An error occurred while finding the resource dictionary

If you are using Visual Studio 2017, try to restart your computer. The problem may be solved.

For VS2017 if assembly is referenced and double-checked that all namings are OK try to close VS and delete .vs directory in solution directory. This will cause you to lose all user settings (startup project, WPF designer zoom, etc.) but will fix this.

I'm using VS2019 getting this error.

I just restart the VS and the error disappeared.

This was a known issue with Visual Studio 2012. See this link at Microsoft Connect. After installing VS2012 Update 1 this issue resolved for me. If you think you're running the most up to date version of VS2012 and still experiencing this issue, make absolutely sure that the update has been applied. I thought I had updated using Windows Update, but then found that I had to tell VS2012 to apply the update. After VS2012 did its thing everything was fine.