Loop through all recipients in the `MailItem.Recipients` collection. For each recipient if the `Recipient.AddressEntry` property is not null, read the `AddressEntry.Members` property (returns AddressList object). If it is not null, recursively process each member of the list.

Like this:



    Private Function GetAllRecipients(ByVal mailItem As MailItem) As List(Of String)
        Dim allRecipients As New List(Of String)
        For Each rec As Recipient In mailItem.Recipients
            If rec.AddressEntry IsNot Nothing Then
                AddAddressEntryMembers(allRecipients, rec.AddressEntry)
            End If
        Next
        Return allRecipients.Distinct.ToList()
    End Function

    Private Sub AddAddressEntryMembers(ByRef allRecipients As List(Of String),
                                       ByVal addressEntry As AddressEntry)
        If addressEntry.Members Is Nothing Then
            allRecipients.Add(addressEntry.Address)
        Else
            For Each member As AddressEntry In addressEntry.Members
                AddAddressEntryMembers(allRecipients, member)
            Next
        End If
    End Sub

To get the number of recipients just call the return the list and call `Count`:



    Dim totalRecipients = GetAllRecipients(mailItem).Count()

I'm writing an Add-In for Outlook 2010 that should get the total number of people the recieving the email.

I can get the email property fairly easily with the following code:



    Dim mailItem As MailItem
    mailItem = Globals.ThisAddIn.Application.ActiveInspector.CurrentItem

[`MailItem`][MailItem] exposes some properties like:

* [`MailItem.Recipients`][MailItem.Recipients]
* [`MailItem.To`][MailItem.To]

However, if the email is going out to a [contact group (formerly distribution lists)][group], `mailItem.Recipients.Count` will only return a value of `1`, even though multiple people will get the email.

Is there a way to query the total number of recipients within a contact group?

On some level it has to be possible, because Outlook will even do some of it for you:

![RecipientCount](http://i.imgur.com/wOVJS8r.png)


[MailItem]: http://msdn.microsoft.com/en-us/library/office/dn320330.aspx
[MailItem.Recipients]: http://msdn.microsoft.com/en-us/library/office/ff865320.aspx
[MailItem.To]: http://msdn.microsoft.com/en-us/library/office/ff860378.aspx

[group]: http://office.microsoft.com/en-us/outlook-help/create-and-edit-a-contact-group-formerly-distribution-lists-HA010354963.aspx

Outlook Add-In Get Recipient Count In Distribution Group

This function gets the parent `PropertyPageSite` Object using Reflection. It must be called in Load event.

**Visual Basic**



    Private Function GetPropertyPageSite() As Outlook.PropertyPageSite
        Dim objType As Type = GetType(System.Object)
        Dim assemblyPath As String = objType.Assembly.CodeBase.Replace("mscorlib.dll", "System.Windows.Forms.dll").Replace("file:///", "")
        Dim assemblyName As String = System.Reflection.AssemblyName.GetAssemblyName(assemblyPath).FullName

        Dim unsafeNativeMethods As Type = Type.[GetType](System.Reflection.Assembly.CreateQualifiedName(assemblyName, "System.Windows.Forms.UnsafeNativeMethods"))
        Dim oleObjectType As Type = unsafeNativeMethods.GetNestedType("IOleObject")

        Dim methodInfo As System.Reflection.MethodInfo = oleObjectType.GetMethod("GetClientSite")
        Dim propertyPageSite As Object = methodInfo.Invoke(Me, Nothing)

        Return DirectCast(propertyPageSite, Outlook.PropertyPageSite)
    End Function

**CSharp**



    private Outlook.PropertyPageSite GetPropertyPageSite()
    {
        Type objType  = typeof(System.Object);
        string assemblyPath = objType.Assembly.CodeBase.Replace("mscorlib.dll", "System.Windows.Forms.dll").Replace("file:///", "");
        string assemblyName = System.Reflection.AssemblyName.GetAssemblyName(assemblyPath).FullName;
        
