I'm working on a Visual Studio add-in that displays a stage in a (windows form) window. Creating a new special project (related to the add-in) makes a new project with all the necessary.dlls for the user to interact with the stage in their own code. In the stage, actors move around and can interact with other actors. The stage has UI that allows users to create new actors. The users are supposed to be able to alter.cs files while the stage exists.

Apr 26, 2018  Applications on Mac differ to those in Windows by only being limited to one instance by default. However, it is possible to open both a second (or more) instance of Visual Studio for Mac and to open a second (or more) solution inside a single Visual Studio for Mac instance.

The reason it's an add-in is so users can create and edit the code of the actors, compile it, and see the results of their actors on the stage immediately. Anyway - this works fine until there are two instances of Visual Studio open. The second one fails to paint anything (I get the red X's in the Windows Form window). I get an error telling me 'It is illegal to call out when inside message filter. (Exception from HRESULT: 0x80010005 (RPC_E_CANTCALLOUT_INEXTERNALCALL)). I've searched around and the most relevant thing I could find is that it may be an issue with COM controls or painting.

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I'm not really sure. Has anyone encountered this error under these circumstances? Is there a fix to get my add-in to run fine over multiple instances of VS?

Visual Studio 2017 allows multiple side by side installations, which makes experimenting with various versions and extensions a breeze: To install those multiple IDEs, you simply head over to grab each of the editions you want to test and just run the installers. NOTE: you can install one of each VS editions (Community, Professional and Enterprise) as well as one of each VS editions from the Preview “channel” from for a total of potentially up to 6 instances to play with! Uninstalling, modifying or even launching the instances is trivial from the Visual Studio Installer UI: Sometimes, however, the installer may not be able to modify, repair or even uninstall a given instance. This may happen when adding/removing VSIXes that mess up the so-called catalog that drives the VS installer.

Fortunately, VS provides a tool, called which can give you about all installed instances. With it and a bit of powershell, I came up with the following script to allow you to easily cleanup any misbehaving VS instance: If you want to run it from any Powershell prompt, you can add the following to your: Running the Cleanup-VS from a prompt will show you something like the following.