How Can I Efficiently Retrieve the Last N Elements of a Collection Using LINQ?

Determining the Last N Elements of a Collection with LINQ

Retrieving a specified number of concluding elements from a collection can be a frequent requirement. While the framework may not offer a dedicated method, an extension method can be beneficial.

Using the TakeLast Extension Method

An efficient method for extracting the last N elements utilizes LINQ's Skip method. The following code illustrates its usage:

collection.Skip(Math.Max(0, collection.Count() - N));

Preserving item order, this technique avoids sorting and ensures compatibility with various LINQ providers. Precaution must be taken to avoid passing negative arguments to Skip, as certain providers (e.g., Entity Framework) may raise exceptions.

Implementing the Extension Method

The code below presents a custom TakeLast extension method:

public static class MiscExtensions
{
    // Ex: collection.TakeLast(5);
    public static IEnumerable<T> TakeLast<T>(this IEnumerable<T> source, int N)
    {
        return source.Skip(Math.Max(0, source.Count() - N));
    }
}

Performance Considerations

Depending on the data structure, counting (via Count()) can result in multiple enumerations. While optimizations exist for certain data types and scenarios, this approach may not be suitable for forward-only enumerables.

Alternative One-Pass Algorithms

In cases where performance is critical, alternative one-pass algorithms can be employed. These approaches utilize a temporary buffer to accumulate items, yielding the last N elements once the end of the collection is reached. Techniques developed by Lasse V. Karlsen and Mark Byers exemplify such algorithms.

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