Delete the previous version of Wisej.Framework.Designer.dll from the project in all directories. That error shows that the designer is loading the previous Wisej.Framework.Design.dll. If you delete it from everywhere it cannot be loaded. You can also check in case it’s in the GAC “gacutil -l Wisej.Framework,Design”, and remove it “gacutil -u Wisej.Framework.Design”. or open “\Windows\assembly” or “\Windows\assembly\GAC_MSIL”. Once the new build is installed, check again with gacutil. You can also reference it in the project.