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StartRuntime StopRuntime
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There are more methods associated with WorkflowRuntime, but the methods shown in Table 2-2 are the ones most commonly used and the ones we ll focus on both here and in the remainder of the book. There are also a number of events WorkflowRuntime will raise at various times during workflow execution, but we ll examine those a bit later in the chapter.
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Part I
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Introducing Windows Workflow Foundation (WF)
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Basically, then, working with WorkflowRuntime involves calling a few simple methods and handling some events of interest. There is a significant limitation WorkflowRuntime imposes, however, which we ll look at next.
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Building a Workflow Runtime Factory
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I mentioned this previously in the chapter, but it is important enough to mention again there can be only a single instance of WorkflowRuntime per AppDomain. And because the majority of .NET applications use only a single AppDomain, it necessarily follows that you can generally use only a single instance of WorkflowRuntime in your application. Whenever I hear use only a single instance, I naturally think of using a combination of the singleton and factory patterns. The singleton pattern, if you re unfamiliar with patterns, is simply a mechanism for assuring that no matter how many times your application requests instances of the singleton object, only one instance of the singleton is ever given out. This is typically done for objects that are considered expensive to create, such as objects that consume a large number of resources or take a significant amount of time to be created. The concept of a singleton, which is to say only a single object is ever created and handed to your application, dovetails nicely with the factory pattern. The factory pattern involves an intermediate object that s used to create instances of other objects. Most of us, for example, don t build our own cars. Instead, we purchase them from the automobile manufacturer, at least indirectly. (Many of us, I m sure, wish we could buy them directly!) The combination of the singleton and factory is powerful because the factory can make sure only a single instance of the singleton object is ever created. This is perfect for our needs, because within our application it s entirely possible that different pieces of the application might try to load and start the workflow runtime (independent application modules, for instance). Let s see how we might create a WorkflowRuntime factory. Creating the WorkflowRuntime factory object 1. We ll need to add a new class to our WorkflowHost poject. To do that, right-click on the project name (WorkflowHost) in the Visual Studio Solution Explorer and select Class from the Add menu item. Tip
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result. Selecting Add Class from the Visual Studio Project menu achieves the same
2
The Workflow Runtime
2. The Add New Item dialog box should now appear, and because we requested that a new class be created, the Class item in the Templates pane should already be selected. Therefore, we ll only need to name the source file (which indirectly also names the object we re creating). Type WorkflowFactory.cs into the Name field and click Add.
3. As we did with the main application source file, Program.cs, we ll need to add the using directive for the workflow assembly to the top of the WorkflowFactory source file. The WorkflowFactory source file should be open for editing because we just created it, but if not, open it for editing using the techniques we used for opening Program.cs in the previous chapter. To add the using directive, locate this code at the top of the WorkflowFactory.cs file:
using System; using System.Collections.Generic; using System.Text;
Part I
Introducing Windows Workflow Foundation (WF)
After the using directive for System.Text, add the following line:
using System.Workflow.Runtime;
4. The using directive introduces the workflow runtime assembly to our source file, but it does little more. We need to add the code to represent the singleton object to the WorkflowFactory class. To do that, locate the WorkflowFactory class definition:
class WorkflowFactory { }
Not much of a class yet! But we ll fix that. Just after the opening curly brace of the class definition, add these lines of code:
// Singleton instance of the workflow runtime. private static WorkflowRuntime _workflowRuntime = null;| // Lock (sync) object. private static object _syncRoot = new object();
5. Notice that the field _workflowRuntime is initialized to null. Our factory will sense this and create a new instance of WorkflowRuntime. If workflowRuntime is not null, our factory won t create a new instance but will hand out the existing instance. To do this, we ll need to add a method designed to create and return our singleton object. Moreover, we ll make the method static so that objects requesting the workflow runtime object don t need to create instances of the factory. To do this, we ll add the following code just after the _syncRoot field:
// Factory method. public static WorkflowRuntime GetWorkflowRuntime() { // Lock execution thread in case of multi-threaded // (concurrent) access. lock (_syncRoot) { // Check for startup condition. if (null == _workflowRuntime) { // Not started, so create instance. _workflowRuntime = new WorkflowRuntime(); } // if // Return singleton instance. return _workflowRuntime; } // lock }
6. Almost there! When the Visual Studio class template builds a new class, it omits the public keyword on the class definition, making it a private class. Because we want other classes to be able to request instances of WorkflowRuntime, we ll need to make the factory class public. While we re at it, we ll also mark the class as static to prevent direct
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