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.NET Forum / Windows Forms / WinForm General / January 2008Order of evaluation in an If statement
Hi, I am running into a problem which is best described with an example: Dim aList as ArrayList = Nothing .. if (aList IsNot Nothing) And (aList.Count > 0) then ... This always gives me an exception. I have also tried switching the order like: if (aList.Count > 0) And (aList IsNot Nothing) then .. and i get the same thing. I have had to break this into separate if statements. Can anyone shed a light on this and explain why this does not work? thanks, Mars >Hi, >I am running into a problem which is best described with an example: [quoted text clipped - 14 lines] >thanks, >Mars If (aList IsNot Nothing) AndAlso (aList.Count > 0) Then Use AndAlso instead of And. And and Or evaluate all parts of the expression, AndAlso and OrElse stop evaluating when the final result is known. Use And and Or only for logical operations on numeric values. Be aware that Iif works the same way - all arguments are always evaluated. Close! AndAlso stops evaluating when the expression(s) to the left evaluates to False. OrElse stops evaluating when the expression(s) to the left evaluates to True. >>Hi, >>I am running into a problem which is best described with an example: [quoted text clipped - 24 lines] > Be aware that Iif works the same way - all arguments are always > evaluated. I am surprised. And works this way in C#, C and C++ so what required this change in VB .NET? Thank you both! Mars. Simple answer = VB was never based or derived from C/C++, it was based on BASIC. The AndAlso and OrElse operators were added in VB.Net 2003 to provide exactly that 'short-circuiting' behaviour. >I am surprised. And works this way in C#, C and C++ so what required > this change in VB .NET? > Thank you both! > Mars. > > *** Sent via Developersdex http://www.developersdex.com *** "Victory" <csharp@devdex.com> schrieb: >I am surprised. And works this way in C#, C and C++ so what required > this change in VB .NET? Actually there is no operator named 'and' in C#, C, and C++, respectively. However, C# has both '&' and '&&' operators, which share semantics with VB's 'And' and 'AndAlso' operators. '|' maps to 'Or' and '||' maps to 'OrElse'.  SignatureM S Herfried K. Wagner M V P <URL:http://dotnet.mvps.org/> V B <URL:http://dotnet.mvps.org/dotnet/faqs/> Thanks! Mars Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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