How to find duplicate items in list<>? [duplicate]

Question

I have:

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };

How can I get only "a","b"?

I tried to do like this:

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };
List<string> test_list = new List<string>(); 

test_list = list.Distinct().ToList();

Now test_list has {"a", "b", "r", "t"}
And then:

test_list = test_list.Except(list).ToList();

So that was my fail point, cause Except() deleted all elements.

Could you help me with solution?

Answer 1

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };

var dups = list.GroupBy(x => x)
    .Where(x => x.Count() > 1)
    .Select(x => x.Key)
    .ToList();