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What Was Wrong With #15?

Thursday, May 3, 2007

Not many takers on WWWTC #15, but Jason finally nailed it.

The bug in the code revolves around how VB allocates arrays. The following code snippet creates an array of 5 System.Char objects, and will output the number 5 to the screen, right?

Dim length As Integer = 5
Dim chars() As Char = New Char(length) {}


 The output is "6", which still astonishes those of us (me, at least) who hail from the land of semicolons. The number I pass to the New clause in VB doesn't specify the number of elements in the array, but the upper bounds of the array. Passing a 5 tells VB to create an array with 6 elements indexed 0 to 5.

I suspect this behavior is a compromise over the definition of an array's lower bound. Does an array start at 0? Does an array start at 1? It's a religious debate, but a VB developer could use the chars array like so:

For i As Integer = 0 To 4

... or like so:

For i As Integer = 1 To 5

... and neither will fail with an index out of bounds exception.

This behavior does cause a problem, however, when you want a precise number of characters and forget how New works. The unit test failed because it expected to get back a string with a length of 6 ("edoCoTedO"), but the actual string had a length of 7 (with a 0 character tacked on the end).

The fix is to create the array by passing the length of the string minus 1.