Jelle Druyts

Pop quiz: what do you think would be the output of the following code?

Dim i As Integer
For i = 0 To 4
    Dim s As String
    s = s & "X"
    Console.WriteLine(s)
Next

Quite possibly, you would expect to see 5 lines each containing a single X, since the variable is restricted to its scope: the For block. Since it cannot be used outside the block, you would expect it to be reinitialized each time the block is executed. Unfortunately, what we actually get is

X
XX
XXX
XXXX
XXXXX

This is because although the scope of the variable s is limited to the For block (which means it cannot be used outside it), the variable actually starts to exist at the start of the method. It's only initialized to its default value once, upon entry into the method. See Local Variables and Local Declaration Statements in the Visual Basic .NET Language Specification for more information on this.

This means you have to explicitly initialize the variable to Nothing (or to your own default value):

Dim i As Integer
For i = 0 To 4
    Dim s As String = Nothing
    s = s & "X"
    Console.WriteLine(s)
Next

If you would try something similar in C#, the compiler will stop with an error (not a warning): "Use of unassigned local variable 's'" which makes it impossible to make mistakes about this.