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When dealing with workflows from other applications, it s important to add fault handling to the workflows to handle any exceptions or faults that might occur. Workflows can have default Fault Handlers defined for various types of exceptions. These Handlers are called any time a specified exception occurs within the workflow. Create a new VB Sequential Workflow Console Application called VBFaultHandlerSequentialConsoleApplication. By default, a FaultHandlersActivity is already created for a new workflow. The FaultHandlersActivity contains only Fault Handler activities. The Fault Handler activity is a composite activity that is, it s made up of other activities. To view and change the FaultHandlersActivity, find the three tabs at the bottom left of the Workflow Designer. The first of the three tabs (from the left) is the View Workflow tab, which allows you to view the workflow model. The second (middle) tab is the Cancel Handler tab. The third tab is the View Faults tab. Click this tab to get to the Faults area to be able to add activities to a FaultHandlersActivity. First, add a Fault Handler activity from the Toolbox to the FaultHandlersActivity that s already within the designer. Open the properties and change the name of this Fault Handler to GeneralFault. Click the ellipse next to the Fault Type property. This opens a window of all types derived from System.Exception, as shown in Figure 7-6.
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CHAPTER 7 INTERACTING WITH WORKFLOW
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Figure 7-6. Select an exception type. From this list, select the exception that will trigger this Fault Handler. For a general, catch-all exception, select System.Exception from within the mscorlib namespace, as shown in Figure 7-7. Add a Code activity to the GeneralFault Fault Handler activity called GeneralFaultCode. Generate Handlers for this Code activity, and within the ExecuteCode sub add MsgBox("General Fault Error"). Add another Fault Handler activity to the FaultHandersActivity. Call this activity WorkflowTerminatedFault and set the FaultType property to WorkflowTerminatedException from within the System.Workflow.ComponentModel namespace. Add a Code activity to this Fault Handler called TerminatedFault. Generate the Handlers for this Code activity and add MsgBox("Terminated Fault"). Click the first tab on the left at the bottom of the designer to View Workflow. Add a Code activity called FirstActivity and generate Handlers. Within the Handler code add MsgBox("First Activity"): Private Sub FirstActivity_ExecuteCode(ByVal sender As System.Object, ByVal e As System.EventArgs) MsgBox("First Activity") end Sub
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Figure 7-7. Choose System.Exception from the mscorlib namespace. A Throw activity is an activity that throws an exception. You must define the exception the activity will throw, but you don t need to define a Fault Handler for that exception if you don t want to. Using this activity is the same as using the Throw part of a Try . . . Catch . . . Throw code block. This can be useful if you re checking for a specific item. If that item isn t there, you ll want to throw an exception. In this case, the activity also shows how the Fault Handler activity is executed. Leave the name of the Throw activity as the default name, and for the FaultType choose DriveNotFound from mscorlib.System.IO. This is just a random exception to see how the Fault Handler works. Execute the workflow. You ll receive a build failed message. The message states that the WorkflowTerminatedException must be added before the Handler for Exception. The reason for this is the Exception type is the default or catch-all type. When a FaultHandlersActivity is triggered, each Fault Handler is evaluated from left to right. The first Fault Handler activity that matches the exception is the one that s executed. Therefore, if you use the Exception type, which is the catch-all, it must be the last in the list of Fault Handler activities. To fix this problem, view the Faults of the workflow again, and move the WorkflowTerminatedFault activity to the leftmost area within the FaultHandlersActivity, as shown in Figure 7-8.
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