I have followed the steps on http://msdn.microsoft.com/en-us/library/ff798499.aspx in order to create a custom sandboxed workflow action, and have deployed this into a site workflow in Microsoft SharePoint online.
However, I regularly get ThreadAbortExceptions which I believe is to do with the 30 second hard limit for sandboxed solutions. On my development machine the code runs pretty fast and never times out, it is quite simple anyway so for it to take longer than 30 seconds would be a bit silly. Microsoft support have told me that the steps I followed do not apply to Microsoft SharePoint Online, even though the steps are for a sandboxed solution which is the only type of solution that you can run in SharePoint Online and that I should use a different method in order to create a custom workflow action.
However, they said that they cannot provide a direct link to the article as it is provided by a third party. Has anyone got any experience of sandboxed workflow actions using SharePoint Online and could they point me in the right direction for how to implement one? I've got a funny feeling the support guy couldn't really explain the poor performance of their sandboxed code service so has fobbed me off by telling me my code must be implemented incorrectly. Please prove me wrong, any help appreciated!