InvalidCastException in Linq to entities query - vb.net

Question

I am making an application for my department at work. I have run into a big problem.
I am trying to get the primary key of a name selected on a ListBox control. This ListBox control is populated from a query from the database when the form loads. Below is the query used to populate the ListBox control.
Dim examinationOfficer As New ExamOfficerPayEntities
Dim findOfficer = From officer In examinationOfficer.Exams_Officer
Order By officer.First_Name Ascending
Select officer.First_Name.ToUpper & " " & officer.Last_Name.ToUpper
lstFullname.DataSource = findOfficer.ToList
lstFullname.DisplayMember = "FullName"
The query works fine and the ListBox is populated as expected. The problem is I am trying to get the value of the primary key of the selected ListBox item. The primary key value I am trying to retrieve is an integer value and it is set as an identity.
Below is a summary of my database.
Table Exam_Officer:
Officer_ID as int which is the primary key and is also set as an identity column
First_Name as nvarchar
last_Name as nvarchar
One other Table is Result_Submitted,
its columns are:
sn as integer primary key and an identity column
Officer_ID as an int and is a foreign key to the Table Exam_Officer
Result_Submitted as nvarchar
Outstanding_Result as nvarchar
This is the code I am trying to use to get the officer_ID from the SelectedItem property of the ListBox control of a Windows form.
Dim selectedOfficer As Exams_Officer = DirectCast(lstFullname.SelectedItem, Exams_Officer)
Dim selectedOfficerID As Integer = selectedOfficer.Officer_ID
The code dosen't work it throws an error on the first line (directcast method)
System.InvalidCastException was unhandled
Message=Unable to cast object of type 'System.String' to type 'EXAMINATION_OFFICER_PAYMENT.Exams_Officer'.
Source=EXAMINATION OFFICER PAYMENT
I am confused and don't know what else to do. Any help is appreciated.

Answer 1

To be honest, I'm surprised it's even displaying at the moment as FullName isn't a property of String. Look at your code:
Dim examinationOfficer As New ExamOfficerPayEntities
Dim findOfficer = From officer In examinationOfficer.Exams_Officer
Order By officer.First_Name Ascending
Select officer.First_Name.ToUpper & " " & officer.Last_Name.ToUpper
You're selecting the full name - so the result of your code is just the names, not the original officers. I suspect you want:
Dim examinationOfficer As New ExamOfficerPayEntities
Dim findOfficer = From officer In examinationOfficer.Exams_Officer
Order By officer.First_Name Ascending
lstFullname.DataSource = findOfficer.ToList
lstFullname.DisplayMember = "FullName"
At that point, you've got a list of officers rather than a list of strings. This is assuming FullName is a property of your officer type though. If it's not, you might want to project to a list where you do the string concatenation in one property, but keep the original officer in another property. For example:
Dim examinationOfficer As New ExamOfficerPayEntities
Dim findOfficer = From officer In examinationOfficer.Exams_Officer
Order By officer.First_Name Ascending
Select New With {
.FullName = officer.First_Name.ToUpper & " " _ & officer.Last_Name.ToUpper,
.Officer = officer
}
lstFullname.DataSource = findOfficer.ToList
lstFullname.DisplayMember = "FullName"
Then later, you'd need to extract the value of the Officer property to get at the officer object itself.

Answer 2

In general the way to do this is to add Officers to your ListBox and override Officer's ToString() method to return FullName, or bind and set DisplayMember to Fullname. Like Jon Skeet, I'm not sure why the code you've posted is working...
Then you can do
Dim selectedOfficer As Exams_Officer = DirectCast(lstFullname.SelectedItem, Exams_Officer)
and Exams_Officer.OfficeID will be okay.
At the moment you are casting a string to an Officer, the above casts an Object to an Officer, which is more likely to work.
Bear in mind however that you need to instance all your officers and that they'll be in memory. So if had a lot of and/or Officer was a hefty object, performance might start to suffer.
The simplest option at that point is a small class / structure with just ID and name in it.