How to Efficiently Remove Elements from One List That Exist in Another?

How to remove an item from one list that is contained in another list

Suppose you have two lists, list1 and list2, and you want to remove all items in list1 that are also present in list2. Using a foreach loop to iterate over list1 is not enough because it is not index based.

Solution: Use Except method

.NET Framework provides the Except method, which returns a new list containing elements from the first list that are not present in the second list. It takes two lists as parameters and operates element by element.

To use Except, you can write the following code:

<code class="language-csharp">List<car> list1 = GetTheList();
List<car> list2 = GetSomeOtherList();
List<car> result = list2.Except(list1).ToList();</code>

The generated result list will only contain items in list2 that are not in list1.

Note: The Except method does not modify the original list. It creates a new list containing the results.

Simplified syntax

You can further simplify the above code using method chaining:

<code class="language-csharp">List<car> result = GetSomeOtherList().Except(GetTheList()).ToList();</code>

This code achieves the same result without using temporary variables.

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