Loading and Unloading Instances in C#.NET

Generator GTIN - 12 in C#.NET Loading and Unloading Instances

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9. Once the load event handler is inserted, Visual Studio will switch you to the code view for the main application form. Because we just added the form s Load event handler, we might as well add the initialization code we ll need. Type the following into the Form1_Load handler method:

_runtime = WorkflowFactory.GetWorkflowRuntime(); _runtime.WorkflowCompleted += new EventHandler<WorkflowCompletedEventArgs>(Runtime_WorkflowCompleted); _runtime.WorkflowTerminated += new EventHandler<WorkflowTerminatedEventArgs>(Runtime_WorkflowTerminated);

We ve seen code like this before that creates the workflow runtime and hooks some of the major events we ll be interested in intercepting. 10. Somewhere we need to declare the _runtime field, so look for the opening brace for the Form1 class. After the opening brace, type this:

protected WorkflowRuntime _runtime = null; protected WorkflowInstance _instance = null;

11. If you try to compile the application at this point, it won t compile. We ll need to add a reference to the Windows Workflow Foundation assemblies as we ve done in previous chapters that process is the same whether we re building a graphical user interface or a console-based application. So add the workflow assembly references for System.Workflow.Runtime, System.Workflow.ComponentModel, and System.Workflow.Activity and then insert the following using declaration at the top of the source file following the other using declarations:

using System.Workflow.Runtime;

12. Although we now have an application that hosts the workflow runtime, it doesn t actually do anything. To make it functional, we ll need to add some code to the button event handlers, starting with button1_Click. Scroll through the main application form s source file until you find button1_Click, and add this code:

button2.Enabled = true; button1.Enabled = false; _instance = _runtime.CreateWorkflow(typeof(PersistedWorkflow.Workflow1)); _instance.Start();

Introducing Windows Workflow Foundation (WF)

This code disables the Start Workflow button, enables the Unload Workflow button, and then starts a new workflow instance. (We ll add the workflow that it will execute shortly.) 13. Next, find the Unload Workflow button s handler, button2_Click, and add the following code. Here, we re using the WorkflowInstance.Unload method to unload the workflow instance and write it to our database. After the workflow instance unloads, we enable the Load Workflow button (the code for which we ll add in the next section). Note that if we sustain an exception while unloading the workflow instance, the Load Workflow button is not enabled. This makes sense...there would be nothing to load if the unload request failed.

button2.Enabled = false; try { _instance.Unload(); button3.Enabled = true; } // try catch (Exception ex) { MessageBox.Show(String.Format("Exception while unloading workflow" + " instance: '{0}'",ex.Message)); } // catch

I mentioned this previously in the chapter, but it s an important point. Keep in mind that WorkflowInstance.Unload is synchronous. That means the thread making the attempt to unload the workflow instance will block (wait) and continue to be blocked until the operation has completed (the instance has unloaded or failed to unload). In this case, that s precisely the behavior I want because I don t want to repeatedly ask the instance whether it unloaded. But in some cases, you ll want to use the nonblocking alternative I mentioned, WorkflowInstance.TryUnload. Later, when you add the final pieces of code and run this application, as you click Unload Workflow, watch closely and you ll see the application freeze briefly as it waits for the workflow to unload.

14. Now we turn our attention to the workflow event handlers, Runtime_WorkflowCompleted and Runtime_WorkflowTerminated. Both of these event handlers will actually perform the same action, which is to reset the application in preparation for another workflow instance execution. Add these methods following the click event handler for button2 (the method containing the code we added in the preceding step):

void Runtime_WorkflowCompleted(object sender, WorkflowCompletedEventArgs e){ WorkflowCompleted(); } void Runtime_WorkflowTerminated(object sender, WorkflowTerminatedEventArgs e) { WorkflowCompleted(); }