Question

Assume I have a class called MyClass that has two properties (int Id and a string Name). I want to populate a List of these MyClass objects from another collection but I want only the unique ones. This other collection is a 3rd party object that has a property named ‘Properties’ that is just an array of values, the first two of which correspond to the Id and Name values I care about. There can be duplicates in this collection so I want only the unique ones.

It seems like this should do the trick but it does not, it returns all the items regardless of dupes. What am I doing wrong here?

List<MyClass> items = (from MyClass mc in collectionOfProps 
select new MyClass() { 
Id = collectionOfProps.Properties[0], 
Name = collectionOfProps.Properties[1] }).Distinct().ToList();

Answer 1

The problem is likely that MyClass does not implement IEquatable<MyClass> as well as override Equals and GetHashCode.

In order to make Distinct() work the way you want, you have to implement IEquatable<T>. Otherwise, it uses the default (reference equality) for checking, which means it would only determine the elements were not distinct if they were the same exact instance.