【代码保留】WorkflowRuntimeFactory

由于工作流运行时WorkflowRuntime在一个应用程序域中只允许存在一个实例，因此可以用单件Singleton模式来限制，并可以用工厂模式来调用，另者由于运行时需要有Start和Stop，因此也可以在工厂内添加相应的方法来启动和关闭WorkflowRuntime.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Workflow.Runtime;

namespace WorkflowHost
{
    public static class WorkflowFactory
    {
        //Singleton instance of the workflow runtime.
        private static WorkflowRuntime _workflowRuntime = null;
        //Lock (sync)object.
        private static object _syncRoot = new object();

        //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)
                {
                    //Provide for shutdown
                    AppDomain.CurrentDomain.ProcessExit += new EventHandler(StopWorkflowRuntime);
                    AppDomain.CurrentDomain.DomainUnload += new EventHandler(StopWorkflowRuntime);
                    //Not started, so create instance.
                    _workflowRuntime = new WorkflowRuntime();
                    //Start the runtime.
                    _workflowRuntime.StartRuntime();
                }//if

                //Return singleton instance.
                return _workflowRuntime;
            }//lock
        }

        //Shutdown method
        static void StopWorkflowRuntime(object sender, EventArgs e)
        {
            if (_workflowRuntime != null)
            {
                if (_workflowRuntime.IsStarted)
                {
                    try
                    {
                        //Stop the runtime
                        _workflowRuntime.StopRuntime();
                    }
                    catch (ObjectDisposedException)
                    { 
                        //Already disposed of, so ignore
                    }//catch
                }//if
            }//if
        }
    }
}