If you are getting this error while using old reports from your legacy application then you can try out the following solutions
- Change in your Web.config all “Microsoft.ReportViewer.WebForms, Version=8.0.0.0,…” to ” Microsoft.ReportViewer.WebForms, Version=9.0.0.0,…” and in controls “Register Assembly=”Microsoft.ReportViewer.WebForms, Version=8.0.0.0,…” to “Register Assembly=”Microsoft.ReportViewer.WebForms, Version=9.0.0.0,…”
- Clean your solution and delete the related dll files and then again rebuild the solution.
Generally doing step 1 will resolve the problem as in legacy applications its configured to use Version=8.0.0.0
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