I am in the midst of a summer project to upgrade an Umbraco iste from 3.0.5 to version 6 in increments. My current version is 4.5.2. After upgrading and moving the site to a new server, I found that a custom control that I wrote a few years ago no longer worked. I recompiled it, targeting the 4.0 .Net framework, but it started throwing the following error: 

Error creating usercontrol (/usercontrols/tvlisting.ascx)

InternalXmlHelper.vb(9): error BC30560: 'ExtensionAttribute' is ambiguous in the namespace 'System.Runtime.CompilerServices'.

I Googled this and found the following article, which seemed to fit my issue: 

http://stackoverflow.com/questions/12104883/bc30560-extensionattribute-is-ambiguous-in-the-namespace-system-runtime-comp

However, I could find no reference anywhere in my project to "ExtensionAttribute". 

Then I found this thread on this forum: http://our.umbraco.org/forum/using/ui-questions/11217-ExtensionAttribute-is-ambiguous-Why-is-it-not-getting-attention

So it looks like this was a known issue in 4.5.1 and 4.5.2. Can I assume it has been subsequently fixed? (I plan to continue upgrading the site to the latest 6.x version later this summer.)