How does the DLookup work in this instance? - ms-access

Question

New user here :)
I got a question about DLookup in the below code. The code is suppose to only allow a selected user to open a form. The user is in the tbl and with the On Error Resume Next the code is working, allowing the user to access the application, however when I comment out the On Error I get the Run-time error '13': Type Mismatch. I was hoping for someone to explain why does it fail with On Error commented out. I understand that If statement requires a condition to evaluate, so how does getting the username allow the application to open the form?
Private Sub Form_Open(Cancel As Integer)
On Error Resume Next
If DLookup("[CitrixID]", "tbl_UserAccess", "[CitrixID] = " & "'" & Environ("username") & "'") Then
Else
MsgBox "You do not have permission to access the application"
DoCmd.Close
End If
End Sub

Answer 1

That's really not the way to do this, but I'll explain what it does.
DLookUp looks up a value from a table. In your case, it's looking up CitrixID from the table CitrixID where CitrixID is equal to the username. It then returns that username as the condition for that If statement.
An If statement, however, expects the condition to be either True, False, Null, or a numeric value (0 = falsy, all other numerals are truthy) which counts as false, not a Windows username, that's why the error occurs.
If you use On Error Resume Next, you're actually jumping into that If statement if the DLookup returns a string, because that's the next thing. However, if it returns Null, because it can't find that username, that doesn't trigger an error, because Null is a valid value which is cast to false.
Some sample code to help you understand this:
Public Sub TestIfResumeNext()
If Null Then
Debug.Print Null
End If
On Error Resume Next
If "A" Then
Debug.Print "A"
End If
If 1 Then
Debug.Print 1
End If
If 0 Then
Debug.Print 0
End If
If -1 Then
Debug.Print -1
End If
End Sub
This will return "A" because of error capturing, 1 and -1 because they're truthy, but not Null because that's a valid falsy value, and not 0 for that same reason.
If you don't want to rely on this error trapping behavior, but want to keep the rest of the logic, you could simply replace DLookUp with DCount. Since 0 is falsy, you don't even need to check if it's 0 (but doing so is a good practice imho).
If DCount("[CitrixID]", "tbl_UserAccess", "[CitrixID] = " & "'" & Environ("username") & "'") Then
Or, with the comparison
If DCount("[CitrixID]", "tbl_UserAccess", "[CitrixID] = " & "'" & Environ("username") & "'") <> 0 Then
Note that you can now shorten it using If Not or If DCount = 0 and remove the Else

Answer 2

DLookup returns Null for "not found" so all you need is to check for this:
Private Sub Form_Open(Cancel As Integer)
If IsNull(DLookup("[CitrixID]", "tbl_UserAccess", "[CitrixID] = '" & Environ("username") & "'")) Then
MsgBox "You do not have permission to access the application."
DoCmd.Close
End If
End Sub