Outlook vb.net如何处理SentItems文件夹的ItemAdd事件

Question

I am trying to handle the ItemAdd event fired when a new item gets added to the SentItems folder in a VB.net vsto add-in. When I try this:

Private WithEvents mySentItems As Outlook.Items
mySentItems = Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail).Items

I get a Declaration Expected error on the second line, which I find super bizarre since I thought I JUST declared it.

If I do this:

 Private WithEvents mySentItems As Outlook.Items = Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail).Items

The add-in compiles but then outlook gets really angry and won't even load the add in after throwing this exception:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> 
System.NullReferenceException: Object reference not set to an instance of an object.

I am doing all of this just after the ThisAddin class declaration before any of the class subs are declared.

Thanks for any help you may be able to provide.

Answer 1

It seems you cannot make assignments outside of a sub or function. I moved the line

mySentItems = Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail).Items

to the ThisAddin_Startup sub and it worked as expected.

Thanks for looking into this if you did, and honestly I don't do this to just post my own answer, I hand't managed to solve this for a day before I finally asked the question.

Answer 2

You can use the following declarations in the code after all Outlook objects are initialized, looks like a property or method returns null (Nothing in VB.NET):

 Private WithEvents mySentItems As Outlook.Items = Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail).Items

Another aspect is that multiple dots are used in the single line of code. It is hard to understand what property or method exactly fires an exception until you break the chain and declare each property and method on a single line of code.

Moreover, you don't release underlying COM objects instantly. Use System.Runtime.InteropServices.Marshal.ReleaseComObject to release an Outlook object when you have finished using it. Then set a variable to Nothing in Visual Basic (null in C#) to release the reference to the object. Read more about that in the Systematically Releasing Objects article.