How Can I Efficiently Find the Difference Between Two Lists of Custom Objects in C#?

Difference extraction of generic custom object lists in C#

Suppose you have two lists, each containing custom objects, and you need a way to get a third list that only contains items that are present in the first list but not in the second element.

For this purpose, the .Except() method is a suitable option. This method assumes that your custom object implements the Equals and GetHashCode methods, thus providing a way to define object equality. Using .Except() is simple:

<code class="language-csharp">var list3 = list1.Except(list2).ToList();</code>

However, if you need to custom define equality based on a specific property (such as ID), you can define a IEqualityComparer<T> implementation. Consider the following example:

<code class="language-csharp">public class IdComparer : IEqualityComparer<CustomObject>
{
    public int GetHashCode(CustomObject co)
    {
        if (co == null)
        {
            return 0;
        }
        return co.Id.GetHashCode();
    }

    public bool Equals(CustomObject x1, CustomObject x2)
    {
        if (object.ReferenceEquals(x1, x2))
        {
            return true;
        }
        if (object.ReferenceEquals(x1, null) ||
            object.ReferenceEquals(x2, null))
        {
            return false;
        }
        return x1.Id == x2.Id;
    }
}</code>

In this case you can use the following code to retrieve the difference between the two lists:

<code class="language-csharp">var list3 = list1.Except(list2, new IdComparer()).ToList();</code>

For cases where duplicates exist, consider creating a set from the second list and use the following method:

<code class="language-csharp">var list3 = list1.Where(x => !set2.Contains(x)).ToList();</code>

This will ensure that duplicates are retained in the results list.

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