TypeName
DoNotRaiseExceptionsInFilterBlocks
CheckId
CA2219
Category
Microsoft.Usage
Breaking Change
NonBreaking
A method contains a filtered exception handler, and the filter raises exceptions.
When an exception filter raises an exception, the exception is caught by the common language runtime, and the filter returns false. This behavior is indistinguishable from the filter executing and returning false intentionally.
Currently, Visual Basic .NET and the Visual C++ support exception filters.
To fix a violation of this rule, do not raise exceptions in an exception filter.
Do not exclude a warning from this rule. There are no scenarios under which an exception raised by an exception filter provides a benefit to the executing code.
The following example shows a type that violates this rule. Note that the filter appears to return false when it throws an exception.
Imports System Module ExceptionFilter Public Class FilterTest ' The following function is used as an exception filter. ' Violates rule: DoNotRaiseExceptionsInFilterBlocks. Public Function AlwaysTrueFilter(label as String) as Boolean Console.WriteLine("In filter for {0}.", label) ' The following exception terminates the filter. ' The common language runtime does not return the exception to the user. Throw New ApplicationException("Filter generated exception.") Return True End Function Public Sub ThrowException() Try Throw New ApplicationException("First exception.") ' Because the filter throws an exception, this catch clause fails to catch the exception. Catch e as ApplicationException When Not AlwaysTrueFilter("First") Console.WriteLine("Catching first filtered ApplicationException {0}", e) ' Because the previous catch fails, this catch handles the exception. Catch e as ApplicationException Console.WriteLine("Catching any ApplicationException - {0}", e.Message) End Try ' The behavior is the same when the filter test is reversed. Try Throw New ApplicationException("Second exception.") ' Change the filter test from When Not to When. ' This catch fails. Catch e as ApplicationException When AlwaysTrueFilter("Second") Console.WriteLine("Catching second filtered ApplicationException {0}", e) ' This catch handles the exception. Catch e as ApplicationException Console.WriteLine("Catching any ApplicationException - {0}", e.Message) End Try End Sub End Class Sub Main() Dim test as FilterTest = New FilterTest() test.ThrowException() End Sub End Module
The example produces the following output.
In filter for First. Catching any ApplicationException - First exception. In filter for Second. Catching any ApplicationException - Second exception.
This check can result in false positives. For example, there appears to be no way to write this code without throwing raising this warning.
Try m_Listener.Start() Catch ex As HttpListenerException When ex.Message IsNot Nothing AndAlso ex.Message = "The process cannot access the file because it is being used by another process" Throw New AsyncListenerException("Could not start Async Listener on " & prefix & "." & vbCrLf & "Check to see if IIS is running.", ex) End Try