How to throw an exception in BLL when no data found in DAL - asp.net

Question

I am trying to throw an exception in my BLL when there is no corresponding carID for the license plate number I've entered in a text box.
My DAL looks like this:
Public Class DALCar
Private dc As New cars_modelDataContext
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim result = (From car In dc.Cars
Where car.License_Plate = licensePlate_input
Select car.License_Plate).Single
Return result
End Function
End Class
And this is my BLL:
Public Class BLLCar
Private DALcar As New DALCar
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim carID As String = DALcar.getCarIdByLicensePlate(chassisNo_input)
End Function
End Class
So when there is no carID with this specific license plate an exception is throwed in my DAL, but how can I throw this exception in my BLL instead of in my DAL?

Answer 1

Use FirstOrDefault instead of Single
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim result = (From car In dc.Cars
Where car.License_Plate = licensePlate_input
Select car.License_Plate).FirstOrDefault
Return result
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim carID As String = DALcar.getCarIdByLicensePlate(chassisNo_input)
If carID = Nothing Then
Throw New Exception(String.Format("Can't find car id for chassisNo : {0}", chassisNo_input))
End If
End Function

Answer 2

Because you're using Enumerable.Single in your LINQ expression. It throws an exception if there is more than one element in the sequence or if the sequence is empty.
If you can assume the sequence will always contains 0 or 1 element then you can replace Single with FirstOrDefault (see later for more on this). It'll return the first element in the sequence or Nothing if sequence is empty.
In this case you can check for Nothing in your BLL and throw the appropriate exception there.
Like this in your DAL:
Public Class DALCar
Private dc As New cars_modelDataContext
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim result = (From car In dc.Cars
Where car.License_Plate = licensePlate_input
Select car.License_Plate).FirstOrDefault
Return result
End Function
End Class
And this in your BLL:
Public Class BLLCar
Private DALcar As New DALCar
Public Function getCarIdByLicenePlate(ByVal licensePlate_input As String) As String
Dim carID As String = DALcar.getCarIdByLicensePlate(chassisNo_input)
If carId = Nothing Then
Throw New ArgumentException("There is no match.")
End If
End Function
End Class
If your query may returns more than one element than you have to consider if this is an error or not. If it's allowed and you want to process (return) the first one then go on with FirstOrDefault. If it's an error then you should return an enumeration from your DAL and to check the number of items in your BLL (otherwise, using Single, you'll still throw inside DAL).