When using GroupBy in linq, how can I get a list of group names?

If the original collection has objects with property prop:

prop = "a";
prop = "开发者_开发百科a";
prop = "b";
prop = "b";
prop = "c";

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

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

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();

UPDATED

My original answer of using Disticnt() alone is insufficient. You need to GroupBy the prop value, and then select the first member of each subset:

myList.GroupBy(i => i.prop).Select(i => i.First()).ToList().ForEach(i => Console.Write(i.prop + ", "));

Demo code

Here's some code that illustrates the groupings.

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var myList = new List<Foo>() { 
                new Foo(){ prop = "a", anotherProp = "z"},
                new Foo(){ prop = "a", anotherProp = "x"},
                new Foo(){ prop = "b", anotherProp = "x"},
                new Foo(){ prop = "b", anotherProp = "y"},
                new Foo(){ prop = "c", anotherProp = "z"}
            };

            // Display groups.
            myList.GroupBy(i => i.prop).ToList().ForEach(j =>
            {
                Console.WriteLine("\t");
                j.ToList().ForEach(k => Console.Write(k.prop + ", "));
            });

            Console.WriteLine();
            Console.WriteLine(new string('-', 25));

            // Display desired output.
            myList.GroupBy(i => i.prop).Select(i => i.First()).ToList().ForEach(i => Console.Write(i.prop + ", "));
            Console.WriteLine();
        }
    }
    public class Foo
    {
        public string prop { get; set; }
        public string anotherProp { get; set; }
    }
}

If you prefer:

stuff.GroupBy(e => e.prop).Select(group => group.Key)

Using group by only gives individual items by your comparison function. if the comparison is done by prop it will return the objects with distinct prop only. all you need to do is iterate over them and select only prop.

List<strings> groups context.YourObjects.Select(o => o.prop).Distinct().ToList();

var q = from a in yourList 
group a by a.prop into b
select b.Key;

List<MyClass> source = getSource();

List<IGrouping<KeyType, MyClass>> groups = source
  .GroupBy(x => x.prop)
  .ToList();

List<KeyType> keys = groups
  .Select(g => g.Key)
  .ToList();