C# – The report definition is not valid

string is an alias in C# for System.String.
So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, it's generally recommended to use string any time you're referring to an object.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

string greet = String.Format("Hello {0}!", place);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

I had the same error and after checking the following link :

https://cmatskas.com/resolving-rdlc-error-the-report-definition-is-not-valid-in-visual-studio-2012/

I found out that the default referenced Microsoft.ReportViewer.WebForms.dll version was for VS 2008 which is Version : 9.0.0 so i searched for the dll and i found it in :

c:\Program Files (x86)\Microsoft Visual Studio 12.0\ReportViewer\Microsoft.ReportViewer.WebForms.dll

after referencing it , it worked successfully