C#.First()与[0] [英] C# .First() vs [0]

问题描述

感兴趣的是，方法是否有任何区别.
因此，我创建了两个片段.

Interested, does approaches has any differences.
So, I created two snippets.

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]; 

我们相信IL，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  

代码片段B产生了一个命令，其IL增大了，但是最后哪种方法更快?

Snippet B produced one command more IL, but which approach faster in the end?

推荐答案

您可以自己检查它:

    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();
    }

释放模式下的输出:

LINQ First():367

按索引:84

以调试模式输出:

LINQ First():401

按索引:177

P.S.

First方法的源代码是:

The source code for method First is:

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;
            }
        }
    }
}

强制转换source as IList<TSource>或创建Enumerator对象很可能是First()相当慢的原因.

The casting operation source as IList<TSource> or creating an Enumerator object is very likely the reason why First() is considerably slower.

这篇关于C#.First()与[0]的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！