Why "Error: Subreport could not be shown" for some reports and not others?

Question

I'm using VS2010 and the built-in visual Report Designer to create RDLC templates for rendering reports with sub-reports as PDF files in an ASP.NET application using a ReportViewer control and the .LocalReport member. The code iterates over a set of records, producing one report (with its sub-reports) for each record.

I noticed recently that for a small number of the reports, one of the sub-reports was failing and giving the "Error: Subreport could not be shown" message. What's puzzling me about this case, in contrast to the many posts about this error that I've read (and previous times I've wrestled with it myself), is that it is only occurring for a subset of cases; from what I've seen elsewhere, the problem is usually all-or-nothing -- this error always appears until a solution is found, then this error never appears.

So... what could cause this error for only a subset of records? I can run the offending sub-report directly without errors; I can open the .xsd file and preview the DataSet for the offending records without errors; I can run the query behind the DataSet in SQL Server Mgt Studio without errors... I'm not sure where else to look for the cause(s) of this problem which only appears when I run the report-with-subreports?

Answer 1

I tracked this down to an out-of-date .xsd file (DataSet) -- somewhere along the way a table column string width was increased, but the DataSet was not updated or regenerated, so it still had the old width limit on that element, e.g., <xs:maxLength value="50" /> in the .xsd XML instead of the new width of 125 characters. The error was being thrown for those cases where at least one record in the subreport had a data value (string) in that column that exceeded the old width of 50.

An important clue came from adding a handler for the DataSet's .Selected event; I was already using the .Selecting event to set the sub-report's parameter (to tie it to the parent record), but I couldn't see anything useful when breaking in that event. However, examining the event args variable in the .Selected event, after the selection should have occurred, I found an Exception ("Exception has been thrown by the target of an invocation") with an InnerException ("Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints"). There was also a stack trace which indicated the point of failure was executing Adapter.Fill(dataTable).

While this turned out to be pretty misleading -- I had no such constraints in place on the tables involved in the query behind the DataSet -- it at least got me focusing on the specific records in the subreports. After much fruitless searching for anomalies in the subreport record data SQL Server Mgt Studio, I eventually started removing the records one-by-one from one of the offending subreport cases, re-running the report each time to see if I had fixed the error. Eventually I removed a subreport record and the report worked -- the remaining subreport records appeared!

Now I had a specific sub-report record to examine more closely. By chance (wish I could call it inspired intuition...), I decided to edit that record in the web app instead of looking at it as I had been in SQL Server. One of the fields was flagged with an alert saying the string value was too long! That was a mystery to me for a moment: if the string value was too long, how could it already be saved in the database?! I double-checked the column definition in the table, and found it was longer than what the web-app front-end was trying to enforce. I then realized that the column had been expanded without updating the app UI, and I suspected immediately that the .xsd file also had not been updated... Bingo!

There are probably a number of morals to this story, and it leaves me with a familiar and unwelcome feeling that I'm not doing some things as intelligently as I ought. One moral: always update (or better and usually simpler, just re-build) your .xsd DataSet files whenever you change a query or table that its based on... easier said than remembered, however. The queasy feeling I have is that there must be some way that I haven't figured out to avoid building brittle apps where a column width that's defined in the database is also separately coded into the UI and/or code-behind to provide user feedback and/or do data validation... suggestions on how to manage that more robustly are welcome!