AsListBenchmarks.cs

public enum TestMode
{
    List,
    IEnumerable,
    Null,
}

// See https://twitter.com/Nick_Craver/status/724331063414235137
[Config(typeof(Config))]
public class Program
{
    static void Main(string[] args)
    {
        var summary = BenchmarkRunner.Run<Program>();
    }

    private class Config : ManualConfig
    {
        public Config()
        {
            Add(Job.Clr.WithLaunchCount(1));
            Add(new GCDiagnoser());
            Add(RPlotExporter.Default);
            Add(CsvMeasurementsExporter.Default); // Without this RPlot Exporter doesn't work!!!
        }
    }

    private List<int> list = Enumerable.Range(0, 100).Select(i => i).ToList();
    private IEnumerable<int> enumerable = Enumerable.Range(0, 100).Select(i => i);
    private IEnumerable<int> listToUse;

    [Params(TestMode.List, TestMode.IEnumerable, TestMode.Null)]
    public TestMode Mode { get; set; }

    [Setup]
    public void Setup()
    {
        switch (Mode)
        {
            case TestMode.List:
                listToUse = list;
                break;
            case TestMode.IEnumerable:
                listToUse = enumerable;
                break;
            case TestMode.Null:
                listToUse = null;
                break;
            default:
                throw new InvalidOperationException("Something went wrong!!");
        }
    }

    [Benchmark]
    public List<int> NickCraver()
    {
        return listToUse.AsListNickCraver();
    }
    
    [Benchmark]
    public List<int> KirkWoll()
    {
        return listToUse.AsListKirkWoll();
    }
}

public static class Extensions
{
    // See https://twitter.com/Nick_Craver/status/724320336691470337
    public static List<T> AsListNickCraver<T>(this IEnumerable<T> source) =>
        source != null && !(source is List<T>) ? source.ToList() : (List<T>)source;

    // See https://twitter.com/kirkwoll/status/724325018662903808
    public static List<T> AsListKirkWoll<T>(this IEnumerable<T> source) =>
        source as List<T> ?? source?.ToList();
}