Uniquely identify Mailitem - outlook

Question

I need to store a model for every used MailItem. For this I've written following Method
private readonly static Dictionary<string, PermitCustomPaneViewmodel> ViewmodelLookup = new Dictionary<string, PermitCustomPaneViewmodel>();
public static PermitCustomPaneViewmodel CreateOrGet(MailItem c)
{
if (c.EntryID == null)
c.Save();
if (!ViewmodelLookup.ContainsKey(c.EntryID))
{
var vm = new PermitCustomPaneViewmodel(c);
c.Unload += () => ViewmodelLookup.Remove(c.EntryID);
ViewmodelLookup.Add(c.EntryID, vm);
}
return ViewmodelLookup[c.EntryID];
}
When the Model already exists, I look it up and return it. If it was not created, I create it and remove the entry after the MailItem will be unloaded.
However I have observed that the MailItem object will not be vailid all the time untill unload is called. In order to reliable identify the MailItem I used the EntryID. The problem now is this only works if the Item is saved.
So currently I save the Item if no EntryID was found. But this automaticly saves the item under draft.
Is there a way to distingush MailItem's that is not saved in a way so it can be used in a Dictionary<,>.

Answer 1

New created items don't have the EntryID property set. Get the ID assigned by the store provider you must save it. If you need to identify a new MailItem object you may consider adding a user property to the item by using the UserProperties.Add method which reates a new user property in the UserProperties collection. For example:
Sub AddUserProperty()
Dim myItem As Outlook.ContactItem
Dim myUserProperty As Outlook.UserProperty
Set myItem = Application.CreateItem(olContactItem)
Set myUserProperty = myItem.UserProperties _
.Add("LastDateSpokenWith", olDateTime)
myItem.Display
End Sub
Be aware, the Entry ID changes when an item is moved into another store, for example, from your Inbox to a Microsoft Exchange Server public folder, or from one Personal Folders (.pst) file to another .pst file. Solutions should not depend on the EntryID property to be unique unless items will not be moved. Basically it works fine as long as the message is staying in its parent folder or it may be changed if the Outlook item is moved to a different folder (depends on the store provider).
You may also consider using the message id from the message MIME header (PR_INTERNET_MESSAGE_ID and PR_TRANSPORT_MESSAGE_HEADERS). But they are not set on newly created items. These properties are available on the message received from an SMTP server or through the SMTP connector.