C#8.0本质论第十六章--使用查询表达式的LINQ

C#8.0本质论第十六章–使用查询表达式的LINQ

像SQL这样的专业查询语言虽然容易阅读和理解，但又缺乏C#语言的完整功能。这正是C#语言设计者在C#3.0中添加查询表达式语法的原因。

本章大部分都类似于SQL，一般不会使用到，在用到的时候再去书里查吧。

16.1查询表达式概述

开发者经常对集合进行赛选来删除不想要的项，以及对集合进行投射将其中的项变成其它形式。

        IEnumerable<string> selection =from word in CSharp.Keywordswhere !word.Contains('*')select word;

16.1.1投射

        IEnumerable<string> fileNames = Directory.GetFiles(rootDirectory, searchPattern);IEnumerable<FileInfo> fileInfos =from fileName in fileNamesselect new FileInfo(fileName);

16.1.2筛选

        IEnumerable<FileInfo> files =from fileName in Directory.EnumerateFiles(rootDirectory, searchPattern)where File.GetLastWriteTime(fileName) <DateTime.Now.AddMonths(-1)select new FileInfo(fileName);

16.1.3查询

        IEnumerable<string> fileNames =from fileName in Directory.EnumerateFiles(rootDirectory, searchPattern)orderby (new FileInfo(fileName)).Length descending,fileNameselect fileName;

16.1.4let子句

        IEnumerable<FileInfo> files =from fileName in Directory.EnumerateFiles(rootDirectory, searchPattern)orderby new FileInfo(fileName).Length, fileNameselect new FileInfo(fileName);

16.1.5分组

    private static void GroupKeywords1(){IEnumerable<IGrouping<bool, string>> selection =from word in CSharp.Keywordsgroup word by word.Contains('*');foreach(IGrouping<bool, string> wordGroupin selection){Console.WriteLine(Environment.NewLine + "{0}:",wordGroup.Key ?"Contextual Keywords" : "Keywords");foreach(string keyword in wordGroup){Console.Write(" " +(wordGroup.Key ?keyword.Replace("*", null) : keyword));}}}

16.1.6使用into实现查询延续

    private static void GroupKeywords1(){IEnumerable<IGrouping<bool, string>> keywordGroups =from word in CSharp.Keywordsgroup word by word.Contains('*');IEnumerable<(bool IsContextualKeyword, IGrouping<bool, string> Items)> selection =from groups in keywordGroupsselect(IsContextualKeyword: groups.Key,Items: groups);foreach ((bool isContextualKeyword, IGrouping<bool, string> items)in selection){Console.WriteLine(Environment.NewLine + "{0}:",isContextualKeyword ?"Contextual Keywords" : "Keywords");foreach (string keyword in items){Console.Write(" " + keyword.Replace("*", null));}}}

16.1.7用多个from子句"平整"序列的序列

var selection =from word in CSharp.Keywordsfrom character in wordselect character;var numbers = new[] { 1, 2, 3 };
IEnumerable<(string Word, int Number)> product =from word in CSharp.Keywordsfrom number in numbersselect (word, number);

16.2查询表达式只是方法调用

令人惊讶的是，在C#3.0中引入查询表达式并未对CLR或者CIL进行任何改动。相反，编译器只是将查询表达式转换成一系列方法调用。

private static void ShowContextualKeywords1()
{IEnumerable<string> selection =from word in CSharp.Keywordswhere !word.Contains('*')select word;// ...
}

编译之后，会转换成一个由System.Linq.Enumerable提供的IEnumerable扩展方法调用。

private static void ShowContextualKeywords2()
{IEnumerable<string> selection =CSharp.Keywords.Where(word => !word.Contains('*'));// ...
}