C# join子句中某个表达式的类型不正确。调用'；时类型推断失败；加入'；

我目前正在尝试将一列从另一个表连接到现有表，并将其返回到MVC4中的视图

var q = from accCo in db.AccControls
        join accCom in db.AccCompanies
        on new { accCo.ControlCode } equals
        new { ControlCode = accCom.Code }

        where accCo.ControlCode == Request.QueryString["ControlCode"]
        orderby accCom.Code
        select new Combined{ AccControls = accCo, AccCompoanies = accCom };

return View(q);

但是我得到了上面的错误

试试看

var q = from accCom in db.AccCompanies
        let accCo = db.AccControls.FirstOrDefault(x => x.ControlCode == accCom.Code)
        where accCo.ControlCode == Request.QueryString["ControlCode"]
              && accCo != null
        orderby accCom.Code
        select new Combined { AccControls = accCo, AccCompoanies = accCom };

---

如果您加入的任何列都可以为null，则需要将其更改为.Value

on new { accCo.ControlCode } equals
            new { ControlCode = accCom.Code.Value }

或者你可以用这个

var q = from accCo in db.AccControls
    join accCom in db.AccCompanies
    where accCom.Code != null
    on new { accCo.ControlCode } equals
    new { ControlCode = accCom.Code }

    where accCo.ControlCode == Request.QueryString["ControlCode"]
    orderby accCom.Code
    select new Combined{ AccControls = accCo, AccCompoanies = accCom };

---

也许是

请求。QueryString[“ControlCode”]

尝试在linq qry外贴花，如下所示：

var code = Request.QueryString["ControlCode"];

var q = from accCo in db.AccControls
        join accCom in db.AccCompanies
        on new { accCo.ControlCode } equals
        new { ControlCode = accCom.Code }

        where accCo.ControlCode == code //EDIT
        orderby accCom.Code
        select new Combined{ AccControls = accCo, AccCompoanies = accCom };

---

accCo.ControlCode或accCom.Code是否可以为空？您是否尝试过在accCo.ControlCode==accom.Code上使用=？或者这些类型不同？accCo.ControlCode不允许空值。accCom.Code确实允许为空。请看我下面的答案：）当然，它们应该来自同一类型才能做到这一点。为什么这会有帮助？这样就消除了连接错误，谢谢，但现在我得到了：String不包含Value的定义，并且找不到接受String类型的第一个参数的扩展方法值，那么您不应该使用.Value，而是筛选空记录