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This article mainly explains "how to use slicing grammar sugar in C#". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn how to use slicing grammar sugar in C#.

One: background 1. Tell a story

Yesterday, I was going to find out what new grammar candy C# 9 can try on github, but I saw a strange way to write it on a document: foreach (var item in myArray [0.5]) , familiar and unfamiliar, friends who have played with python are so familiar with this [0.5] that they even encountered it in C#. Happy ha, look at the new grammar of C# 8, ironic. 8 did not even play well to do 9, I have a strong desire to explore, always want to see what is supported by the bottom of this thing.

Two:. The usage of grammatical sugar

From the semantics of myArray [0.. 5] introduced earlier, we can also see that this is an operation of segmenting array, so how many sharding ways are there? Let's introduce the following one. In order to facilitate the demonstration, I'll define an array with the following code:

Var myarr = new string [] {"10", "20", "30", "40", "50", "60", "70", "80", "90", "100"}; 1. Extract the first three elements of arr

If you use linq, you can use Take (3), and if you use slicing, it is [0.. 3]. The code is as follows:

Static void Main (string [] args) {var myarr = new string [] {"10", "20", "30", "40", "50", "60", "70", "80", "90", "100"}; / / 1. Get the first three elements of the array var query1= myarr [0.3]; var query2= myarr.Take (3). ToList (); Console.WriteLine ($"query1= {string.Join (", ", query1)}"); Console.WriteLine ($"query2= {string.Join (", ", query2)}");}

two。 Extract the last three elements of arr

How can I get this? It is fine to use-3 directly in python. In C #, you need to use ^ to indicate starting from the end. The code is as follows:

Static void Main (string [] args) {var myarr = new string [] {"10", "20", "30", "40", "50", "60", "70", "80", "90", "100"}; / / 1. Get the last three elements of the array 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. Extract three positional elements of index = 4, 5, 5, 6 from array

If you use linq, you need to use Skip + Take double combination, if you use slicing operation, it is too easy.

Static void Main (string [] args) {var myarr = new string [] {"10", "20", "30", "40", "50", "60", "70", "80", "90", "100"}; / / 1. Get the elements 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)}");}

Judging from the output of the above cutting interval [4. 7], this is a left-closed and right-open interval, so pay special attention to it.

4. Get the third to last element and the second element in array

In terms of requirements, it is to get elements 80 and 90. If you understand the previous two uses, I am sure you will write this quickly. The code is as follows:

Static void Main (string [] args) {var myarr = new string [] {"10", "20", "30", "40", "50", "60", "70", "80", "90", "100"}; / / 1. Get the penultimate element 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)}");}

three。 Inquiry principle

Through the previous four examples, I think we all know how to play, and the next step is to see what the interior is supported with. Here we use DnSpy to dig.

1. From myarr [0.. 3]

The decompiled code with dnspy is as follows:

/ / pre-compilation var query1 = myarr [0.3]; / / after compilation: string [] query = RuntimeHelpers.GetSubArray (myarr, new Range (0,3))

From the compiled code, we can see that the original array for getting the slice is obtained by calling RuntimeHelpers.GetSubArray, and then I simplify this method. The code is as follows:

Public static T [] GetSubArray (T [] array, Range range) {ValueTuple offsetAndLength = range.GetOffsetAndLength (array.Length); int item = offsetAndLength.Item1; int item2 = offsetAndLength.Item2; T [] array3 = new T [item2]; Buffer.Memmove (Unsafe.As (array3.GetRawSzArrayData ()), Unsafe.Add (Unsafe.As (array.GetRawSzArrayData ()), item), (ulong) item2) Return array3;}

As you can see from the above code, the last child array is done by Buffer.Memmove, but the cutting position for the child array is implemented by the GetOffsetAndLength method, so continue to trace the code:

Public readonly struct Range: IEquatable {public Index Start {get;} public Index End {get;} public Range (Index start, Index end) {this.Start = start; this.End = end } public ValueTuple 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 (num, num2-num);}}

After reading the code above, you may have two doubts:

1) what do start.IsFromEnd and end.IsFromEnd mean.

In fact, after looking at the above code logic, you can see that IsFromEnd indicates whether the starting point starts on the left or right. It's as simple as that.

2) I don't see how start.IsFromEnd and end.IsFromEnd are assigned values.

In the constructor of the Index class, depending on the true or false inserted in the previous layer when going to new Index, the code is as follows:

The flow of this example is roughly as follows: new Range (1Magne3)-> operator Index (int value)-> FromStart (value)-> new Index (value). You can see that the optional parameters are not assigned at the end of new.

two。 Explore myarr [^ 3.]

The example just now does not assign a value to the optional parameter, so let's see if it is assigned when this example is new Index?

/ / pre-compilation: var query1 = myarr [^ 3..]; / / after compilation: string [] query = RuntimeHelpers.GetSubArray (myarr, Range.StartAt (new Index (3, true)

See, when you new Index this time, you give IsFromEnd = true, which means that the calculation starts at the end, and then you combine the GetOffsetAndLength method just now. I think you should straighten out this logic.

At this point, I believe you have a deeper understanding of "how to use sliced grammar sugar in C#". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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