读取Outlook已发送邮件文件夹时遇System.InvalidCastException异常求助

已发送邮件文件夹里并非只有MailItem对象，还可能包含会议请求（MeetingItem）等其他类型的Outlook项，而TypeOf在处理COM包装的System.__ComObject时，无法准确识别对象的实际接口类型，导致类型转换失败。

可以通过以下两种方法解决：

方法一：使用TryCast安全转换 #

Imports Microsoft.Office.Interop
Imports Microsoft.Office.Interop.Outlook 

Dim moApp As Outlook.Application
Dim moNS As Outlook.NameSpace

moApp = New Outlook.Application
moNS = moApp.GetNamespace("MAPI")

Dim oFolder As Outlook.MAPIFolder
oFolder = moNS.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail)

For Each i As Object In oFolder.Items
    ' 用TryCast尝试转换，失败返回Nothing
    Dim mailItem = TryCast(i, Outlook.MailItem)
    If mailItem IsNot Nothing Then
        MessageBox.Show("To: " & mailItem.To)
    End If
Next

方法二：检查对象的Class属性 #

通过Outlook项的Class属性判断类型，OlObjectClass.olMail对应邮件项：

Imports Microsoft.Office.Interop
Imports Microsoft.Office.Interop.Outlook 

Dim moApp As Outlook.Application
Dim moNS As Outlook.NameSpace

moApp = New Outlook.Application
moNS = moApp.GetNamespace("MAPI")

Dim oFolder As Outlook.MAPIFolder
oFolder = moNS.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail)

For Each i As Object In oFolder.Items
    ' 先判断是否为邮件类对象
    If i.Class = Outlook.OlObjectClass.olMail Then
        Dim mailItem = DirectCast(i, Outlook.MailItem)
        MessageBox.Show("To: " & mailItem.To)
    End If
Next

内容的提问来源于stack exchange，提问作者Liliana