Error: The tag 'DesignOnlyResourceDictionary' does not exist in XML namespace

[MaryGraceMcClellan]

I am receiving an error when I try to build an Add-in solution using the ArcGIS Pro SDK. I have set the target framework for .NET 4.8 and I have ArcGIS Pro 2.8 installed. The error is as follows: 

The tag 'DesignOnlyResourceDictionary' does not exist in XML namespace 'clr-namespace:Esri.ArcGISPro.Extensions;assembly=Esri.ArcGISPro.Extensions'. 

The error is occurring on my .XAML file. 

Has anyone else run into this issue? 

[Wolf · Accepted solution]

In 2.8 all Pro assemblies were switched from mixed (x86 and x64) to 64 bit only.  Unfortunately the XAML designer loader cannot load x64 bit assemblies.   Here is a knowledge base article to that regard: Error: Could not load file or assembly 'ArcGIS.Desktop.Framework, Culture=neutral' or one of its dep...

Hopefully the upcoming release of Visual Studio 2022 will fix this issue. 

[WolfgangRuschke]

Unfortunately it's the same problem in 3.0 with .NET 6.0.5 and VS 2022:

XDG0062 Could not load file or assembly 'ArcGIS.Desktop.Extensions, Version=13.0.0.0, Culture=neutral, PublicKeyToken=8fc3cc631e44ad86'

[profchamberlain]

I also have this issue. I've ran through the solution that is posted earlier, but I am still getting errors when trying to build. What is the actual solution?

[Wolf]

Unfortunately, the XAML designer (which is part of VS 2022) is somehow not able to find the ArcGIS Pro assemblies referenced in the XAML.  You see these errors only when the XAML designer is open.  When you close the XAML designer, the errors go away, and the application is able to find and load all required assemblies.   I will check once more with the framework dev team to see if there's a workaround for this VS issue.

[profchamberlain]

As I was producing the migration of my extension to 3.0, I noticed that my installation was for the user not for the machine. So the ArcPro bin was not found. I uninstalled and reinstalled (for all users) and that solved several errors, including most of the ones identified above. I also see what you mean by the XAML designer being opened (that's when most of these errors occur). However, there is still one issue that might be related.:

• Error MC3074 The tag 'DesignOnlyResourceDictionary' does not exist in XML namespace 'clr-namespace:ArcGIS.Desktop.Extensions;assembly=ArcGIS.Desktop.Extensions'.
• It refers to C:\Users\<username>\Source\Repos\<software path>\SettingsDockpane.xaml

Is this related to the problems above?