标签: linq-query-syntax

在阅读Jon Skeet的" Odd query expressions "后,我尝试了下面的代码.我期望最后的LINQ查询转换为int query = proxy.Where(x => x).Select(x => x);不编译因为Where返回一个int.代码编译并将"Where(x => x)"打印到屏幕并将查询设置为2.从不调用Select,但需要在那里编译代码.怎么了？

using System;
using System.Linq.Expressions;

public class LinqProxy
{
    public Func<Expression<Func<string,string>>,int> Select { get; set; }
    public Func<Expression<Func<string,string>>,int> Where { get; set; }
}

class Test
{
    static void Main()
    {
        LinqProxy proxy = new LinqProxy();

        proxy.Select = exp => 
        { 
            Console.WriteLine("Select({0})", exp);
            return 1;
        };
        proxy.Where = exp => 
        { 
            Console.WriteLine("Where({0})", exp);
            return 2;
        };

        int query = from …

你会如何使用LINQ查询语法编写这个完全相同的查询？

var q2 = list.GroupBy(x => x.GroupId)
             .Select(g => g
                 .OrderByDescending(x => x.Date)
                 .FirstOrDefault());

我虽然这应该工作,但它没有:

var q1 = from x in list
         group x by x.GroupId into g
         from y in g
         orderby y.Date descending
         select g.FirstOrDefault();

如果你想玩它,这是一个测试程序:

public class MyClass
{
    public int Id { get; set; }
    public string GroupId { get; set; }
    public DateTime Date { get; set; }

    public override bool Equals(object obj)
    {
        var item = obj as MyClass;
        return item == …

我的数据库中有一个多对多关系设置，如下所示：

User
-------
Id (PK, Identity)
First
Last
...various other fields

Skill
-------
Id (PK, Identity)
Description

UserSkill
-----------
UserId (PK, FK on User.Id)
SkillId (PK, FK On Skill.Id)

当我在 DbContext 上运行此 LINQ 查询时：

from u in Users 
from s in u.Skills 
where s.Id == 5 
select new 
{
    u.Id,
    s.Description
})

生成的 SQL 包含所有内部联接，这就是我想要的：

SELECT 
[Extent1].[UserId] AS [UserId], 
[Extent2].[Description] AS [Description]
FROM  [dbo].[UserSkill] AS [Extent1]
INNER JOIN [dbo].[Skill] AS [Extent2] ON [Extent1].[SkillId] = [Extent2].[Id]
WHERE 5 = [Extent2].[Id]

但是，当我添加一个简单的额外 …

我有一个场景，我需要加入两个表：

一种

|---------------------|------------------|
|          ID         |        Name      |
|---------------------|------------------|
|          1          |        John      |
|---------------------|------------------|
|          2          |        Matt      |
|---------------------|------------------|
|          3          |        Emma      |
|---------------------|------------------|

乙

|---------------------|------------------|
|          ID         |        Text      |
|---------------------|------------------|
|          1          |  blah blah John  |
|---------------------|------------------|
|          2          | this is some data|
|---------------------|------------------|
|          3          | My name is Jeff  |
|---------------------|------------------|

我需要使用 LINQ 的查询语法来连接这两个表。

左表需要是表A。

虽然我需要根据“文本”列是否包含表 A 中“名称”列中的文本来加入。

代码应如下所示：

var result = from ta in A
             join tb in B on …

下面的代码提供了两种方法,它们生成总和小于100的整数对,并且它们根据它们与(0,0)的距离按降序排列.

    //approach 1
    private static IEnumerable<Tuple<int,int>>  ProduceIndices3()
    {
        var storage = new List<Tuple<int, int>>();
        for (int x = 0; x < 100; x++)
        {
            for (int y = 0; y < 100; y++)
            {
                if (x + y < 100)
                    storage.Add(Tuple.Create(x, y));
            }
        }
        storage.Sort((p1,p2) =>
           (p2.Item1 * p2.Item1 + 
           p2.Item2 * p2.Item2).CompareTo(
           p1.Item1 * p1.Item1 +
           p1.Item2 * p1.Item2));
        return storage;
    }

    //approach 2
    private static IEnumerable<Tuple<int, int>> QueryIndices3()
    {
        return from x in Enumerable.Range(0, 100)
               from y in …

I'm currently trying to make a Web Api with EF Core, and i'm running into some problems joining the tables i've got together. I'm working with the following Database Diagram:

And the data i'm currently getting back from my API looks this this:

[  
   {  
      "postId":1,
      "postDate":"2018-10-21T21:56:43.9838536",
      "content":"First entry in posts!",
      "user":{  
         "userId":1,
         "creationDate":"2018-10-21T21:56:36.3539549",
         "name":"Hansel"
      },
      "comments":[  
         {  
            "commentId":1,
            "postDate":"0001-01-01T00:00:00",
            "content":"Nice!",
            "user":null
         },
         {  
            "commentId":2,
            "postDate":"0001-01-01T00:00:00",
            "content":"Cool, here's another comment",
            "user":null
         },
         {  
            "commentId":3,
            "postDate":"0001-01-01T00:00:00",
            "content":"and the last one for …

我有这个非常奇怪的问题,我无法理解.也许有人可以指出我做错了什么.

