ "a", "b", "c" }

Answer 1

eg.

public class Foo
{
    public string PropertyA { get; set; }
    public string PropertyB { get; set; }
}

following code to group:

var foos = new List<Foo>();
var groupings = from foo in foos
                group foo by foo.PropertyA
                into groupedFoos
                select groupedFoos;
/*
// the same as
var groupings = foos.GroupBy(foo => foo.PropertyA);
*/
var keys = from grouping in groupings
           select grouping.Key;

.GroupBy() will return IEnumerable<IGrouping<TKey, TSource>>

if you just want the distinct properties, you can still go for .Distinct(), eg:

var keys = (from foo in foos
            select foo.PropertyA).Distinct();