VSTO Outlook插件处理收件人时偶发InvalidComObjectException异常排查

问题背景 #

开发VSTO Outlook加载项，在邮件撰写窗口添加按钮移除特定收件人，按钮点击事件及辅助代码如下：

按钮点击事件代码 #

private void ClickButton(object sender, System.Windows.RoutedEventArgs e)
{
    List<Outlook.Recipient> toList = new List<Outlook.Recipient>();
    List<Outlook.Recipient> CCList = new List<Outlook.Recipient>();
    List<Outlook.Recipient> BCCList = new List<Outlook.Recipient>();

    Outlook.Recipient recipient = null;
    Outlook.Recipients recipients = this.MyMailItem?.Recipients;
    for (int i = recipients?.Count ?? 1; i > 0; i -= 1)
    {
        recipient = recipients[i];
        if (!this.recipientsList.Contains(recipient))
        {
            if (recipient.Type == (int)Outlook.OlMailRecipientType.olTo)
            {
                toList.Add(recipient);
            }
            else if (recipient.Type == (int)Outlook.OlMailRecipientType.olCC)
            {
                CCList.Add(recipient);
            }
            else if (recipient.Type == (int)Outlook.OlMailRecipientType.olBCC)
            {
                BCCList.Add(recipient);
            }
        }

        Marshal.ReleaseComObject(recipient);
        recipient = null;
    }

    this.MyMailItem.To = null;
    this.MyMailItem.CC = null;
    this.MyMailItem.BCC = null;

    if (toList != null && toList.Count > 0)
    {
        MyMailItem.To = string.Join(";", this.GetRecipientsAsString(toList));
    }

    if (CCList != null && CCList.Count > 0)
    {
        MyMailItem.CC = string.Join(";", this.GetRecipientsAsString(CCList));
    }

    if (BCCList != null && BCCList.Count > 0)
    {
        MyMailItem.BCC = string.Join(";", this.GetRecipientsAsString(BCCList));
    }

    this.recipientsList.Clear();
}

辅助方法代码 #

private List<string> GetRecipientsAsString(List<Outlook.Recipient> recipientsList)
{
    List<string> recList = null;
    if (recipientsList?.Count > 0)
    {
        recList = new List<string>();
        foreach (Outlook.Recipient recipient in recipientsList)
        {
            recList.Add(string.IsNullOrWhiteSpace(recipient.Name) ? recipient.Address : recipient.Name);
        }
    }

    return recList;
}

异常信息 #

非必现抛出异常：

COM object that has been separated from its underlying RCW cannot be used.

异常触发位置：GetRecipientsAsString方法中访问recipient.Name或recipient.Address的代码行。

错误原因 #

核心问题是COM对象释放时机错误：

• 在循环中，你将recipient添加到toList/CCList/BCCList后，立刻调用Marshal.ReleaseComObject(recipient)释放了该COM对象的RCW（Runtime Callable Wrapper）。
• 后续调用GetRecipientsAsString时，尝试访问已被释放的Recipient对象的属性，此时RCW已与底层COM对象分离，触发异常。
• 非必现是因为.NET垃圾回收器的时机不确定，偶尔可能还未回收RCW，但大部分场景下释放后无法再访问。

解决方案 #

方案1：延迟释放COM对象 #

将Marshal.ReleaseComObject的调用时机延后，直到所有使用Recipient对象的操作完成后，再统一释放列表中的所有对象：

修改后的点击事件代码：

private void ClickButton(object sender, System.Windows.RoutedEventArgs e)
{
    List<Outlook.Recipient> toList = new List<Outlook.Recipient>();
    List<Outlook.Recipient> CCList = new List<Outlook.Recipient>();
    List<Outlook.Recipient> BCCList = new List<Outlook.Recipient>();

    Outlook.Recipient recipient = null;
    Outlook.Recipients recipients = this.MyMailItem?.Recipients;
    if (recipients == null) return;

    for (int i = recipients.Count; i > 0; i -= 1)
    {
        recipient = recipients[i];
        if (!this.recipientsList.Contains(recipient))
        {
            switch ((Outlook.OlMailRecipientType)recipient.Type)
            {
                case Outlook.OlMailRecipientType.olTo:
                    toList.Add(recipient);
                    break;
                case Outlook.OlMailRecipientType.olCC:
                    CCList.Add(recipient);
                    break;
                case Outlook.OlMailRecipientType.olBCC:
                    BCCList.Add(recipient);
                    break;
            }
        }
        // 暂时不释放，保留引用供后续使用
    }

    this.MyMailItem.To = null;
    this.MyMailItem.CC = null;
    this.MyMailItem.BCC = null;

    List<string> toStrings = GetRecipientsAsString(toList);
    List<string> ccStrings = GetRecipientsAsString(CCList);
    List<string> bccStrings = GetRecipientsAsString(BCCList);

    if (toStrings?.Count > 0)
    {
        MyMailItem.To = string.Join(";", toStrings);
    }

    if (ccStrings?.Count > 0)
    {
        MyMailItem.CC = string.Join(";", ccStrings);
    }

    if (bccStrings?.Count > 0)
    {
        MyMailItem.BCC = string.Join(";", bccStrings);
    }

    // 统一释放所有Recipient对象
    foreach (var rec in toList.Concat(CCList).Concat(BCCList))
    {
        Marshal.ReleaseComObject(rec);
    }
    toList.Clear();
    CCList.Clear();
    BCCList.Clear();

    this.recipientsList.Clear();
}

方案2：直接存储字符串（更推荐） #

避免持有Recipient COM对象引用，在循环中直接提取需要的名称/地址存入字符串列表，彻底消除COM对象生命周期问题：

修改后的代码（可移除GetRecipientsAsString方法）：

private void ClickButton(object sender, System.Windows.RoutedEventArgs e)
{
    List<string> toList = new List<string>();
    List<string> CCList = new List<string>();
    List<string> BCCList = new List<string>();

    Outlook.Recipient recipient = null;
    Outlook.Recipients recipients = this.MyMailItem?.Recipients;
    if (recipients == null) return;

    for (int i = recipients.Count; i > 0; i -= 1)
    {
        recipient = recipients[i];
        try
        {
            if (!this.recipientsList.Contains(recipient))
            {
                // 直接提取字符串，无需持有COM对象引用
                string recStr = string.IsNullOrWhiteSpace(recipient.Name) ? recipient.Address : recipient.Name;
                switch ((Outlook.OlMailRecipientType)recipient.Type)
                {
                    case Outlook.OlMailRecipientType.olTo:
                        toList.Add(recStr);
                        break;
                    case Outlook.OlMailRecipientType.olCC:
                        CCList.Add(recStr);
                        break;
                    case Outlook.OlMailRecipientType.olBCC:
                        BCCList.Add(recStr);
                        break;
                }
            }
        }
        finally
        {
            // 立即释放COM对象，避免内存泄漏
            Marshal.ReleaseComObject(recipient);
            recipient = null;
        }
    }

    this.MyMailItem.To = toList.Count > 0 ? string.Join(";", toList) : null;
    this.MyMailItem.CC = CCList.Count > 0 ? string.Join(";", CCList) : null;
    this.MyMailItem.BCC = BCCList.Count > 0 ? string.Join(";", BCCList) : null;

    this.recipientsList.Clear();
}

内容的提问来源于stack exchange，提问作者Willy