IEqualityComparer and Linq Distinct - Hard Code GetHashCode()
.net, c#, linq, linq-to-objects
Solution
When I run linq's Distinct method on the array, nothing is filtered out. Anyone know why?
Yes - `Equals` is going to return `false` for any two distinct objects. Within your `Equals` implementation, you're calling the `CustomObject.GetHashCode` method (which isn't overridden), not your custom comparer's `GetHashCode` method. If you expected `Equals` to call your custom `GetHashCode` method, you'd need to change it to:
public bool Equals(CustomObject x, CustomObject y)
{
return GetHashCode(x) == GetHashCode(y);
}
Or given the implementation of `GetHashCode(CustomObject)`, you can just simplify it to:
public bool Equals(CustomObject x, CustomObject y)
{
return true;
}
Note that your `GetHashCode` method was being called by `Distinct()`, but it was then calling your `Equals` method to check that the objects were really equal... and at that point you were saying that they weren't (barring a massive coincidence).
Problem
I have an array of `CustomObject` s and a custom `IEqualityComparer<CustomObject>`. I have hard coded the `IEqualityComparer.GetHashCode()` method to return a constant `42`. When I run linq's `Distinct` method on the array, nothing is filtered out. Anyone know why? Note: I know there are a number of questions on here about this issue, however, the ones I've seen (C# Distinct on IEnumerable<T> with custom IEqualityComparer, Distinct() with lambda?, etc) only say to make sure to implement `GetHashCode`. None of them explain, why it doesn't work. Code: ``` public class CustomObject { public string Name { get; set; } } public class CustomObjectEqualityComparer : IEqualityComparer<CustomObject> { public bool Equals(CustomObject x, CustomObject y) { //Every CustomObject should now be 'equal'! return x.GetHashCode() == y.GetHashCode(); } public int GetHashCode(CustomObject obj) { //Every CustomObject should now be 'equal'! return 42; } } ``` Test: ``` [TestFixture] public class TestRunner { private CustomObject[] customObjects = { new CustomObject {Name = "Please"}, new CustomObject {Name = "Help"}, new CustomObject {Name = "Me"}, new CustomObject {Name = "Stack"}, new CustomObject {Name = "Overflow"}, }; [Test] public void DistinctTest() { var distinctArray = customObjects.Distinct(new CustomObjectEqualityComparer()).ToArray(); //Since every CustomObject is 'Equal' there should only be //1 element in the array. Assert.AreEqual(1, distinctArray.Length); } } ``` This is the output I get from running the test: ``` Expected: 5 But was: 1 ``` If I debug the test, I can see that `GetHashCode` is being called, so why isn't `Distinct` filtering out all the 'duplicates'?