When creating a Web application with XtraReports, please keep in mind that a report is generated on a server and only the HTML or PDF version of it is sent to the client. The ShowDesigner method of the XtraReport class is unavailable on a client.
If you wish to allow the users of your web application to customize reports, you will need to create a new Windows Forms application which can be launched from the web (see No-Touch Web Deployment (.NET 1.x)). The primary goal of this application is to allow your customers to design reports via the XtraReports End-User Designer.
After a report has been modified and saved to a file on a client system, you will need to upload it to your web server to use in your Web Forms application. You can then implement this feature: users of your web application can select the name of a customized report in a list box and the corresponding report is generated and displayed in the ReportViewer component.
We advise that you store user-defined report layouts in a database on your server instead of separate files. You can create a special data table for this task, where report layouts are stored in a BLOB column (the "image" data type in MS SQL Server). To load a report from a database, you can use the LoadState method with a parameter of type Stream.