Can all ands and ors be replaced by AndAlso and OrElse?

[Napivo1972]

I was wondering. Do I need to take care when replacing ands and ors? Can anyone give me an example where I better do not replace and and or?

[PlausiblyDamp]

Both AndAlso and OrElse (that sounds weird read out loud) give a performance benefit by only evaluating the minimum number of arguments.

 

e.g.

given the following 2 simple functions

   Private Function Test1() As Boolean
       MessageBox.Show("Test1")
       Return False
   End Function

   Private Function Test2() As Boolean
       MessageBox.Show("Test2")
       Return True
   End Function

 

if called from an if statement like

       If Test1() And Test2() Then
           MessageBox.Show("Both true")
       End If

you should get both the messageboxs from Test1 and Test2 displayed - despite the fact that because Test1 returns false the overall result can never be true.

 

Changing the if statement to

       If Test1() AndAlso Test2() Then
           MessageBox.Show("Both true")
       End If

you only get the first messagebox as it realises the result will always be false so there is no reason to even evalute the second condition.

 

However if Test2 performed some required functionality then that would also be skipped - this is the only time you would need to avoid AndAlso; however I would personally class that as a bad coding style anyway - relying on side-effects like that to get required code to execute is hard to debug / maintain and gives no real benefit.

[Napivo1972]

Thank you for this enlightening explanation. I understand now.