无法转换'System .__ ComObject'类型的COM对象

Question

当我在代码下面进行投射时，我收到了错误

代码：

string subject = ((Microsoft.Office.Interop.Outlook.MailItem)myInbox.Items[temp])
    .Subject.ToString();

  

无法将“System .__ ComObject”类型的COM对象强制转换为接口   输入'Microsoft.Office.Interop.Outlook.MailItem'。这个操作   失败，因为QueryInterface调用了COM组件   与IID'{00063034-0000-0000-C000-000000000046}'的接口失败   出现以下错误：未支持此类接口（例外情况）   HRESULT：0x80004002（E_NOINTERFACE））。

Answer 1

您需要先检查项目类型。 Outlook文件夹可能包含各种类型的项目：

Object selObject = this.Application.ActiveExplorer().Selection[1];
        if (selObject is Outlook.MailItem)
        {
            Outlook.MailItem mailItem =
                (selObject as Outlook.MailItem);
            itemMessage = "The item is an e-mail message." +
                " The subject is " + mailItem.Subject + ".";
            mailItem.Display(false);
        }
        else if (selObject is Outlook.ContactItem)
        {
            Outlook.ContactItem contactItem =
                (selObject as Outlook.ContactItem);
            itemMessage = "The item is a contact." +
                " The full name is " + contactItem.Subject + ".";
            contactItem.Display(false);
        }
        else if (selObject is Outlook.AppointmentItem)
        {
            Outlook.AppointmentItem apptItem =
                (selObject as Outlook.AppointmentItem);
            itemMessage = "The item is an appointment." +
                " The subject is " + apptItem.Subject + ".";
        }
        else if (selObject is Outlook.TaskItem)
        {
            Outlook.TaskItem taskItem =
                (selObject as Outlook.TaskItem);
            itemMessage = "The item is a task. The body is "
                + taskItem.Body + ".";
        }
        else if (selObject is Outlook.MeetingItem)
        {
            Outlook.MeetingItem meetingItem =
                (selObject as Outlook.MeetingItem);
            itemMessage = "The item is a meeting item. " +
                 "The subject is " + meetingItem.Subject + ".";
        }

有关详细信息，请参阅How to: Programmatically Determine the Current Outlook Item。

Answer 2

string subject = myInbox.Items[temp])
    .Subject.ToString();

没有必要使用frist检查myinbox对象的主题属性是非字符串，然后如果它是字符串格式，则需要将其强制转换，然后不需要强制转换它。