起因

.Net Core 3.0已经支持C# 8.0的语法和特性,也迎来了切片这个特性,用起来还是很方便的.和go语言有差异.

在官方文档叫索引和范围,这个叫法有点不是很好,这里还是称切片贴切.主要从Array/Span/ReadOnlySpan获取一个元素或集合范围提供简洁的语法.就是我们俗称的语法糖.

主要通过两个运算符 ^ 和 ..

先看看在C#怎么使用切片

private static void Main(string[] args)
{
    int[] arr1 = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };


    var arr2 = arr1[3..6];        //3..6获取下标3和下标6范围的元素
    Print("[3..6]", arr2);

    arr2 = arr1[..6];               //..6 获取下标0到下标6之间的元素
    Print("[..6]", arr2);

    var elem = arr1[^1];       //获取最后一个元素
    Print("[^1]", elem);

    arr2 = arr1[0..^1];           //获取下标0和arr1.Length之间的元素,不等同0...arr1.Length
    Print("[0..^0]", arr2);

    arr2 = arr1[0..arr1.Length];
    Print("[0..arr1.Length]", arr2);

    //Console.ReadKey();
}

private static void Print(string name, int[] a)
{
    Console.Write(name + ":");
    for (int i = 0; i < a.Length; i++)
    {
        Console.Write($"{a[i]} ");
    }
    Console.Write(Environment.NewLine);
}

private static void Print(string name, int a)
{
    Console.Write(name + ":" + a + Environment.NewLine);
}

我们看看内部是如何实现的

精简一下上边的代码

private static void Main(string[] args)
{
    int[] arr1 = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    var arr2 = arr1[3..6];        //3..6获取下标3和下标6范围的元素
    Print("[3..6]", arr2);
}

查看生成后的代码

private static void Main(string[] args)
{
    int[] numArray = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    //1. [3..6] 在编译的时候,自动转换 new Range(3, 6)
    //2. 通过GetSubArray返回子数组
    int[] subArray = RuntimeHelpers.GetSubArray(numArray, new Range(3, 6));
    Program.Print("[3..6]", subArray);
}

//源码:可以看这里: https://source.dot.net/#System.Private.CoreLib/RuntimeHelpers.cs,92cb32f07e6de356
public static T[] GetSubArray(T[] array, Range range)
{
    ValueTuple offsetAndLength = range.GetOffsetAndLength((int)array.Length);
    int item1 = offsetAndLength.Item1;
    int item2 = offsetAndLength.Item2;
    T t = default(T);
    if (t == null && !(typeof(T[]) == array.GetType()))
    {
        T[] tArray = (T[])Array.CreateInstance(array.GetType().GetElementType(), item2);
        Array.Copy(array, item1, tArray, 0, item2);   //从源数据中拷贝到新数组中
        return tArray;
    }
    if (item2 == 0)
    {
        return Array.Empty();
    }
    T[] tArray1 = new T[item2];

    //通过指针从源数据中拷贝数据到新数组中
    Buffer.Memmove(ref Unsafe.As(ref tArray1.GetRawSzArrayData()),
                        ref Unsafe.Add(ref Unsafe.As(ref array.GetRawSzArrayData()), item1),
        (ulong)item2);
    return tArray1;
}

既然看了一下生成后的代码.顺便看一下生成后的IL代码:

//[3..6]解析
////将3推送到栈上
//IL_0015: ldc.i4.3
////调用 System.Index op_Implicit (3)
//IL_0016: call valuetype[System.Runtime]System.Index[System.Runtime] System.Index::op_Implicit(int32)
////等同于System.Index index1 =3;


////将6推送到栈上        
//IL_001b: ldc.i4.6
//IL_001c: call valuetype[System.Runtime]System.Index[System.Runtime] System.Index::op_Implicit(int32)
////创建Range实例 : new Range(3,6)
//IL_0021: newobj instance void[System.Runtime] System.Range::.ctor(valuetype[System.Runtime] System.Index,  valuetype[System.Runtime] System.Index)
////调用GetSubArray
//IL_0026: call int32[] [System.Runtime] System.Runtime.CompilerServices.RuntimeHelpers::GetSubArray(!!0[],  valuetype[System.Runtime] System.Range)
//IL_002b: stloc.1