There seems to be an error in the intellisense engine for controls in webprojects included through the web.config. When in the same assembly/namespace controls and usercontrols are mixed and using the same tagprefix the intellisense seems to stop functioning for the specified tagprefix and controls within the prefix.
<add assembly="ErrorExampleMixedControls" tagPrefix="MC" namespace="ErrorExampleMixedControls.Controls"/>
<add src="~/Controls/TestUserControl.ascx" tagPrefix="MC" tagName="TestUserControl" />
1) The above construction will cause the intellisense to stop working for the specified tagprefix and cause warnings about invalid tagprefixes (however the code will work as expected).
2) When disabling the line for the .ascx the intellisense is working again (suggesting controls AND webusercontrols). However this will cause errors when running, if these controls are not included through the add with the ascx reference null pointers will be encountered when accessing controls within the element.
3) When disabling the assembly line only the ascx control is suggested by intellisense (which is right since the assembly is not included anymore).
These types of projects were working without problems in VS2010 (and still are). Due to the problems with intellisense the VS2012 software is unfortunately not usable in various webprojects of me and my customers.
The problems only appear in the IDE. When compiling and running the scenario with both add statements in the web.config the application is working as expected.
I have attached a test project to reproduce the problems above. This is tested in the RTM version of VS 2012.