Why is this an issue?

Both the Enumerable.First extension method and the LinkedList<T>.First property can be used to find the first value in a LinkedList<T>. However, LinkedList<T>.First is much faster than Enumerable.First. For small collections, the performance difference may be minor, but for large collections, it can be noticeable. The same applies for the Last property as well.

Applies to:

• LinkedList<T>.First
• LinkedList<T>.Last

What is the potential impact?

We measured a significant improvement both in execution time and memory allocation. For more details see the Benchmarks section from the More info tab.

How to fix it

The First and Last properties are defined on the LinkedList class, and the extension method call can be replaced by calling the propery instead.

Code examples

Noncompliant code example

Function GetFirst(data As LinkedList(Of Integer)) As Integer
    Return Enumerable.First(data)
End Function

Function GetLast(data As LinkedList(Of Integer)) As Integer
    Return Enumerable.Last(data)
End Function

Compliant solution

Function GetFirst(data As LinkedList(Of Integer)) As Integer
    Return data.First.Value
End Function

Function GetLast(data As LinkedList(Of Integer)) As Integer
    Return data.Last.Value
End Function

Resources

Documentation

• LinkedList<T>

Benchmarks

The results were generated by running the following snippet with BenchmarkDotNet:

private LinkedList<int> data;
private Random random = new Random();

[Params(100_000)]
public int Size { get; set; }

[Params(1_000)]
public int Runs { get; set; }

[GlobalSetup]
public void Setup() =>
    data = new LinkedList<int>(Enumerable.Range(0, Size).Select(x => random.Next()));

[Benchmark(Baseline = true)]
public void LastMethod()
{
    for (var i = 0; i < Runs; i++)
    {
        _ = data.Last();                // Enumerable.Last()
    }
}

[Benchmark]
public void LastProperty()
{
    for (var i = 0; i < Runs; i++)
    {
        _ = data.Last;                  // Last property
    }
}

Hardware configuration:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2846/22H2/2022Update)
11th Gen Intel Core i7-11850H 2.50GHz, 1 CPU, 16 logical and 8 physical cores
  [Host]               : .NET Framework 4.8 (4.8.4614.0), X64 RyuJIT VectorSize=256
  .NET 7.0             : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  .NET Framework 4.6.2 : .NET Framework 4.8 (4.8.4614.0), X64 RyuJIT VectorSize=256