CodeQL documentation

Rethrowing exception variable

ID: cs/rethrown-exception-variable
Kind: problem
Severity: warning
Precision: very-high
Tags:
   - maintainability
   - language-features
   - exceptions
Query suites:
   - csharp-security-and-quality.qls

Click to see the query in the CodeQL repository

Rethrowing an exception variable will lose the stack trace in the original exception, and replace it with the stack trace from the throw statement. This will make debugging the root cause of the exception more difficult, for example if the stack trace is written to a log file.

Recommendation

Consider using throw; to rethrow the original exception. Not only is this simpler, but it will retain the original stack information.

Example

This example shows an exception handler which sets the status to UnexpectedException if an exception is thrown. However it throws ex which discards the original stack trace containing the source of the error.

try
{
  Run();
status = Status.Success;
}
catch (Exception ex)
{
  status = Status.UnexpectedException;
  throw ex;    // BAD
}

The fix is to replace the throw statement as follows:

try
{
  Run();
status = Status.Success;
}
catch
{
  status = Status.UnexpectedException;
  throw;    // GOOD
}

Since the variable ex is no longer needed, the catch variable has been removed.

References

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