昨天在 github 上准备找找 C# 9 又有哪些新语法糖可以试用,不觉在一个文档上看到一个很奇怪的写法: foreach (var item in myArray[0..5]) 哈哈,熟悉又陌生,玩过python的朋友对这个 [0..5] 太熟悉不过了,居然在 C# 中也遇到了,开心哈,看了下是 C# 8 的新语法,讽刺讽刺,8 都没玩熟就搞 9 了,我的探索欲比较强,总想看看这玩意底层是由什么支撑的。
var myarr = new string[] { "10", "20", "30", "40", "50", "60", "70", "80", "90", "100" };1. 提取 arr 前3个元素
static void Main(string[] args) { var myarr = new string[] { "10", "20", "30", "40", "50", "60", "70", "80", "90", "100" }; //1. 获取数组 前3个元素 var query1 = myarr[0..3]; var query2 = myarr.Take(3).ToList(); Console.WriteLine($"query1={string.Join(",", query1)}"); Console.WriteLine($"query2={string.Join(",", query2)}"); }
static void Main(string[] args) { var myarr = new string[] { "10", "20", "30", "40", "50", "60", "70", "80", "90", "100" }; //1. 获取数组 最后3个元素 var query1 = myarr[^3..]; var query2 = myarr.Skip(myarr.Length - 3).ToList(); Console.WriteLine($"query1={string.Join(",", query1)}"); Console.WriteLine($"query2={string.Join(",", query2)}"); }3. 提取 array 中index = 4,5,6 的三个位置元素
static void Main(string[] args) { var myarr = new string[] { "10", "20", "30", "40", "50", "60", "70", "80", "90", "100" }; //1. 获取数组 中 index=4,5,6 三个位置的元素 var query1 = myarr[4..7]; var query2 = myarr.Skip(4).Take(3).ToList(); Console.WriteLine($"query1={string.Join(",", query1)}"); Console.WriteLine($"query2={string.Join(",", query2)}"); }
4. 获取 array 中倒数第三和第二个元素
从要求上来看就是获取元素 80 和 90,如果你理解了前面的两个用法,我相信这个你会很快的写出来,代码如下:static void Main(string[] args) { var myarr = new string[] { "10", "20", "30", "40", "50", "60", "70", "80", "90", "100" }; //1. 获取 array 中倒数第三和第二个元素 var query1 = myarr[^3..^1]; var query2 = myarr.Skip(myarr.Length - 3).Take(2).ToList(); Console.WriteLine($"query1={string.Join(",", query1)}"); Console.WriteLine($"query2={string.Join(",", query2)}"); }
//编译前 var query1 = myarr[0..3]; //编译后: string[] query = RuntimeHelpers.GetSubArray<string>(myarr, new Range(0, 3));
public static T[] GetSubArray<[Nullable(2)] T>(T[] array, Range range) { ValueTuple<int, int> offsetAndLength = range.GetOffsetAndLength(array.Length); int item = offsetAndLength.Item1; int item2 = offsetAndLength.Item2; T[] array3 = new T[item2]; Buffer.Memmove<T>(Unsafe.As<byte, T>(array3.GetRawSzArrayData()), Unsafe.Add<T>(Unsafe.As<byte, T>(array.GetRawSzArrayData()), item), (ulong)item2); return array3; }
public readonly struct Range : IEquatable<Range> { public Index Start { get; } public Index End { get; } public Range(Index start, Index end) { this.Start = start; this.End = end; } public ValueTuple<int, int> GetOffsetAndLength(int length) { Index start = this.Start; int num; if (start.IsFromEnd) { num = length - start.Value; } else { num = start.Value; } Index end = this.End; int num2; if (end.IsFromEnd) { num2 = length - end.Value; } else { num2 = end.Value; } return new ValueTuple<int, int>(num, num2 - num); } }
//编译前: var query1 = myarr[^3..]; //编译后: string[] query = RuntimeHelpers.GetSubArray<string>(myarr, Range.StartAt(new Index(3, true)));
看到没有,这一次 new Index 的时候,给了 IsFromEnd = true , 表示从末尾开始计算,大家再结合刚才的 GetOffsetAndLength 方法,我想这逻辑你应该理顺了吧。