Question

For example, one could avoid a possible run-time null exception in the case below by instantiating s as it is declared. For example, Dim s as String = "" Is there a reason for us to allow the possibility of a run-time exception? Perhaps it does not make sense for s to be ""? This is not a homework question. I am just curious.

Public Function X() As String 
    Dim s as String 
    Dim sMethod As String = X 
    Try 
        Dim vowels() As String = {"a", "e", "i", "o", "u"}
        For Each vowel As String In vowels
            s = "0"
        Next
    Catch ex As Exception 
        Throw System.Exception(ex)
    End Try
    Return s 
End Function

• Editing to throw exception

Visual Studio seems to be smart enough to tell us that “A null reference exception could result at runtime.” which means some capable, professional developers ignored this warning. This leads me to think that must be for a reason. The developers have since moved on though so I can’t ask them. It might be something fundamental that I can’t understand. Please help me understand this if you will. (I can’t be the only person wondering about this.)

Answer 1

Suppose you don’t initialize your String variable. And there goes some processing. In the end you expect get a definitive value, which you return via a Return statement. If, for some reason, you forgot to account for a possible execution path, VS will warn you about possible null-reference exception. Then you may think to yourself: “oh yeah, how did I forget that?”

For example, you have a Function that converts a Integer into a word (limited practical use, so consider it just for demo purposes):

Function ConvertIntToWord(a As Integer) As String
  Dim retString As String
  If a = 0
    retString = "Zero  "
  ElseIf a = 1
    retString = "One  " 
  End If
  Return retString
End Function

In the above lines, you were unlucky to forget about the Else path. Then suppose you want to run further processing on a resulting string:

ConvertIntToWord(5).TrimEnd

With 5 passed as an argument, you will get an exception at this point. If you instead listened to VS, you would have noticed a warning – underlined retString in Return retString line. So you could fix this problem without running your code.

Now suppose you initialize Dim retString As String = "". You will no longer see the warning, and have to debug your program to solve this issue. Depending on the complexity of your program, it may take much more effort.

Bottom line – it is always better to spot problems in compile time, rather than run-time.

EDIT: Forgetting to initialize a variable can be intentional, for example why using TryGetValue. For this method, value is passed uninitialized, according to documentation. However, VS does not know about this, so it will continue to show a warning, until you explicitly assign it to Nothing:

Dim retString As String = Nothing

So this is a way to tell VS that yes, you know it is uninitialized, but for this particular scenario, it is intentional.