        Type unsafeNativeMethods = Type.GetType(System.Reflection.Assembly.CreateQualifiedName(assemblyName, "System.Windows.Forms.UnsafeNativeMethods"));
        Type oleObjectType = unsafeNativeMethods.GetNestedType("IOleObject");
        
        System.Reflection.MethodInfo methodInfo = oleObjectType.GetMethod("GetClientSite");
        Object propertyPageSite = methodInfo.Invoke(this, null);
        
        return (Outlook.PropertyPageSite)propertyPageSite;
    }

From [How to implement OL PropertyPage][propertypage] / [Customize Outlook Options Dialog][customize]

[propertypage]: http://www.outlookcode.com/codedetail.aspx?id=978
[customize]: http://social.msdn.microsoft.com/Forums/vstudio/en-US/f7dd6f82-13e7-4a28-8316-9c535eaa7518/customize-outlook-options-dialog?forum=vsto#c45f5fb1-c15d-4a5e-bf0e-7cb677462fdd

The outlook [`PropertyPage`][PropertyPage] Interface requires a Read Only Boolean Property `Dirty`.  

When this is set to `true`, the Apply button from the property page options dialog will become enabled.

![Apply](http://i.imgur.com/JxKsTeZ.png)

According to this [walkthrough][walkthrough], you have to call [`OnStatusChange`][OnStatusChange] to notify the UI that the value of `Dirty` has changed.

Purportedly, this is available by calling the following:



    Dim ppSite As Outlook.PropertyPageSite = Parent
    ppSite.OnStatusChange()

But `Parent` always returns nothing so I have no mechanism to tell the UI when I have updated the dirty flag.

How can I do this?

---

I'm using the basic steps produced in this [discussion][discussion] to setup the options page.  
Here's a full implementation of my code:

### Added New User Control called `SendReminderOptions`:



    <ComVisible(True)>
    Public Class SendReminderOptions : Inherits UserControl : Implements Outlook.PropertyPage
    
        Const captionDispID As Integer = -518
    
        Private _dirty As Boolean = False
        Public ReadOnly Property Dirty As Boolean Implements Microsoft.Office.Interop.Outlook.PropertyPage.Dirty
            Get
                Return _dirty
            End Get
        End Property
    
        Public Sub SetDirty(newValue As Boolean)
            _dirty = newValue
            Dim ppSite As Outlook.PropertyPageSite = Parent
            ppSite.OnStatusChange()
        End Sub
    
        <DispId(captionDispID)> _
        Public ReadOnly Property PageCaption() As String
            Get
                Return "Send Reminder Options"
            End Get
        End Property
    
        Public Sub GetPageInfo(ByRef HelpFile As String, ByRef HelpContext As Integer) Implements Microsoft.Office.Interop.Outlook.PropertyPage.GetPageInfo
    
        End Sub
    
        Public Sub Apply() Implements Microsoft.Office.Interop.Outlook.PropertyPage.Apply
    
        End Sub
    
    End Class

### Added the following code to `ThisAddIn`



    Private Sub ThisAddIn_Startup() Handles Me.Startup
        Dim myOutlook As Outlook.Application = Globals.ThisAddIn.Application
        AddHandler myOutlook.OptionsPagesAdd, AddressOf AddOptionsPage
    End Sub

    Private Sub AddOptionsPage(ByVal pages As PropertyPages)
        pages.Add(New SendReminderOptions(), "Options")
    End Sub

[PropertyPage]: http://msdn.microsoft.com/en-us/library/microsoft.office.interop.outlook.propertypage.aspx
[discussion]: http://social.msdn.microsoft.com/Forums/vstudio/en-US/f7dd6f82-13e7-4a28-8316-9c535eaa7518/customize-outlook-options-dialog?forum=vsto#3e9cf09c-8056-46d3-bb09-2da019cf8ba6
[walkthrough]: http://msdn.microsoft.com/en-us/library/office/ff864286.aspx
[OnStatusChange]: http://msdn.microsoft.com/en-us/library/office/ff869275(v=office.14).aspx