Could not find type 'Microsoft.Office.Tools.Ribbon.RibbonTab' error prevents designer from opening

Question

I have a VSTO for Excel project with a custom ribbon tab. The solution builds without errors. I can run the solution. It launches Excel and displays the custom ribbon tab and task pane without any issue.

When I try to open the designer for the custom ribbon tab in Visual Studio 2019, I get the following error:

Could not find type 'Microsoft.Office.Tools.Ribbon.RibbonTab'. Please make sure the assembly that contains this type is referenced. If this type is a part of a development project, make sure that the project has been successfully built using the settings for your current platform or Any CPU.

• The solution builds and runs without any issues.
• When I expand the error info, There is no stack trace or error line information available for this error.
• The build settings are set to 'Any CPU'.
• The assemblies that contain 'Microsoft.Office.Tools.Ribbon.RibbonTab' are included in the project references. These are Microsoft.Office.Tools.Common and Microsoft.Office.Tools.Common.v4.0.Utilities.
• I have tried to Clean, Rebuild, and Restart Visual Studio 2019. This had no effect.
• I could not find anything definitive when I Check Windows Forms Design-Time error list.

Would anyone know how to address this issue so that I can open the custom ribbon in the designer?

Is VSTO still supported by Microsoft? All documentation appears to be several years old.

Answer 1

This is a Visual Studio issue, not a C#/Build/DLL linkage issue.

Make sure your Visual Studio setup (do the Modify in the installer (#1) if already installed) has these two components checked:

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Is VSTO still supported by Microsoft?

They are moving to an online office365 addin (my words) type VSTO. As to new features, not recently. One can still put in a Visual Studio support ticket if you have the right subscription.

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I have never had any issues with ribbons, but sometimes the winforms res files in designer do not show and I use the JetBrains Rider editor to view those controls when VS fails.