Outlook VSTO设置SendUsingAccount仅生效一次的问题排查

我希望Outlook VSTO插件在从“Public”账户发送邮件时，将“发件人”设置为默认账户。首次新建邮件时功能正常，但后续操作失效。

原始代码 #

public partial class ThisAddIn
{
    private Account defaultAcct = null;
    private Inspectors inspectors;
 
    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
        inspectors = this.Application.Inspectors;
        inspectors.NewInspector += new InspectorsEvents_NewInspectorEventHandler(Inspectors_NewInspector);

        Accounts accounts = Application.Session.Accounts;
        foreach (Account account in accounts)
        {
            var displayName = account.DisplayName;
            if(displayName.Contains("example.com"))
            {
                defaultAcct = account;
                break;
            }
        }
    }
    
    void Inspectors_NewInspector(Inspector Inspector)
    {
        if (Inspector.CurrentItem is MailItem)
        {
            MailItem mailItem = Inspector.CurrentItem as MailItem;

            if (mailItem.Sent == false)
            {
                if (mailItem.SendUsingAccount != null)
                {
                    if (mailItem.SendUsingAccount.DisplayName == "Public")
                    {
                        MessageBox.Show("In Public, set from to: " + defaultAcct.DisplayName); // shows every time
                        mailItem.SendUsingAccount = defaultAcct; // Only gets set the first time
                    }
                }

            } 
        }
    }
}

更新说明 #

测试时修改了代码，Subject每次都能正常设置，但SendUsingAccount仍仅首次生效：

void Inspectors_NewInspector(Inspector Inspector)
{
    if (Inspector.CurrentItem is MailItem)
    {
        MailItem mailItem = Inspector.CurrentItem as MailItem;
        mailItem.SendUsingAccount = defaultAcct;
        mailItem.Subject = "test";
    }
}

问题原因及解决方法 #

问题出在NewInspector事件触发时机过早：当Outlook新建邮件时，NewInspector事件先触发，但此时MailItem的SendUsingAccount可能还未被Outlook完全初始化，或者后续Outlook会根据当前选中的账户重新覆盖这个值，导致你的设置被冲掉。而Subject没有这个问题，因为它不会被Outlook自动重置。

解决步骤： #

1. 在NewInspector事件中，不为MailItem直接设置SendUsingAccount，而是绑定它的Open事件
2. 在Open事件处理函数中再执行SendUsingAccount的设置逻辑，此时MailItem已经完全加载，设置不会被覆盖
3. 为避免重复绑定事件，可使用字典记录已处理过的MailItem

修改后的代码示例 #

public partial class ThisAddIn
{
    private Account defaultAcct = null;
    private Inspectors inspectors;
    // 记录已绑定Open事件的MailItem，避免重复绑定
    private Dictionary<MailItem, MailItemEvents_OpenEventHandler> _mailItemEvents = new Dictionary<MailItem, MailItemEvents_OpenEventHandler>();
 
    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
        inspectors = this.Application.Inspectors;
        inspectors.NewInspector += Inspectors_NewInspector;

        Accounts accounts = Application.Session.Accounts;
        foreach (Account account in accounts)
        {
            if(account.DisplayName.Contains("example.com"))
            {
                defaultAcct = account;
                break;
            }
        }
    }
    
    void Inspectors_NewInspector(Inspector Inspector)
    {
        if (Inspector.CurrentItem is MailItem mailItem && !mailItem.Sent)
        {
            // 检查是否已绑定过事件
            if (!_mailItemEvents.ContainsKey(mailItem))
            {
                MailItemEvents_OpenEventHandler openHandler = null;
                openHandler = (sender, ref bool cancel) =>
                {
                    var currentItem = sender as MailItem;
                    if (currentItem != null && defaultAcct != null)
                    {
                        // 检查当前发件账户是否为Public
                        if (currentItem.SendUsingAccount != null && currentItem.SendUsingAccount.DisplayName == "Public")
                        {
                            currentItem.SendUsingAccount = defaultAcct;
                        }
                        // 解绑事件，避免内存泄漏
                        currentItem.Open -= openHandler;
                        _mailItemEvents.Remove(currentItem);
                    }
                };
                mailItem.Open += openHandler;
                _mailItemEvents.Add(mailItem, openHandler);
            }
        }
    }

    private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
    {
        // 清理事件绑定
        foreach (var kvp in _mailItemEvents)
        {
            kvp.Key.Open -= kvp.Value;
        }
        _mailItemEvents.Clear();
    }
}

额外注意点 #

• 确保defaultAcct正确获取到了目标账户，可在Startup中加日志或弹窗验证
• 处理Outlook的对象生命周期，及时解绑事件避免内存泄漏
• 若使用的是Outlook 2016及以上版本，也可以尝试监听Application.ItemLoad事件，逻辑类似但触发时机更晚

内容的提问来源于stack exchange，提问作者Barry Ralphs