wf HandleExternalEventActivity

using System;
using System.Threading;
using System.Windows.Forms;
using System.Workflow.Runtime;
using System.Workflow.Activities;
namespace Microsoft.Samples.Workflow.Communication.HostCommunication
{
    // Class defines the message passed between the local service and the workflow
    [Serializable]
    internal class VotingServiceEventArgs : ExternalDataEventArgs
    {
        private string aliasValue;
        public VotingServiceEventArgs(Guid instanceID, string alias)
            : base(instanceID)
        {
            this.aliasValue = alias;
        }
        public string Alias
        {
            get 
            {
                return this.aliasValue;
            }
        }
    }
    // Workflow communication interface which defines the contract
    // between a local service and a workflow
    [ExternalDataExchange]
    internal interface IVotingService
    {
        event EventHandler<VotingServiceEventArgs> ApprovedProposal;
        event EventHandler<VotingServiceEventArgs> RejectedProposal;
        void CreateBallot(string alias);
    }
    // Local service that implements the contract on the host side
    // i.e. it implements the methods and calls the events, which are
    // implemented by the workflow
    internal class VotingServiceImpl : IVotingService
    {
        public event EventHandler<VotingServiceEventArgs> ApprovedProposal;
        public event EventHandler<VotingServiceEventArgs> RejectedProposal;
        // Called by the workflow to create a new ballot, this method
        // creates a new thread which shows a voting dialog to the user
        public void CreateBallot(string alias)
        {
            Console.WriteLine("Ballot created for {0}.", alias);
            ShowVotingDialog(new VotingServiceEventArgs(WorkflowEnvironment.WorkflowInstanceId, alias));
        }
        public void ShowVotingDialog(VotingServiceEventArgs votingEventArgs)
        {
            DialogResult result;
            string alias = votingEventArgs.Alias;
            // Show the voting dialog to the user and depending on the response
            // raise the ApproveProposal or RejectProposal event back to the workflow
            result = MessageBox.Show(string.Format("Approve Proposal, {0}?", alias), string.Format("{0} Ballot", alias), MessageBoxButtons.YesNo);
            if (result == DialogResult.Yes)
            {
                EventHandler<VotingServiceEventArgs> approvedProposal = this.ApprovedProposal;
                if (approvedProposal != null)
                    approvedProposal(null, votingEventArgs);
            }
            else
            {
                EventHandler<VotingServiceEventArgs> rejectedProposal = this.RejectedProposal;
                if (rejectedProposal != null)
                    rejectedProposal(null, votingEventArgs);
            }
        }
    }
}

在这个文件中首先定义了interface IVotingService，那么这个接口的作用是什么呢？接口定义了在工作流和本地交互时，使用的数据或者是函数，接口中定义的event可能是直接在接口中定义的或者是在workflow中定义的。

event EventHandler<VotingServiceEventArgs> ApprovedProposal;
event EventHandler<VotingServiceEventArgs> RejectedProposal;
void CreateBallot(string alias);

然后在这个文件中定义了一个实现类，将上面的接口实现，interface只是在说我能够干什么？但是实现类却是真正工作的类（说白了就是实际的worker）。

    internal class VotingServiceImpl : IVotingService
    {
        public event EventHandler<VotingServiceEventArgs> ApprovedProposal;
        public event EventHandler<VotingServiceEventArgs> RejectedProposal;

        // Called by the workflow to create a new ballot, this method
        // creates a new thread which shows a voting dialog to the user
        public void CreateBallot(string alias)
        {
            Console.WriteLine("Ballot created for {0}.", alias);
            ShowVotingDialog(new VotingServiceEventArgs(WorkflowEnvironment.WorkflowInstanceId, alias));
        }

        public void ShowVotingDialog(VotingServiceEventArgs votingEventArgs)
        {
            DialogResult result;
            string alias = votingEventArgs.Alias;

            // Show the voting dialog to the user and depending on the response
            // raise the ApproveProposal or RejectProposal event back to the workflow
            result = MessageBox.Show(string.Format("Approve Proposal, {0}?", alias), string.Format("{0} Ballot", alias), MessageBoxButtons.YesNo);
            if (result == DialogResult.Yes)
            {
                EventHandler<VotingServiceEventArgs> approvedProposal = this.ApprovedProposal;
                if (approvedProposal != null)
                    approvedProposal(null, votingEventArgs);
            }
            else
            {
                EventHandler<VotingServiceEventArgs> rejectedProposal = this.RejectedProposal;
                if (rejectedProposal != null)
                    rejectedProposal(null, votingEventArgs);
            }
        }
    }

所以说整个的过程是这样的：

1.首先需要定义一个接口，主要适用于workflow和本地服务通信使用。

    [ExternalDataExchange]
    internal interface IVotingService
    {
        event EventHandler<VotingServiceEventArgs> ApprovedProposal;
        event EventHandler<VotingServiceEventArgs> RejectedProposal;
        void CreateBallot(string alias);
    }

2.定义一个实现类，实现接口中定义的方法。

3.最工作流中使用HandleExternalEventActivity活动时，该活动可以invoke一个函数，当特定的外部事件触发时，调用这个invoke的函数。