基本上,我只是尝试使用Linq搜索项目到Sitecore.

所以,我的班级看起来像(我也在使用玻璃)

[SitecoreType(TemplateId = "{TEMPLATE_GIUD}")]
public class MyMappedClass : SharedFieldClass
{
    [SitecoreField(FieldName = "mylist")]
    public virtual IEnumerable<SharedFieldClass> MyMultilistField { get; set; }


    [SitecoreField(FieldName = "field1")]
    [IndexField("field1")]
    public virtual MyKeyValue field1 { get; set; }    
}
[SitecoreType]
public class MyKeyValue
{
    public virtual Sitecore.Data.ID Id {get;set;}    
    public virtual string MyValue{get;set;}
}

因此,当我执行以下查询时,它可以正常工作.

    var results = context.GetQueryable<SearchResultItem>()
                  .Where(c => ((string)c["field1"]) == "{GUID}").GetResults();

但是,当我执行以下操作时,它返回0结果.

List<MyMappedClass> results = context.GetQueryable<MyMappedClass>()
                              .Where(c => c.field1.MyValue == "{GUID}").ToList();

我看过这个链接.我已经按照此处描述的第二个过程使用Glass来使用Sitecore7 Search("SharedFieldClass"包含所有基本索引字段).

这是一个非常明显的场景,我相信很多人已经做过了,我在做一些愚蠢的事情.

提前致谢.

/ …

我有一个查询，它检索一堆商店的销售额，并将它们作为列表返回。在我的报告请求屏幕上，用户可以按多个 id 进行过滤 - ShopOwnerId、ShopRegionId、ShopTypeCode 等。

我的查询的结构是获取选定日期之间的所有销售额，然后根据选择进行过滤。这显然是非常低效的。:

private List<Sales> GetFilteredListOfSales(Request reportreq)
{
    ModelContainer ctn = new ModelContainer();
    List<ShopSale> shopsSales = new List<shopsale>();

    // If no filters are selected
    //
    if (reportreq.RegionalId == null && reportreq.OwnerId == null && reportreq.ShopTypeCode == null)
    {
        shopsSales = (from sale in ctn.ShopSales
                    where sale.DateSold >= reportreq.FromDate && sale.DateSold <= reportreq.ToDate
                    select sale).ToList();
    }


    // If the regional ID has a value...
    // 
    if (reportreq.RegionalId.HasValue)
    {
        shopsSales = (from sale in ctn.ShopSales
                  where sale.Shop.Owner.RegionalId == …

我知道这被问了很多次,我在网上搜索了大部分解决方案,但似乎没有任何东西可以帮我.我有一个这种结构的表:

ID | ScheduleId | Filename | Description
 1 |     10     |          |  ....
 2 |     10     | test.txt |  .....

我希望Filename通过传递ScheduleId 来获取最后一个非空(例如,在这种情况下获取"test.txt").

我尝试了很多东西,似乎没有任何东西让我得到文件名.这是最后一个:

var tempFileName = objContext.SchedulesAndFiles
                           .Where(x => x.ScheduleId == scheduleId)
                           .OrderByDescending(x => x.ScheduleId)
                           .Take(1).Select(x => x.Filename);

这不起作用,虽然我明白为什么它不:

var tempFileName = from e in objContext.SchedulesAndFiles
                   where e.ScheduleId == scheduleId 
                   orderby e.ScheduleId descending
                   select e.Filename;

调用.Last()或.LastOrDefault()抛出异常(The query operator 'LastOrDefault' is not supported.)

我有一个任务,我必须加入两个相同类型的列表(客户).他们有类似的条目,我必须避免重复.

这是我的客户类:

class Customer
{
  private String _fName, _lName;
  private int _age, _cusIndex;
  private float _expenses;

  public Customer(String fName, String lName, int age, float expenses, int cusIndex)
  {
    this._fName = fName;
    this._lName = lName;
    this._age = age;
    this._expenses = expenses;
    this._cusIndex = cusIndex;
  }
}

所以我有两个List<Customer>名字customers1和customers2.我需要在不使用Collections方法的情况下加入这两个方法(比如customer1.Union(customer2).ToList();使用Linq查询).

这是我写的Linq查询:

var joined = (from c1 in customers1
              join c2 in customers2
              on c1.CusIndex equals c2.CusIndex
              select new {c1, c2});

但是这给了我出现在两个列表上的成员.但我需要所有人,而不是重复.有什么办法吗？