如何实现IEnumerable以便在LinQ查询中使用自定义类？ [英] How do I implement IEnumerable so that I can use a custom class in a LinQ query?

问题描述

我使用Entity Framework连接到SQL Server数据库（Northwind） （2008R2）。然后我使用LinQ查询使用Customers表中的3列数据填充Asp.Net Gridview控件。 这一切都可以在没有和使用Lambdas
的情况下工作但是........

I am connecting to a sql server Database (Northwind) (2008R2) using Entity Framework. Then I am using a LinQ Query to populate an Asp.Net Gridview control with 3 columns of data from the Customers table.  This all works without and with Lambdas but ........

 我在没有和使用Lambdas的情况下尝试过这个LinQ查询的几个版本。 在我的阅读中，似乎如果我在查询中使用自定义类对象，这为我提供了更多功能（我是否正确理解了该部分？）。  
我在（网络表单）代码后面添加了一个（普通）类对象，其中包含我在查询中使用的3列的3个属性。 我可以在不使用Lambdas的LinQ查询中使用此类对象。 但是，当我尝试在 DOES 的查询中使用类对象
时，使用Lambda我收到以下错误消息：

 I have tried a few versions of this LinQ query without and with Lambdas.  In my readings it appears that if I use a custom class object in the query this provides me with more versatility (am I understanding that part correctly?).   I added a (plain) class object to the (web form) code behind with 3 properties for the 3 columns I am using in the query.  I can use this class object in the LinQ query which does not use the Lambdas.  But when I try to use the class object in the query that DOES use the Lambda I receive the following error message:

>>无法使用集合初始值设定项初始化GridviewProject.CustomerSet，因为它没有实现System.Collections.IEnumerable<<

>>Cannot initialize type GridviewProject.CustomerSet with a collection initializer because it does not implement System.Collections.IEnumerable<<

我尝试在类上实现IEnumerable，但是然后我收到没有添加功能的错误消息。  LinQ查询如下：

I tried implementing IEnumerable on the class, but then I get the error message that there is no Add function.  The LinQ queries are below:

- 没有Lambdas的LinQ查询

--LinQ query Without the Lambdas

private void GetCustomersPageWise(int pageIndex)
{
   //--LinQ query without the Lamda -- I can use the CustomerSet class here
   var dataset1 = (from rs in db.Customers
                  select new CustomerSet
                  {
                      CustomerId = rs.CustomerID,
                      ContactName = rs.ContactName,
                      CompanyName = rs.CompanyName
                  }).ToList();

   //--LinQ query with the Lamda -- I get the error message here if I try using
   //--CustomerSet class in the query -- how to implement IEnumerable?

// - 如果我不使用CustomerSet类，则此Lambda查询正常工作
var dataset2 = db.Customers
.Select（x => new {x.CustomerID， x.ContactName，x.CompanyName}）。ToList（）;

GridView1.DataSource = dataset1;
GridView1.DataBind（）;
}

公共类CustomerSet
{
public string CustomerId {get;组; }
公共字符串ContactName {get;组; }
public string CompanyName {get;组; }
}

//--this Lambda query works OK if I don't use CustomerSet class var dataset2 = db.Customers .Select(x => new { x.CustomerID, x.ContactName, x.CompanyName }).ToList(); GridView1.DataSource = dataset1; GridView1.DataBind(); } public class CustomerSet { public string CustomerId { get; set; } public string ContactName { get; set; } public string CompanyName { get; set; } }

我问我的问题主要是了解如何在这种情况下实施IEnumerable。 但我也想知道在LinQ查询中使用像CustomerSet这样的类有什么好处？ 有优势吗？ 这个优势是什么
以及我在项目中如何/在哪里使用这个优势？

I ask my question mainly to understand how to go about implement IEnumerable in this scenario.  But I am also wondering what is the advantage of using a class like CustomerSet in the LinQ Query?  Is there an advantage?  What is this advantage and how/where would I use this advantage in my project?

如果重要，这里是Gridview控件的标记：

In case it matters, here is the markup for the Gridview control:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" PageSize="10" AllowPaging="true"   
                                                       OnPageIndexChanging="GridView1_PageIndexChanging">
   <Columns>
      <asp:BoundField HeaderText="CustomerId" DataField="CustomerId" />
      <asp:BoundField HeaderText="ContactName" DataField="ContactName" />
      <asp:BoundField HeaderText="CompanyName" DataField="CompanyName" />
   </Columns>
</asp:GridView>

任何建议赞赏

Any suggestions appreciated

Rich P

推荐答案

试试这个：

var dataset2 = db
                 .Customers
                 .Select(x => new CustomerSet { CustomerId = x.CustomerID, ContactName = x.ContactName, CompanyName = x.CompanyName })
                 .ToList();
 

您还可以编写'new CustomerSet（x）'或'new CustomerSet（x .CustomerID，x.ContactName，x.CompanyName）'如果为
CustomerSet 类定义相应的构造函数。

You can also write 'new CustomerSet( x )' or 'new CustomerSet( x.CustomerID, x.ContactName, x.CompanyName )' if you define the corresponding constructors for CustomerSet class.

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