I don’t think there is a way to indicate the data source at run time because the data source is part of the report definition (.rdl) and it has some protection around it, preventing a method like find-and-replace. However, there is the functions available in SSRS (an example search for ReportingService2010.cs) for us in .NET applications that allow you to set the data source. I know that I used to be in a project where we deployed using our own created .NET application, which uses these calls to set the data source for publication. This was for SSRS 2005.
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