Neither Equals nor GetHashCode get called on IEqualityComparer

Question

I'm comparing two List<Dictionary<string, object>> with my own IEqualityComparer<Dictionary<string, object>> implementation, but neither GetHashCode nor Equals method get called.

Here's my own IEqualityComparer implementation.

public class TestEqualityComparer : IEqualityComparer<Dictionary<string, object>>
{
    public bool Equals(Dictionary<string, object> a, Dictionary<string, object> b)
    {
        return true; // breakpoint here
    }

    public int GetHashCode(Dictionary<string, object> obj)
    {
        return 0; // breakpoint here
    }
}

And here's a actual comparing code.

var a = new List<Dictionary<string, object>>();
var b = new List<Dictionary<string, object>>();

a.Add(new Dictionary<string, object> { ["id"] = 1, ["msg"] = "aaaaa" });
a.Add(new Dictionary<string, object> { ["id"] = 2, ["msg"] = "bbbbb" });
a.Add(new Dictionary<string, object> { ["id"] = 3, ["msg"] = "ccccc" });

b.Add(new Dictionary<string, object> { ["id"] = 1, ["msg"] = "zzzzz" });
b.Add(new Dictionary<string, object> { ["id"] = 2, ["msg"] = "bbbbb" });
b.Add(new Dictionary<string, object> { ["id"] = 4, ["msg"] = "ddddd" });

var except = a.Except(b, new TestEqualityComparer());

When I ran the above code, breakpoints never got triggered. What's the problem?

Answer 1

Since LINQ uses deferred execution the contents of except collection will not be determined unless you decide to iterate over it, hence no calls to your IEqualityComparer .

To force evaluation of your Except statement you can either iterate over it with foreach or append ToList/ToArray to your statement, like so:

var except = a.Except(b, new TestEqualityComparer()).ToList(); // ToList forces processing of LINQ query