Compiler warning: null reference exception

Question

I have the following code in Visual Studio 2005.

    Dim OutFile As System.IO.StreamWriter
    Try
        OutFile = New System.IO.StreamWriter(Filename)
            


        

Answer 1

The accepted answer is correct, of course, but it doesn't explain why or when explicit initialization may matter.

VB.NET usually assigns a default value (0 or Nothing) when a variable is declared, but there are corner cases where it doesn't.

Consider this simple console application:

Sub Main()
    For i As Integer = 1 To 5
        Dim number As Integer
        If i = 3 Then number = 3

        Console.Write(number)
    Next
End Sub

What's the output look like? You might expect that number gets set to 0 for every iteration of the loop, and it only gets set to 3 on the third iteration of the loop. Then for the fourth and fifth iteration, it'd be 0 again. So the output is 00300, right? Not so. The output of this code is actually

00333

That's because in VB.NET, the lifetime of a variable declared in a loop is for the whole loop, not for one iteration of the loop (Not what you'd expect, huh?). But if you explicitly set the value of number to 0 at its declaration, like so

Dim number As Integer = 0

then the output looks like

00300

So it's usually safe to assume VB.NET will set the default value when you Dim a variable, but it's always safest to set it explicitly to 0 or Nothing to get the expected behavior.

Answer 2

Its a question of scope, the initialisation of the outfile object is happening in a block of code not visible to the fianlly block.

Answer 3

Dim OutFile As System.IO.StreamWriter
OutFile = Nothing
Try
    OutFile = New System.IO.StreamWriter(Filename)
   // Do stuff with OutFile
Catch Ex As Exception
   // Handle Exception
Finally
   If OutFile IsNot Nothing Then OutFile.Close()
End Try

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