Difference between declaring variables before or in loop?

Question

I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference? A (q

Answer 1

This is a gotcha in VB.NET. The Visual Basic result won't reinitialize the variable in this example:

For i as Integer = 1 to 100
    Dim j as Integer
    Console.WriteLine(j)
    j = i
Next

' Output: 0 1 2 3 4...

This will print 0 the first time (Visual Basic variables have default values when declared!) but i each time after that.

If you add a = 0, though, you get what you might expect:

For i as Integer = 1 to 100
    Dim j as Integer = 0
    Console.WriteLine(j)
    j = i
Next

'Output: 0 0 0 0 0...