The WebForms viewer works like the HTML5 Viewer in a client-server pair with the Reporting REST Service. Even if the Reporting REST Service is added in the same WebForms project, the service does not operate in the context of the client project. To use data modified in the WebForms project, it has to be saved in the data source, from where the Reporting REST service can access it.
The old ASP.NET WebForms ReportViewer displays content rendered through the Reporting HTML Rendering mechanism.
As of Telerik Reporting Q3 2015, the old ASP.NET WebForms ReportViewer control is obsolete. The main reasons for this decision were that the viewer:
tableelements in its structure.
Utilizes the ASP.NET Session to preserve resources between requests.
Is suitable for WebForms projects only.
The HTML5 ASP.NET Web Forms Report Viewer is built on and requires ASP.NET 4 Web Forms or newer framework.
A reference to Telerik.ReportViewer.Html5.WebForms.dll.
The WebForms wrapper is designed to request all Kendo UI CSS and JS files, including the viewer's HTML, CSS and JS templates, from the resources of the Reporting REST Service. If you will not add changes in the default styles of the viewer, you need only a link to jQuery of version greater than 1.9.1.