The report definition is not valid
c#, rdlc, report
Solution
Just stumbled upon this question since I was having the same problem. In my case, I had a reference to an older version of ReportViewer.WinForms. When I switched versions, everything worked fine. Might not be the same case as yours, but just thought I'd chime in in case anyone else finds this page.
Problem
I have converted an `.rdlc` rerort from SSRS 2005 to SSRS 2008. Now when I am going to build that project I have got error. The report definition is not valid. Details: The report definition has an invalid target namespace 'http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition' which cannot be upgraded. Please give me solution for this.