Type Inference failed in a call to 'join' on nullable and non-nullable int

问题

In my Linq, I am trying to make an inner join to a nullable field. Employee and Department have a relation, Department may have an EmployeeID or may have a null. So what would be my join, if i want only the records that satisifed the inner join (no result for null EmployeeIDs):

var result = from emp in employees
             join dept in departments
             on new { Source = emp.EmployeeID }
             equals new { Source = dept.EmployeeID };

I am getting an exception:

The type of one of the expressions in the join clause is incorrect. Type Inference failed in a call to 'join'.

Thanks

回答1:

What if you reverse your join and put a little where in there?

var result = from department in departments
             where department.EmployeeID != null
             join employee in employees
             on department.EmployeeID.Value equals employee.EmployeeID
             select new { employee, department };

回答2:

To compare Int? and Int, append .Value to the nullable property:

var result = from emp in employees
             join dept in departments
             on new { Source = emp.EmployeeID }
             equals new { Source = dept.EmployeeID.Value };

回答3:

Check the type on emp.EmployeeID and dept.EmployeeID. You might be missing a cast if they are different.

something like:

on new { Source = emp.EmployeeID }
equals new { Source = **(int)**dept.EmployeeID };

Looks like emp.EmployeeID is of type int and dept.EmployeeID is of type nullable<int>.

回答4:

I had the same issue, where my charge_codes.CompanyId was nullable but my order_items.CompanyId was NOT nullable.

So I had to get my charge codes into their own ananomous type and make it not be nullable.

var chargeCodes = from s in db.Charge_Codes
where s.CompanyID != null
select new { CompanyID = (int)s.CompanyID, 
             Charge_CodeID = s.Charge_CodeID, 
             Revenue_Code_Id = (int)s.Revenue_CodeID, };



//now my chargeCodes contains an anonymous with a non nullable CompanyID and 
//a non nullable Revenue_CodeID 

//use chargeCodes here
var query = from oi in db.Order_Items
join cc in chargeCodes on 
new {oi.CompanyID, oi.Charge_CodeID} equals new {cc.CompanyID, cc.Charge_CodeID}

回答5:

Found a useful answer from another link at https://social.msdn.microsoft.com/Forums/en-US/bf98ec7a-cb80-4901-8eb2-3aa6636a4fde/linq-join-error-the-type-of-one-of-the-expressions-in-the-join-clause-is-incorrect-type-inference?forum=linqprojectgeneral

To join multi-valued keys you need to construct an anonymous typ on both sides of the 'equals' that is the same type. The anonymous type initializer expression infers both type and name of members from the expression you supply. In your case the names of the members are different so the types end up being different so C# cannot figure out the common type between the two.

on new { VC.Make, VC.Model } equals new { MD.MakeID, MD.RefNum }

should be

on new { VC.Make, CV.Model } equals new { Make = MD.MakeID, Model = MD.RefNum }

Using the name = value syntax in the initializer you can specify the name the compiler uses when creating the type. If all members types & names are the same then the anonymous types are the same type.

回答6:

In my scenario I had this error on a join using multiple columns. The property names were different and one of them was also nullable. All I did was creating a name to those properties and adding the ".Value" on the nullable value so the LINQ Join could correctly associate those properties.

var query = from y in Context.Table1
        join y in Context.Table2 on new { Field1 = x.Field1.Value, Field2 = x.Field2 }
        equals new { Field1 = y.Field1DiffName, Field2 = y.Field2 }

I hope it helps whoever is facing this issue.

来源：https://stackoverflow.com/questions/7345039/type-inference-failed-in-a-call-to-join-on-nullable-and-non-nullable-int

标签

linq

entity-framework

nullable

non-nullable