Expressions can be evaluated only once the report processing is started.
My suggestion is to test settings the SubReport.ReportSource
and a custom function
in which the sub report is obtained, modified and returned in an InstanceReportSource.
To design reports with user functions in the Standalone Report Designer tool, functions will have to be created in an external assembly extending the tool's configuration - Extending Report Designer
(there are tutorials linked at the bottom).
On a side note, you can also use a custom data access layer in which you can specify the connection string in code. Then this data access layer can be used on designing and deploying reports.
I hope the provided information is helpful.
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