Question

我有一个表名TestNotifications，它有一个CompanyID和一个TestCompanyID。这些ID链接到具有companyName列的Companies表。我需要获得公司和testCompany的companyName。下面的代码不起作用，我得到一个不能隐式转换错误。任何帮助表示赞赏。

testNotifications = from t in db.CT_TestNotifications
    join c in db.CT_Companies on t.CompanyID equals c.CompanyID
    join tc in db.CT_Companies on t.TestCompanyID equals tc.CompanyID 
    select new
    {
        t.TestNotificationID,
        c.CompanyName,
        //tc.CompanyName
        TestCompanyName = tc.CompanyName
    };

这是错误：

Cannot implicitly convert type 'System.Linq.IQueryable<AnonymousType#1>' to
'System.Linq.IQueryable<CT_TestNotification>'. An explicit conversion exists 
(are you missing a cast?)

Answer 1

您投射的是匿名类型，但testNotifications期望CT_TestNotification。

尝试在CT_TestNotification中创建select的实例：

testNotifications = from t in db.CT_TestNotifications
    join c in db.CT_Companies on t.CompanyID equals c.CompanyID
    join tc in db.CT_Companies on t.TestCompanyID equals tc.CompanyID
    select new CT_TestNotification       // Here's the major difference
    {
        PropName = t.TestNotificationID, // PropName must be changed to the
        PropName = c.CompanyName,        // properties of your actual class
        //tc.CompanyName
        TestCompanyName = tc.CompanyName
    };

Answer 2

查看您的条件连接是否属于相同类型 - IE是t.CompanyID是int和c.CompanyID。

Answer 3

我怀疑问题在于您如何声明testNotifications。如果你做了类似下面的事情，你会看到错误：

IQueryable<CT_TestNotifications> testNotifications;

testNotifications = from t in db.CT_TestNotifications 
    join c in db.CT_Companies on t.CompanyID equals c.CompanyID 
    join tc in db.CT_Companies on t.TestCompanyID equals tc.CompanyID  
    select new 
    { 
        t.TestNotificationID, 
        c.CompanyName, 
        //tc.CompanyName 
        TestCompanyName = tc.CompanyName 
    };

如果您在开始时声明了testNotifications，然后继续查询，那么您也会遇到同样的问题，因为您尝试将值的类型从已知类型更改为匿名类型。：

var testNotifications = db.CT_TestNotifications;
    testNotifications = from t in testNotifications 
        join c in db.CT_Companies on t.CompanyID equals c.CompanyID 
        join tc in db.CT_Companies on t.TestCompanyID equals tc.CompanyID  
        select new 
        { 
            t.TestNotificationID, 
            c.CompanyName, 
            //tc.CompanyName 
            TestCompanyName = tc.CompanyName 
        };

由于您要投射匿名类型，因此需要使用类型推断而不是显式声明变量：

var testNotifications = from .. select ..;

LINQ在同一列上连接多个表

3 个答案: