How Can I Efficiently Find Distinct Elements Between Two Lists of Custom Objects in C#?

Customize object comparison to differentiate lists

When dealing with two generic lists containing custom objects, extracting unique elements in one list (especially those that are not present in the other list) is a useful operation.

The

.Except() method provides an efficient solution for this. It accepts two list arguments and returns a new list containing items that are in the first list but not found in the second list.

Override equality and default comparison

If your custom object type overrides the Equals() and GetHashCode() methods, you can use .Except() as follows:

var list3 = list1.Except(list2).ToList();

In this case, the comparison of objects relies on their overridden methods. Any equality differences will be reflected in the differences between the lists.

Custom equality comparator

However, if your equality criterion requires a custom implementation, you can define your own IEqualityComparer<T>. Here's an example of using ID as a basis for comparison:

public class IdComparer : IEqualityComparer<customobject>
{
    // ... (实现如提供的答案中所示)
}

You can then use this custom comparator with .Except() via:

var list3 = list1.Except(list2, new IdComparer()).ToList();

This customized comparison will ensure that the .Except() operation meets your specific equality requirements.

Keep duplicates

It is worth noting that .Except() will exclude duplicate elements. If you need to keep duplicates, another way is to convert the second list to HashSet and use this:

var list3 = list1.Where(x => !set2.Contains(x)).ToList();

This method will retain all unique elements in the resulting list list3, including duplicates.

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