C#.First()vs [0]

Question

有兴趣,方法有什么不同.
所以,我创建了两个片段.

Snippet A 
List<int> a = new List<int>();
a.Add(4);
a.Add(6);
int b = a.First(); 

和

Snippet B 
List<int> a = new List<int>();
a.Add(4);
a.Add(6);
int b = a[0]; 

在IL,我们相信,所以

Snippet A IL
IL_0000:  nop         
IL_0001:  newobj      System.Collections.Generic.List<System.Int32>..ctor
IL_0006:  stloc.0     // a
IL_0007:  ldloc.0     // a
IL_0008:  ldc.i4.4    
IL_0009:  callvirt    System.Collections.Generic.List<System.Int32>.Add
IL_000E:  nop         
IL_000F:  ldloc.0     // a
IL_0010:  ldc.i4.6    
IL_0011:  callvirt    System.Collections.Generic.List<System.Int32>.Add
IL_0016:  nop         
IL_0017:  ldloc.0     // a
IL_0018:  call        System.Linq.Enumerable.First
IL_001D:  stloc.1     // b
IL_001E:  ret        

和

Snippet B IL
IL_0000:  nop         
IL_0001:  newobj      System.Collections.Generic.List<System.Int32>..ctor
IL_0006:  stloc.0     // a
IL_0007:  ldloc.0     // a
IL_0008:  ldc.i4.4    
IL_0009:  callvirt    System.Collections.Generic.List<System.Int32>.Add
IL_000E:  nop         
IL_000F:  ldloc.0     // a
IL_0010:  ldc.i4.6    
IL_0011:  callvirt    System.Collections.Generic.List<System.Int32>.Add
IL_0016:  nop         
IL_0017:  ldloc.0     // a
IL_0018:  ldc.i4.0    
IL_0019:  callvirt    System.Collections.Generic.List<System.Int32>.get_Item
IL_001E:  stloc.1     // b
IL_001F:  ret  

Snippet B产生了一个更多的IL命令,但最终接近哪个更快？

Answer 1

你可以自己检查一下:

    static void Main()
    {
        List<long> resultsFirst = new List<long>();
        List<long> resultsIndex = new List<long>();

        Stopwatch s = new Stopwatch();

        for (int z = 0; z < 100; z++)
        {
            List<int>[] lists = new List<int>[10000];

            int temp = 0;

            for (int i = 0; i < lists.Length; i++)
                lists[i] = new List<int>() { 4, 6 };                

            s.Restart();

            for (int i = 0; i < lists.Length; i++)
                temp = lists[i].First();

            s.Stop();

            resultsFirst.Add(s.ElapsedTicks);

            s.Restart();

            for (int i = 0; i < lists.Length; i++)
                temp = lists[i][0];

            s.Stop();

            resultsIndex.Add(s.ElapsedTicks);
        }

        Console.WriteLine("LINQ First()  :   " + resultsFirst.Average());
        Console.WriteLine(Environment.NewLine);
        Console.WriteLine("By index      :   " + resultsIndex.Average());

        Console.ReadKey();
    }

发布模式下的输出:

LINQ First():367

按指数:84

调试模式下的输出:

LINQ First():401

按索引:177

PS

方法First的源代码是:

public static TSource First<TSource>(this IEnumerable<TSource> source)
{
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
    {
        if (list.Count > 0)
        {
            return list[0];
        }
    }
    else
    {
        using (IEnumerator<TSource> enumerator = source.GetEnumerator())
        {
            if (enumerator.MoveNext())
            {
                return enumerator.Current;
            }
        }
    }
}

转换操作source as IList<TSource>或创建Enumerator对象很可能是因为First()它相当慢的原因.

Answer 2

该Enumerable.First方法定义为

public static TSource First<TSource>(this IEnumerable<TSource> source) 
{
    if (source == null) throw Error.ArgumentNull("source");
    IList<TSource> list = source as IList<TSource>;
    if (list != null) {
        if (list.Count > 0) return list[0];
    }
    else {
        using (IEnumerator<TSource> e = source.GetEnumerator()) {
            if (e.MoveNext()) return e.Current;
        }
    }
    throw Error.NoElements();
}

因此，对于 aList<T>它最终在空检查和强制转换后使用索引器。看起来不多，但是当我测试性能时，First它比索引器慢 10 倍（for循环，10 000 000 次迭代，发布版本：第一个 - 100 毫秒，索引器 - 10 毫秒）。