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The old ASP.NET WebForms ReportViewer control is obsolete as of Telerik Reporting Q3 2015. The main reasons for this decision: the viewer uses extensively IFRAME and TABLE elements in its structure, the viewer uses the ASP.NET Session to preserve resources between requests, the viewer is suitable only for WebForms projects.
The article discusses how to troubleshoot comonly met issues result the above listed reasons.
The old ASP.NET WebForms ReportViewer displays content rendered through the Reporting HTML Rendering nmechanism.
The old ASP.NEt WebForms ReportViewer depends on the ASP.NET Session. If the session expires the page with the viewer will have to be refreshed.
Other possible reasons for getting a session Expired message are discussed in the Report is unavailable or session has expired error message KB article.
The old ASP.NET WebForms ReportViewer depends on the ASP.NET Session. Depending on the session state mode of the application reports may require additional settings - Design Considerations for Out-proc Session State.
The old ASP.NET WebForms ReportViewer displays content delivered by its Telerik.ReportViewer.axd handler. Verify the handler is registered in both system.Web and system.webServer sections - How to: Add report viewer to a web page, and that the viewer is added in a valid AJAX Scenario.
In case of using Visual Studio 2013 or greater version, disable the BrowserLink feature..
The old ASP.NET Webforms ReportViewer does not display reports or the page area is messed on iPad or other mobile device.
The old ASP.NET WebForms ReportViewer displays content via IFRAME and TABLE elements in browser. This may result in unexpected layout in modern HTML5-compliant browsers. The solution is to use an older DOCTYPE declaration - Blank report in ASP.NET ReportViewer on iPad/iPhone.
The old ASP.NET WebForms ReportViewer displays content delivered by its Telerik.ReportViewer.axd handler. Check if the handler is registered correctly and if the browser's proxy and security settings allow it to deliver content - The styles of the web report viewer's toolbar are messed up.
In case of using authorization, verify that the Telerik.ReportViewer.axd handler's path is allowed for the user - HOW TO: Control Authorization Permissions in an ASP.NET Application.
The ASP.NET WebForms ReportViewer does not display a report that uses Table/List/Crosstab item in Internet Explorer.
In case data visualized by the Table/List/Crosstab item results in a great number of HTML elements, Internet Explorer may fail to visualize the content. solutions are suggested in Design Considerations for HTML Rendering (Browsers Limitations).
Error message on adding a viewer in Visual Studio: "The ReportViewer control failed to setup the http handler in the web.config file. You may experience troubles when viewing the report. Exception details: Unable to cast COM object of type 'System.__ComObject' to class type 'Telerik.Reporting.Vs.Common.AssemblyReference'. Instances of types that represent COM components cannot be cast to types that do not represent COM components; however they can be cast to interfaces as long as the underlying COM component supports QueryInterface calls for the IID of the interface."
Web Sites handle assembly references differently and Telerik Reporting Wizards and Designers are not designed to work with such settings. You can manually add viewers or upgrade the Web Site. Reports should be designed in an external ClassLibrary project referenced by the Web Site project.