linq

.net连接mysql

由 坚强是说给别人听的谎言 提交于 2021-02-12 07:16:01
首先在官网下载，mysql-connect-net，用于使用mysql的驱动程序，我在下载mysql-connect-net.msi. installer后，执行安装程序的时候一直无法安装成功，最简单的方法是直接下载.zip文件后解压，无须安装。 官网地址:http://dev.mysql.com/downloads/file/?id=463757 解压文件后， 出现了好几个文件夹，其中有v4和v4.5两个文件夹，对应vs的不同版本 VS2010使用V4.0下的dll文件 VS2012/2013/2015使用v4.5下的dll文件 其中有一个帮助手册十分有用： Documentation文件夹下的ConnectorNET.chm中包含了连接mysql数据库的API。 MySqlConnection类用来连接数据库 Constructor: 构造函数 MySqlConnection(String) Initializes a new instance of the MySqlConnection class when given a string containing the connection string. methods: 打开数据库 Open Opens a database connection with the property settings specified

Compare a Datatable and string array in c#

由 大憨熊 提交于 2021-02-11 17:52:24
问题 I simply want to compare the first col of the datatable called "name" to items in the array. They can be multiple rows in the datatable with the same name. After it has compared all items in the array it should delete the rows that were NOT from the datatable. The output result can be the datatable itself or list array (prefer this) however this should retain all the columns from datatable. Possibly want to use linq query. code: DataTable dt = new DataTable("TestTable"); dt.Columns.Add(new

初学c#-2019.1.17

由 こ雲淡風輕ζ 提交于 2021-02-11 17:38:33
using System; // using 关键字——引用.net框架类库中的现有资源 using System.Collections.Generic; // System命名空间——提供了访问 using System.Linq; using System.Text; using System.Threading.Tasks; namespace test_01 { class Program { static void Main(string[] args) { Console.WriteLine("Hello world!"); // 静态方法——写到控制台窗口上 Console.ReadKey77(); // 告诉代码在结束前等待按键 } } } // 不同于c的一个数据类型 decimal类型 ——不遵守四舍五入规则的十进制数，默认为0.0m // 内置引用类型——object 根类型 .net框架中所有类型都由其派生而来 // 类型转换用类 Convert ——Convert.To类型() // foreach语句 /* foreach(数据类型 变量名 in 数组名) * { * 语句； * } */ 来源： oschina 链接： https://my.oschina.net/u/4321246/blog/3672145

LINQ JOIN Child Collection

由 最后都变了- 提交于 2021-02-11 15:51:56
问题 I have a meetingRepository class that returns IEnumerable and an attendeeRepository class that returns IEnumerable<Attendee> public class meetingRepository { IEnumerable<Meeting> GetAll() { //return all Meetings } } public class attendeeRepository { IEnumerable<Attendee>GetAll() { //return all Attendees } } public class Meeting { public int Id { get; set; } public DateTime Date { get; set; } public string FilePath { get; set; } public int Duration { get; set; } public IEnumerable<Attendee>

List<string> complex sorting

由 不想你离开。 提交于 2021-02-11 15:14:33
问题 I have a List<string> of sizes, say XS, S, M, L, XL, XXL, UK 10, UK 12 etc What I want is to force the order to be that of above, regardless of the order of items in the list, I think I need a IComparable operator but unsure. Ideally I want to have another List with the correct order in which it can reference it's 'place' in the list and re-sort itself, if it doesn't exist it will default to A-Z 回答1: Create an array of sizes in the order you want them to be in, then sort the shirts by the

List<string> complex sorting

由 喜你入骨 提交于 2021-02-11 15:11:47
问题 I have a List<string> of sizes, say XS, S, M, L, XL, XXL, UK 10, UK 12 etc What I want is to force the order to be that of above, regardless of the order of items in the list, I think I need a IComparable operator but unsure. Ideally I want to have another List with the correct order in which it can reference it's 'place' in the list and re-sort itself, if it doesn't exist it will default to A-Z 回答1: Create an array of sizes in the order you want them to be in, then sort the shirts by the

How do I merge two seperate '.ToList()"?

由 こ雲淡風輕ζ 提交于 2021-02-11 13:40:59
问题 There are several posts where people are asking how to merge generic lists. However, in my case I have two seperate linq statements where I am returning two separate database results .ToList() var appleList = (from al in db.Apples where al.Id == id select al).ToList(); var bananaList = (from bl in db.Bananas where bl.Id == id select bl).ToList(); I tried merging these list like so: appleList.Concat(bananaList); but this causes the following error: 'List' does not contain a definition for

SelectMany nested in parent's lambda vs SelectMany after SelectMany

由 流过昼夜 提交于 2021-02-11 13:27:19
问题 Those two examples give the same results, but different syntax tells me that are executed in a completely different way. Where is the difference? Which way should be preferred? 1st continents .SelectMany(continent => continent.Countries) .SelectMany(country => country.Cities) 2nd continents .SelectMany(continent => continent.Countries.SelectMany(country => country.Cities)) EDIT: Let's not talk about deferred executions of IEnumerable, because it is not important here. Please assume that each

SelectMany nested in parent's lambda vs SelectMany after SelectMany

由 末鹿安然 提交于 2021-02-11 13:25:30
问题 Those two examples give the same results, but different syntax tells me that are executed in a completely different way. Where is the difference? Which way should be preferred? 1st continents .SelectMany(continent => continent.Countries) .SelectMany(country => country.Cities) 2nd continents .SelectMany(continent => continent.Countries.SelectMany(country => country.Cities)) EDIT: Let's not talk about deferred executions of IEnumerable, because it is not important here. Please assume that each

Execution flow of a linq query

由 ￡可爱￡侵袭症＋ 提交于 2021-02-11 12:49:41
问题 case1 var numbers = new List<int>(); numbers.Add (1); IEnumerable<int> query = numbers.Select (n => n * 10); // Build query numbers.Add (2); //Use or execute query case2 var numbers = new List<int>() { 1, 2 }; numbers.Add(4); List<int> query = numbers .Select (n => n * 10) .ToList(); // Executes immediately into a List<int> numbers.Add(3); numbers.Clear(); //Use or execute query Why in the first case query contains both 1,2 In second case query contains only 1,2,4 but not 3,is it because we
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