Below is a quote from my response in your other support ticket on the same question, where we should consider the expression is required in the context of the Report Server application.
"This adds new information to your previous threads about using a custom function.
The Report Server is not designed for loading external assemblies with data and functions.
Yes, it can be configured manually by copying the DLL on the server in the corresponding executing folders, and adding the settings in Report Server\Telerik.ReportServer.Web\Web.config
(for the Report Server preview) and Report Server\Services\Telerik.ReportServer.ServiceAgent.exe.config
(for the Scheduler service) - both require restarting the Report Server web site in IIS and the Windows Service of the scheduler. Note that we do not recommend these manual settings.
The approach we can suggest you to select all values is to make the multivalue report parameter's AllowNull to true and interpret it as 'select all' in the data-retrieval method e.g
. How to use MultiValue Report Parameter in a SQL query.
You will have all values selected initially. Then the editor of the multivalue parameter has 'clear selection' and 'select all' options."
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