C# 使用 Index 和 Range 简化集合操作

Intro

有的语言数组的索引值是支持负数的，表示从后向前索引，比如：arr[-1]

从 C# 8 开始，C# 支持了数组的反向 Index，和 Range 操作，反向 Index 类似于其他语言中的负索引值，但其实是由编译器帮我们做了一个转换，Range 使得我们对数组截取某一部分的操作会非常简单，下面来看一下如何使用吧

Sample

使用 ^ 可以从集合的最后开始索引元素，如果从数组的最后开始索引元素，最后一个元素应该是 1 而不是0如：arr[^1]

使用 .. 可以基于某个数组截取集合中的某一段创建一个新的数组，比如 var newArray = array[1..^1]，再来看一下下面的示例吧

int[] someArray = new int[5] { 1, 2, 3, 4, 5 };
int lastElement = someArray[^1]; // lastElement = 5
lastElement.Dump();someArray[3..5].Dump();someArray[1..^1].Dump();someArray[1..].Dump();someArray[..^1].Dump();someArray[..2].Dump();

输出结果如下：

output

Index

那么它是如何实现的呢，索引值引入了一个新的数据结构 System.Index，当你使用 ^ 运算符的时候，实际转换成了 Index。

Index:

public readonly struct Index : IEquatable<Index>
{public Index(int value, bool fromEnd = false);/// <summary>Create an Index pointing at first element.</summary>public static Index Start => new Index(0);/// <summary>Create an Index pointing at beyond last element.</summary>public static Index End => new Index(~0);//// Summary://     Gets a value that indicates whether the index is from the start or the end.//// Returns://     true if the Index is from the end; otherwise, false.public bool IsFromEnd { get; }//// Summary://     Gets the index value.//// Returns://     The index value.public int Value { get; }//// Summary://     Creates an System.Index from the end of a collection at a specified index position.//// Parameters://   value://     The index value from the end of a collection.//// Returns://     The Index value.public static Index FromEnd(int value);//// Summary://     Create an System.Index from the specified index at the start of a collection.//// Parameters://   value://     The index position from the start of a collection.//// Returns://     The Index value.public static Index FromStart(int value);//// Summary://     Returns a value that indicates whether the current object is equal to another//     System.Index object.//// Parameters://   other://     The object to compare with this instance.//// Returns://     true if the current Index object is equal to other; false otherwise.public bool Equals(Index other);//// Summary://     Calculates the offset from the start of the collection using the given collection length.//// Parameters://   length://     The length of the collection that the Index will be used with. Must be a positive value.//// Returns://     The offset.public int GetOffset(int length);//// Summary://     Converts integer number to an Index.//// Parameters://   value://     The integer to convert.//// Returns://     An Index representing the integer.public static implicit operator Index(int value);
}

如果想要自己自定义的集合支持 Index 这种从数组最后索引的特性，只需要加一个类型是 Index 的索引器就可以了，正向索引也是支持的，int 会自动隐式转换为 Index，除了显示的增加 Index 索引器之外，还可以隐式支持，实现一个 int Count {get;} 的属性（属性名叫 Length 也可以），在实现一个 int 类型的索引器就可以了

写一个简单的小示例：

private class TestCollection
{public IList<int> Data { get; init; }public int Count => Data.Count;public int this[int index] => Data[index];//public int this[Index index] => Data[index.GetOffset(Data.Count)];
}
var array = new TestCollection()
{Data = new[] { 1, 2, 3 }
};
Console.WriteLine(array[^1]);
Console.WriteLine(array[1]);

Range

Range 是在 Index 的基础上实现的，Range 需要两个 Index 来指定开始和结束

public readonly struct Range : IEquatable<Range>
{/// <summary>Represent the inclusive start index of the Range.</summary>public Index Start { get; }/// <summary>Represent the exclusive end index of the Range.</summary>public Index End { get; }/// <summary>Construct a Range object using the start and end indexes.</summary>/// <param name="start">Represent the inclusive start index of the range.</param>/// <param name="end">Represent the exclusive end index of the range.</param>public Range(Index start, Index end){Start = start;End = end;}/// <summary>Create a Range object starting from start index to the end of the collection.</summary>public static Range StartAt(Index start) => new Range(start, Index.End);/// <summary>Create a Range object starting from first element in the collection to the end Index.</summary>public static Range EndAt(Index end) => new Range(Index.Start, end);/// <summary>Create a Range object starting from first element to the end.</summary>public static Range All => new Range(Index.Start, Index.End);/// <summary>Calculate the start offset and length of range object using a collection length.</summary>/// <param name="length">The length of the collection that the range will be used with. length has to be a positive value.</param>/// <remarks>/// For performance reason, we don't validate the input length parameter against negative values./// It is expected Range will be used with collections which always have non negative length/count./// We validate the range is inside the length scope though./// </remarks>public (int Offset, int Length) GetOffsetAndLength(int length);
}

如何在自己的类中支持 Range 呢？

一种方式是自己直接实现一个类型是 Range 的索引器

另外一种方式是隐式实现，在自定义类中添加一个 Count 属性，然后实现一个 Slice 方法，Slice 方法有两个 int 类型的参数，第一个参数表示 offset，第二个参数表示 length

来看下面这个示例吧，还是刚才那个类，我们支持一下 Range：

private class TestCollection
{public IList<int> Data { get; init; }//public int[] this[Range range]//{//    get//    {//        var rangeInfo = range.GetOffsetAndLength(Data.Count);//        return Data.Skip(rangeInfo.Offset).Take(rangeInfo.Length).ToArray();//    }//}public int Count => Data.Count;public int[] Slice(int start, int length){var array = new int[length];for (var i = start; i < length && i < Data.Count; i++){array[i] = Data[i];}return array;}
}

新的操作符 (^ and ..) 都只是语法糖，本质上是调用 Index、Range

Index 并不是支持负数索引，从最后向前索引只是编译器帮我们做了一个转换，转换成从前到后的索引值，借助于它，我们很多取集合最后一个元素的写法就可以大大的简化了，就可以从原来的 array[array.Length-1] => array[^1]

Range 在创建某个数组的子序列的时候就非常的方便， newArray = array[1..3],需要注意的是，Range 是"左闭右开"的，包含左边界的值，不包含右边界的值

还没使用过 Index/Range 的快去体验一下吧，用它们优化数组的操作吧~~

References

https://docs.microsoft.com/en-us/dotnet/csharp/tutorials/ranges-indexes
https://docs.microsoft.com/en-us/dotnet/api/system.index?view=net-5.0
https://docs.microsoft.com/en-us/dotnet/api/system.range?view=net-5.0
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/ranges
https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Index.cs
https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Range.cs
https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/ArraySegment.cs