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John NoCookies
John NoCookies

Reputation: 1061

C# method to remove duplicates from a List<T>

I need a C# method to remove duplicates from a List<T> using a custom comparison operation. In .NET 4. Is there one or do I have to write it myself?

Upvotes: 2

Views: 1780

Answers (2)

Dan Tao
Dan Tao

Reputation: 128317

You could go with Jon's answer, or if you really want to remove duplicates from an existing list, something like this would work:

public static void RemoveDuplicates<T>(this IList<T> list, IEqualityComparer<T> comparer = null)
{
    comparer = comparer ?? EqualityComparer<T>.Default;

    var uniques = new HashSet<T>(comparer);
    for (int i = list.Count - 1; i >= 0; --i)
    {
        if (!uniques.Add(list[i]))
        {
            list.RemoveAt(i);
        }
    }
}

Upvotes: 6

Jon Skeet
Jon Skeet

Reputation: 1500385

Assuming your comparison operation is IEqualityComparer<T> or can be converted to it, you're fine with LINQ:

var newList = oldList.Distinct(customComparer).ToList();

Obviously that creates a new list rather than removing elements from the old one, but in most cases that's okay. You could always completely replace the contents of the old list with the new list afterwards if not...

Upvotes: 7

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