Question

Is it possible to GroupBy in LINQ, using a collection property?

e.g.

void Main()
{
    var t1 = new Test() { Children = new List<string>() { "one", "two" } };
    var t2 = new Test() { Children = new List<string>() { "one", "two" } };
    var t3 = new Test() { Children = new List<string>() { "one", "three" }        };

    var tests = new List<Test>() { t1, t2, t3 };
    var anon =  from t in tests
                select new
                {
                    Children = t.Children
                };

    anon.GroupBy(t => t.Children).Dump();
}

public class Test
{
    public List<string> Children {get;set;}
}

In this example, I would hope for two groups:

Key: List() { “one”, “two” } Value: t1, t2

Key: List() { “one”, “three” } Value: t3

My understanding is that anonymous types are compared not by reference, but by comparing equality on their public properties.

However, the actual result is three groups:

Key: List() { “one”, “two” } Value: t1

Key: List() { “one”, “two” } Value: t2

Key: List() { “one”, “three” } Value: t3

If this is not possible, is there a way to get the result I want?

Hopefully explained this clearly…

Answer 1

The problem is that the lists are not exactly identical. It’s comparing equality for grouping, and you have two new List<string>s, which aren’t exactly equal. You can, however, join the strings by hash code, which would produce a correct result:

tests.GroupBy(t => String.Join(string.Empty, t.Children.Select(c => c.GetHashCode().ToString())));