score:84

Accepted answer

Note: My computer is running .Net 4.5 RC, so it's possible that my results are affected by this.

Measuring the time it takes to execute a method just once is usually not very useful. It can be easily dominated by things like JIT compilation, which are not actual bottlenecks in real code. Because of this, I measured executing each method 100× (in Release mode without debugger attached). My results are:

  • Aggregate(): 9 ms
  • Sum(lambda): 12 ms
  • Sum(): 6 ms

The fact that Sum() is the fastest is not surprising: it contains a simple loop without any delegate invocations, which is really fast. The difference between Sum(lambda) and Aggregate() is not nearly as prominent as what you measured, but it's still there. What could be the reason for it? Let's look at decompiled code for the two methods:

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)
{
    if (source == null)
        throw Error.ArgumentNull("source");
    if (func == null)
        throw Error.ArgumentNull("func");

    TAccumulate local = seed;
    foreach (TSource local2 in source)
        local = func(local, local2);
    return local;
}

public static int Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector)
{
    return source.Select<TSource, int>(selector).Sum();
}

As you can see, Aggregate() uses a loop but Sum(lambda) uses Select(), which in turn uses an iterator. And using an iterator means there is some overhead: creating the iterator object and (probably more importantly) one more method invocation for each item.

Let's verify that using Select() is actually the reason by writing our own Sum(lambda) twice, once using Select(), which should behave the same as Sum(lambda) from the framework, and once without using Select():

public static int SlowSum<T>(this IEnumerable<T> source, Func<T, int> selector)
{
    return source.Select(selector).Sum();
}

public static int FastSum<T>(this IEnumerable<T> source, Func<T, int> selector)
{
    if (source == null)
        throw new ArgumentNullException("source");
    if (selector == null)
        throw new ArgumentNullException("selector");

    int num = 0;
    foreach (T item in source)
        num += selector(item);
    return num;
}

My measurements confirm what I thought:

  • SlowSum(lambda): 12 ms
  • FastSum(lambda): 9 ms

score:0

Only a decade late, but...

I've worked on a Linq replacement (nuget, github), which is (mostly!) a drop in replacement for System.Linq (just by adding the nuget package and then changing using System.Linq to using Cistern.ValueLinq) but using value types and some tricks under the hood.

Anyway, as far as Sum goes, it uses SIMD instructions under the hood (still honouring overflow) where it can.

Results below are running on a machine which is old as this original stackoverflow question, so modern machines should even have better results (although for a non SIMD able IEnumerable you do are getting some overhead)

public enum ContainerTypes { Enumerable, Array, List, }

[MemoryDiagnoser]
public class Benchmarks
{
    IEnumerable<int> _data;

    [Params(ContainerTypes.Array, ContainerTypes.Enumerable, ContainerTypes.List)]
    public ContainerTypes ContainerType { get; set; } = ContainerTypes.Enumerable;

    [GlobalSetup]
    public void SetupData()
    {
        var data = System.Linq.Enumerable.Range(0, 1000);
        _data = ContainerType switch
        {
            ContainerTypes.Enumerable => data,
            ContainerTypes.Array => System.Linq.Enumerable.ToArray(data),
            ContainerTypes.List => System.Linq.Enumerable.ToList(data),
            _ => throw new Exception("Unknown ContainerType")
        };
    }

    [Benchmark(Baseline = true)] public int System_Linq_Sum() => System.Linq.Enumerable.Sum(_data);
    [Benchmark] public int System_Linq_Sum_Predicate() => System.Linq.Enumerable.Sum(_data, c => c);
    [Benchmark] public int System_Linq_Aggregate() => System.Linq.Enumerable.Aggregate(_data, 0, (result, element) => result + element);
    [Benchmark] public int Cistern_ValueLinq_Sum() => Cistern.ValueLinq.Enumerable.Sum(_data);
    [Benchmark] public int Cistern_ValueLinq_Sum_Predicate() => Cistern.ValueLinq.Enumerable.Sum(_data, c => c);
    [Benchmark] public int Cistern_ValueLinq_Aggregate() => Cistern.ValueLinq.Enumerable.Aggregate(_data, 0, (result, element) => result + element);

    static void Main(string[] args) => BenchmarkRunner.Run<Benchmarks>();
}

Method ContainerType Mean Error StdDev Median Ratio RatioSD Gen 0 Gen 1 Gen 2 Allocated
System_Linq_Sum Enumerable 6.025 us 0.1155 us 0.1501 us 6.041 us 1.00 0.00 0.0076 - - 40 B
System_Linq_Sum_Predicate Enumerable 8.731 us 0.1727 us 0.3681 us 8.742 us 1.45 0.08 - - - 40 B
System_Linq_Aggregate Enumerable 8.534 us 0.1683 us 0.3514 us 8.657 us 1.42 0.07 - - - 40 B
Cistern_ValueLinq_Sum Enumerable 6.907 us 0.1369 us 0.3061 us 6.911 us 1.15 0.07 0.0076 - - 40 B
Cistern_ValueLinq_Sum_Predicate Enumerable 9.769 us 0.1907 us 0.1784 us 9.823 us 1.62 0.04 - - - 40 B
Cistern_ValueLinq_Aggregate Enumerable 9.295 us 0.1847 us 0.4962 us 9.396 us 1.52 0.08 - - - 40 B
System_Linq_Sum Array 6.731 us 0.1343 us 0.3653 us 6.814 us 1.00 0.00 0.0076 - - 32 B
System_Linq_Sum_Predicate Array 9.432 us 0.1873 us 0.5065 us 9.685 us 1.41 0.11 - - - 32 B
System_Linq_Aggregate Array 9.494 us 0.1890 us 0.4707 us 9.759 us 1.41 0.11 - - - 32 B
Cistern_ValueLinq_Sum Array 1.404 us 0.0279 us 0.0710 us 1.436 us 0.21 0.02 - - - -
Cistern_ValueLinq_Sum_Predicate Array 4.064 us 0.0811 us 0.0996 us 4.087 us 0.61 0.04 - - - -
Cistern_ValueLinq_Aggregate Array 3.549 us 0.0709 us 0.1496 us 3.584 us 0.53 0.04 - - - -
System_Linq_Sum List 11.779 us 0.2344 us 0.3048 us 11.854 us 1.00 0.00 - - - 40 B
System_Linq_Sum_Predicate List 14.227 us 0.2842 us 0.6919 us 14.601 us 1.22 0.05 - - - 40 B
System_Linq_Aggregate List 13.852 us 0.2761 us 0.7418 us 14.249 us 1.17 0.06 - - - 40 B
Cistern_ValueLinq_Sum List 1.437 us 0.0288 us 0.0835 us 1.471 us 0.12 0.01 - - - -
Cistern_ValueLinq_Sum_Predicate List 3.672 us 0.0732 us 0.1941 us 3.771 us 0.32 0.02 - - - -
Cistern_ValueLinq_Aggregate List 3.597 us 0.0718 us 0.1880 us 3.698 us 0.31 0.02 - - - -

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