Perusing the forums over the last few months, it’s clear that there’s some confusion about the various VSTO loader and runtime components. At the time of writing, there are 4 versions of VSTO in existence, including 4 sets of design-time tooling and 3.5 versions of the VSTO runtime. I say 3.5 versions, because the VSTO 2005 SE runtime was an in-situ replacement for the VSTO 2005 runtime, and both are considered VSTO v2 (2005 SE was purely additive to 2005). Other versions – that is, VSTO 2003 (v1) and VSTO 2008 (v3) – are SxS with v2.

Right now, each VSTO solution runs against the version of the VSTO runtime that it was built against, regardless of the version of Office that is hosting it. It can be confusing to figure out which version will be used, so here’s a breakdown. The 4 versions of VSTO in existence at the time of writing are:

·         VSTO 2003 (VSTO v1 runtime), which supported doc-level customizations for Excel and Word 2003 only, in a very simple fashion.

·         VSTO 2005 (VSTO v2 runtime), which provided very much broader and deepter runtime support for Excel/Word 2003 doc-level customizations, and introduced app-level add-in support for Outlook 2003.

·         VSTO 2005 SE (VSTO v2 runtime), which maintained the v2 runtime support for Excel/Word 2003 doc-level customizations, and replaced the app-level add-in support – extending it to cover Excel, Outlook, PowerPoint, Visio and Word 2003, and Excel, InfoPath, Outlook, PowerPoint, Visio and Word 2007.

·         VSTO 2008 (VSTO v3 runtime), which maintained the v2 runtime support for Excel/Word 2003 doc-level customizations, and added v3 runtime support for Excel/Word 2007 doc-level customizations. Also maintained the v2 runtime support for app-level add-ins for Excel, Outlook, PowerPoint, Visio and Word 2003, and added Project 2003. Also added v3 runtime support for app-level add-ins for Excel, InfoPath, Outlook, PowerPoint, Project, Visio and Word 2007. Plus, added design-time support for SharePoint 3.0 Workflow.

The runtime versions are used as follows, first the doc-level solution types:

·         A VSTO 2003 v1 doc-level solution will always run with the VSTO v1 runtime. You could only create a VSTO 2003 solution for Excel/Word 2003, but the solution should continue to run without change in Excel/Word 2007 – and it will continue to use the VSTO v1 runtime.

·         A VSTO 2005 v2 doc-level solution will always run with the VSTO v2 runtime. There’s no such thing as a VSTO 200 SE doc-level solution, because VSTO 2005 SE was additive to VSTO 2005, adding only app-level add-in support. Again, a VSTO 2005 doc-level solution built against Excel/Word 2003 should continue to run unchanged in Excel/Word 2007 and will continue to use the VSTO v2 runtime.

·         A VSTO 2008 doc-level solution for Excel/Word 2003 will use the VSTO v2 runtime. It should also continue to run without change in Excel/Word 2007 and will continue to use the VSTO v2 runtime.

·         A VSTO 2008 doc-level solution for Excel/Word 2007 will use the VSTO v3 runtime. It will not run in Excel/Word 2003.

Similar patterns hold true for app-level add-ins:

·         A VSTO 2005 v2 Outlook add-in will run against the VSTO 2005 v2 runtime so long as VSTO 2005 SE is not installed. If VSTO 2005 SE is installed, then the add-in will run against the VSTO 2005 SE v2 runtime.

·         A VSTO 2005 SE v2 add-in for Excel, Outlook, PowerPoint, Visio or Word 2003 will run against the VSTO 2005 SE v2 runtime. It will not run if the VSTO 2005 SE runtime is not installed. It will continue to run without change in Excel/Outlook/PowerPoint/Visio/Word 2007, and it will continue to use the VSTO 2005 SE v2 runtime.

·         A VSTO 2005 SE v2 add-in for Excel, InfoPath, Outlook, PowerPoint, Visio or Word 2007 will run against the VSTO 2005 SE v2 runtime. It will not run if the VSTO 2005 SE runtime is not installed. It will not run in Office 2003 because it requires the Office 2007 PIAs which are not designed to be used with Office 2003.

·         A VSTO 2008 add-in for Excel, Outlook, PowerPoint, Project, Visio or Word 2003 will run against the VSTO 2005 SE v2 runtime. It will not run if the VSTO 2005 SE runtime is not installed. It will continue to run without change in Excel/Outlook/PowerPoint/Project/Visio/Word 2007, and it will continue to use the VSTO 2005 SE v2 runtime.

·         A VSTO 2008 add-in for Excel, InfoPath, Outlook, PowerPoint, Project, Visio or Word 2007 will run against the VSTO 2008 v3 runtime. It will not run if the VSTO 2008 runtime is not installed. It will not run in Office 2003 because it requires the Office 2007 PIAs which are not designed to be used with Office 2003.

The different versions of the VSTO runtime include different loader components. The load sequence therefore varies according to the version of the solution. For doc-level customizations, the load sequences are:

·         VSTO 2005: Otkloadr.dll à VSTOLoader.dll à VSTORuntime.dll à solution

·         VSTO 2005 SE Office 2003: Otkloadr.dll à VSTOLoader.dll à VSTORuntime.dll à solution

·         VSTO 2005 SE Office 2007: Otkloadr.dll à VSTOLoader.dll à VSTORuntime.dll à solution

·         VSTO 2008 Office 2003: Otkloadr.dll à VSTOLoader.dll à VSTORuntime.dll à solution

·         VSTO 2008 Office 2007: VSTOEE.dll à VSTOLoader.dll à solution

The load sequences for app-level add-ins are:

·         VSTO 2005: AddinLoader.dll à solution

·         VSTO 2005 SE Office 2003: AddinLoader.dll àVSTORuntime.dll à solution

·         VSTO 2005 SE Office 2007:  VSTOEE.dll àAddInLoader.dll à VSTORuntime.dll à solution

·         VSTO 2008 Office 2003: AddInLoader.dll à VSTORuntime.dll à solution

·         VSTO 2008 Office 2007: VSTOEE.dll à VSTOLoader.dll à solution

Why do we have so many runtime versions, you might ask? Why is it so seemingly complicated? Well, the runtimes are designed for the opposite purpose – the intention is to make life simple for users – users should not need to worry about which version is used, and we have multiple versions to ensure that all solutions will continue to run without change in the face of new versions of Office. A major factor in this is the requirement that a deployed solution should continue to work without any user/admin intervention even though the user might upgrade the version of Office on their machine. They might have multiple solutions deployed which were built over time against multiple versions of VSTO – and all of them should just continue to work.