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How to prevent the debugger from intercepting an exception meant for someone else Level 300
DevForce Express		 May 9, 06
Problem:
We write a validation method that should throw an exception if the test fails. The caller will intercept this exception and display a suitable message. Unfortunately, when we run it in Visual Studio, the debugger stops where the code throws the exception, before the caller has a chance to deal with it. We wonder frantically what is wrong before realizing that we can continue executing. Isn't there a way to tell the debugger to let that exception through? Yes there is.
Analysis:
Here's an example from a DevForce application. Our LoginManager tests if the username and password are valid. If not, it will throw an exception which the client Login Dialog should catch and display as shown here:
 

C#:

private static void VerifyCredentials(SecureUser pUser,
  string pUsername, string pPassword, string pDomain) {

  if (pUser.IsNullEntity) {
   throw new LoginException(
    LoginExceptionType.InvalidUserName, null, pUsername);
  }

  if (pPassword != pUser.UserPassword) {
   throw new LoginException(
LoginExceptionType.InvalidPassword, null, pUsername);
  }
}

VB .Net:

Private Shared Sub VerifyCredentials(ByVal pUser As SecureUser, _
  ByVal pUsername As String, ByVal pPassword As String, _
  ByVal pDomain As String)

  If (pUser.IsNullEntity) Then
  Throw New LoginException( _
    LoginExceptionType.InvalidUserName, Nothing, pUsername)
  End If

  If (Not pPassword = pUser.UserPassword) Then
  Throw New LoginException( _
    LoginExceptionType.InvalidPassword, Nothing, pUsername)
  End If
End Sub

We run it within Visual Studio, configured for debugging. We deliberately enter the incorrect username. The debugger halts unexpectedly in the middle of the LoginManager like so:

This message is destined for the client but we're stuck here inside LoginManager.

If we understand what is happening, we know we can press [F5] to continue. But this halt is disconcerting at the least and more often than not we'll waste a few hours rooting around for a bug that is not there.

Solution:

Let's add an attribute to our method:

C#:

  // Let the client catch exceptions.
  [System.Diagnostics.DebuggerHidden()]
  private static void VerifyCredentials(SecureUser pUser,

VB .Net:

  ' Let the client catch exceptions.
  <System.Diagnostics.DebuggerHidden()> _
  Private Shared Sub VerifyCredentials(ByVal pUser As SecureUser, _

There is progress: the debugger no longer stops us inside the VerifyCredentials method.

But now the debugger stops us in the calling method, Login. The attribute was only effective for the VerifyCredentials method, not its caller.

Unfortunately, we can't decorate the call to VerifyCredentials with the attribute.

  // It won't compile. We can't decorate method call with an attribute.
  [System.Diagnostics.DebuggerHidden()]
  VerifyCredentials(aUser, username, password, domain);

But we can add the attribute to the calling method:

In C#:

  // Let the client catch exceptions.
  [System.Diagnostics.DebuggerHidden()]
  public IPrincipal Login(ILoginCredential pCredential, ...

VB .Net:

  ' Let the client catch exceptions.
  <System.Diagnostics.DebuggerHidden()> _
  Public Function Login(ByVal pCredential As ICredential, _

Now when we run it, the debugger lets the exception escape both VerifyCredentials and Login. The Login Dialog catches it and displays the message as intended, as shown at the top of this tip.

Caveat: You will not be able to debug into a method tagged with this attribute! Remember to comment out the attribute before setting a breakpoint in Login or VerifyCredentials.


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