{"id":1033,"date":"2009-11-03T18:32:00","date_gmt":"2009-11-03T18:32:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vbteam\/2009\/11\/03\/spot-the-bug-much-ado-about-nothing-jonathan-aneja\/"},"modified":"2024-07-05T12:42:42","modified_gmt":"2024-07-05T19:42:42","slug":"spot-the-bug-much-ado-about-nothing-jonathan-aneja","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/vbteam\/spot-the-bug-much-ado-about-nothing-jonathan-aneja\/","title":{"rendered":"Spot the Bug! – Much Ado about Nothing… (Jonathan Aneja)"},"content":{"rendered":"

Microsoft has this neat mailing list called \u201cSpot the Bug\u201d where developers can send interesting snippets of code that look correct but actually have subtle bugs in them.  The puzzles are a lot of fun and I\u2019ve always thought it\u2019d be a fun thing to try here on the team blog.  Over the past year or so I\u2019ve been keeping a list of interesting bug reports and emails where people have been tripped up by some of VB\u2019s hidden subtleties (though admittedly many of these could apply to C# as well).<\/font><\/span><\/p>\n

 <\/font><\/span><\/p>\n

The format\u2019s gonna be a bit of an experiment \u2013 let me know if you prefer to see the answer in the same post or posted a day later.  Alright let\u2019s get started\u2026<\/font><\/span><\/p>\n

<\/font><\/span> <\/p>\n

<\/font><\/p>\n

Spot the Bug:<\/strong> <\/font><\/p>\n

The following code is intended to print \u201cYes\u201d \u2013 will it?<\/font><\/p>\n

 <\/font><\/p>\n

        <\/span>Dim<\/span> x As<\/span> Integer<\/span>? = Nothing<\/span><\/span><\/p>\n

 <\/span><\/p>\n

        <\/span>If<\/span> x = Nothing<\/span> Then<\/span><\/span><\/p>\n

            <\/span>MsgBox(“Yes: x contains null”<\/span>)<\/span><\/p>\n

        <\/span>Else<\/span><\/span><\/p>\n

            <\/span>MsgBox(“No: x does not contain null, and has a real value”<\/span>)<\/span><\/p>\n

        <\/span>End<\/span> If<\/span><\/span><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/b><\/p>\n

Answer:  <\/span><\/b>No!  <\/span>We\u2019re using the equality operator to compare a nullable integer with Nothing \u2013 this results in null propagation<\/i>, i.e. the result of the expression is Nothing.  <\/span>An If-statement attempts to convert its condition to a Boolean, so basically we\u2019re left with this:<\/font><\/font><\/p>\n

 <\/font><\/p>\n

        <\/span>If<\/span> Nothing<\/span> Then<\/span><\/span><\/p>\n

 <\/span><\/p>\n

        <\/span>Else<\/span><\/span><\/p>\n

            <\/span>‘Obviously we’ll always land here<\/span><\/span><\/p>\n

        <\/span>End<\/span> If<\/span><\/span><\/p>\n

 <\/font><\/p>\n

This can be really surprising when working with nullables, so we\u2019ve decided to add a warning for this case in VB10:<\/font><\/p>\n

 \"New<\/font><\/p>\n

<\/font><\/font><\/span><\/p>\n

 <\/font><\/b><\/p>\n

The warning explains the fix \u2013 use \u201cIs\u201d instead of = to avoid the null propagation and get the semantics you expect:<\/font><\/p>\n

        <\/span>Dim<\/span> x As<\/span> Integer<\/span>? = Nothing<\/span><\/span><\/p>\n

 <\/span><\/p>\n

        <\/span>If<\/span> x Is<\/span> Nothing<\/span> Then<\/span><\/span><\/p>\n

            <\/span>MsgBox(“Yes: x contains null”<\/span>)<\/span><\/p>\n

        <\/span>Else<\/span><\/span><\/p>\n

            <\/span>MsgBox(“No: x does not contain null, and has a real value”<\/span>)<\/span><\/p>\n

        <\/span>End<\/span> If<\/span><\/span><\/p>\n

<\/span><\/span> <\/p>\n

<\/p>\n

Check back for a similar case but with a bit of a twist tomorrow…<\/p>\n

<\/font><\/font><\/span><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Microsoft has this neat mailing list called \u201cSpot the Bug\u201d where developers can send interesting snippets of code that look correct but actually have subtle bugs in them.  The puzzles are a lot of fun and I\u2019ve always thought it\u2019d be a fun thing to try here on the team blog.  Over the past year […]<\/p>\n","protected":false},"author":260,"featured_media":8818,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[195],"tags":[83,143,165,166,167],"class_list":["post-1033","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-visual-basic","tag-jonathan-aneja","tag-spot-the-bug","tag-vb2005","tag-vb2008","tag-vb2010"],"acf":[],"blog_post_summary":"

Microsoft has this neat mailing list called \u201cSpot the Bug\u201d where developers can send interesting snippets of code that look correct but actually have subtle bugs in them.  The puzzles are a lot of fun and I\u2019ve always thought it\u2019d be a fun thing to try here on the team blog.  Over the past year […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/1033","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/users\/260"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/comments?post=1033"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/1033\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media\/8818"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media?parent=1033"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/categories?post=1033"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/tags?post=1033"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}