KB Article KA18603
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Could not load file or assembly "Microsoft.Windows.Design.Extensibility ..." or Could not load file or assembly "DevExpress X ..." when using AjaxControlToolkit

Description:
This KB Article addresses the following question:
Why the following exceptions may be thrown when using DevExpress ASP.NET products together with the AjaxControlToolkit
- Could not load file or assembly "Microsoft.Windows.Design.Extensibility …"
- Could not load file or assembly "DevExpress X …"

Answer:
This problem usually occurs when the "DevExpress.*.Design.dll" assembly files exist in the application Bin folder. These files are used for development purposes only and usually are registered in the GAC via the DXperience installer.
Note that the "DevExpress.*.Design.dll" assembly files cannot be distributed according to our EULA terms.
To resolve this issue,
- Remove all "DevExpress.*.Design.dll" assembly files from the application Bin folder and exclude them from the deployment;
- Clear the required "Temporary ASP.NET Files" directory:
  c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\
  c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\
- Rebuild the application;
- Redeploy the application without the "DevExpress.*.Design.dll" assembly files.

See also
The "Could not load file or assembly 'X'" error message appears

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