Question

也许是一个简单的问题，但无法轻易找到它，请原谅我=）我尝试选择多个列。我使用的陈述是：

var dataset2 = from recordset in entities.processlists 
               where recordset.ProcessName == processname 
               select recordset.ServerName, recordset.ProcessID, recordset.Username;

显然，这甚至都没有编译。什么是正确的语法？我也试过基于方法，甚至强硬这个语法似乎是正确的，当访问它时抛出'无法强制转换类型'匿名类型'来键入'AIM.PInfo'。 LINQ to Entities仅支持转换EDM原语或枚举类型。异常。

有什么想法吗？

var dataset = entities.processlists
             .Where(x => x.environmentID == environmentid && x.ProcessName == processname && x.RemoteIP == remoteip && x.CommandLine == commandlinepart)
             .Select(x => new { x.ServerName, x.ProcessID, x.Username })
             .Cast<PInfo>().ToList();

Answer 1

实际上，编译器不知道如何将此匿名类型（new { x.ServerName, x.ProcessID, x.Username }部分）转换为PInfo对象。

var dataset = entities.processlists
    .Where(x => x.environmentID == environmentid && x.ProcessName == processname && x.RemoteIP == remoteip && x.CommandLine == commandlinepart)
    .Select(x => new { x.ServerName, x.ProcessID, x.Username }).ToList();

这为您提供了之后可以使用的对象（匿名类型）的列表，但是您无法将其返回或传递给另一个方法。

如果你的PInfo对象具有正确的属性，它可以是这样的：

var dataset = entities.processlists
    .Where(x => x.environmentID == environmentid && x.ProcessName == processname && x.RemoteIP == remoteip && x.CommandLine == commandlinepart)
    .Select(x => new PInfo 
                 { 
                      ServerName = x.ServerName, 
                      ProcessID = x.ProcessID, 
                      UserName = x.Username 
                 }).ToList();

假设PInfo至少有这三个属性。

这两个查询都允许您仅提取所需的列，但使用现有类型（如第二个查询中）允许您将此数据发送到应用的其他部分。

Answer 2

您可以选择匿名类型，例如

var dataset2 = 
    (from recordset in entities.processlists 
    where recordset.ProcessName == processname 
    select new
    {
        serverName = recordset.ServerName,
        processId = recordset.ProcessID, 
        username = recordset.Username
    }).ToList();

或者您可以创建一个代表您选择的新类，例如

public class MyDataSet
{
    public string ServerName { get; set; }
    public string ProcessId { get; set; }
    public string Username { get; set; }
}

然后您可以执行以下操作

 var dataset2 = 
    (from recordset in entities.processlists 
    where recordset.ProcessName == processname 
    select new MyDataSet
    {
        ServerName = recordset.ServerName,
        ProcessId = recordset.ProcessID, 
        Username = recordset.Username
    }).ToList();

Answer 3

为什么不在.Select：

中创建新对象

.Select(x => new PInfo{ 
    ServerName = x.ServerName, 
    ProcessID = x.ProcessID, 
    UserName = x.Username }).ToList();

Answer 4

您要么选择匿名类型：

var dataset2 = from recordset 
               in entities.processlists 
               where recordset.ProcessName == processname 
               select new 
               {
                recordset.ServerName, 
                recordset.ProcessID, 
                recordset.Username
               };

但是你不能把它转换成另一种类型，所以我猜你想要这样的东西：

var dataset2 = from recordset 
               in entities.processlists 
               where recordset.ProcessName == processname 

               // Select new concrete type
               select new PInfo
               {
                ServerName = recordset.ServerName, 
                ProcessID = recordset.ProcessID, 
                Username = recordset.Username
               };

Answer 5

var test_obj = from d in repository.DbPricing
join d1 in repository.DbOfficeProducts on d.OfficeProductId equals d1.Id
join d2 in repository.DbOfficeProductDetails on d1.ProductDetailsId equals d2.Id
    select new
    {
    PricingId = d.Id,
    LetterColor = d2.LetterColor,
    LetterPaperWeight = d2.LetterPaperWeight
    };


http://www.cybertechquestions.com/select-across-multiple-tables-in-entity-framework-resulting-in-a-generic-iqueryable_222801.html

Answer 6

获取指定类型的数据是正确的方法：

var dataset = entities.processlists
         .Where(x => x.environmentID == environmentid && x.ProcessName == processname && x.RemoteIP == remoteip && x.CommandLine == commandlinepart)
         .Select(x => new { x.ServerName, x.ProcessID, x.Username })
         .ToList() /// To get data from database
         .Select(x => new PInfo()
              { 
                   ServerName = x.ServerName, 
                   ProcessID = x.ProcessID, 
                   Username = x.Username 
              });

有关更多信息，请参阅： The entity cannot be constructed in a LINQ to Entities query

Answer 7

以下是代码示例：

var dataset = entities.processlists
    .Where(x => x.environmentID == environmentid && x.ProcessName == processname && x.RemoteIP == remoteip && x.CommandLine == commandlinepart)
    .Select(x => new PInfo 
                 { 
                      ServerName = x.ServerName, 
                      ProcessID = x.ProcessID, 
                      UserName = x.Username 
                 }) AsEnumerable().
               Select(y => new PInfo
               {
                   ServerName = y.ServerName,
                   ProcessID = y.ProcessID,
                   UserName = y.UserName 
               }).ToList();

使用Entity Framework选择多个列

7 个答案: