What is the scope of a block-level variable in VB.NET?

Question

Consider the following code:

For i As Integer = 0 To 10
   Dim str As String = str & " Kratika "
Next

When I attach the debugger and check the value of str at i = 10, I see the following:

 Kratika  Kratika  Kratika  Kratika  Kratika  Kratika  Kratika  Kratika  Kratika  Kratika  Kratika

Why is it concatenating the previous value? I would expect that the variable would be set to kratika each time, because each time it will declare a new String object, right?

score 4 · Accepted Answer · answered Feb 08 '13 at 05:49

This is expected behavior.

As you mention in a comment to another answer, it's probably easier to see using integers rather than string concatenation:

Sub Main()
  For i As Integer = 0 To 5
     Dim j As Integer = j + 1
     Console.WriteLine(j.ToString())
  Next
End Sub

The output is as follows:

To learn why, we turn to the documentation, specifically the section on scope. The j variable that you've declared is at block scope, because it is declared inside of the For block. However, variables that are declared at block scope still retain their values for the entire duration of their containing procedure. As the documentation says in a note:

Even if the scope of a variable is limited to a block, its lifetime is still that of the entire procedure. If you enter the block more than once during the procedure, each block variable retains its previous value. To avoid unexpected results in such a case, it is wise to initialize block variables at the beginning of the block.

So what's happening is that each time you re-enter the For block, j still has its previous value. Because the right-side of the equals sign is evaluated first, that old value of j is incremented by 1, and then stored in j (effectively erasing the old contents of j).

@CodeGray would you please review this post http://stackoverflow.com/questions/14937392/object-of-interop-word-document-class-is-null-on-windows-server-2008-word-open — vikas, Feb 19 '13 at 04:01

score 0 · Answer 2 · answered Feb 08 '13 at 05:19

0

Hi you have use str & " Kratika " inside the loop.If you expect only " Kratika " every time Then use only Dim str As String = " Kratika " inside the loop.

Now check again

answered Feb 08 '13 at 05:19

user2001117

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thanks for reply my doubt is why it is adding previous value of string `kratika` – vikas Feb 08 '13 at 05:30
yes but with declaring you are using & operator IT always concat with previous value. – user2001117 Feb 08 '13 at 05:34
Dim str As String = " Kratika " – user2001117 Feb 08 '13 at 05:35
ok if I will use `Dim j As Integer = j + 1` inside the `for loop' then what should I expect – vikas Feb 08 '13 at 05:38
if u have assign j=1 then it should be 1,2,3 lik that – user2001117 Feb 08 '13 at 05:47
He already knows what *does* happen. He ran it in a debugger and checked that for himself before asking the question (which is good). What's he's asking here is ***why*** that happens. – Cody Gray - on strike Feb 08 '13 at 05:52

What is the scope of a block-level variable in VB.NET?

2 Answers2