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How to: Limit the Size of an Attachment to an Outlook Email Message

This topic describes how you can create a managed add-in for Outlook that cancels sending email if the total attachment size is greater than a fixed limit.

Provided by:  Ken Getz, MCW Technologies, LLC

A given email message can contain one or more file attachments, and you may want to limit the total attachment size in email messages that you send. The sample code in this topic demonstrates how you can handle the ItemSend event in an Outlook add-in, and in the event handler, cancel the sending of the email message if the combined size of all the attachments is larger than a specific value (2 MB, in this example).

The Outlook ItemSend event receives as its parameters a reference to the item being sent, and a Boolean variable that is passed by reference and that allows you to cancel the send operation. It is up to your own code in the event handler to determine whether you want to cancel the event; you do so by setting the Cancel parameter to True if you do wish to cancel the event.

In this example, to determine whether the total attachment size is larger than a specific size, the code loops through each attachment in the item's Attachments collection. For each item, the code retrieves the Size property, summing as it loops. If the sum ever exceeds the size of the maxSize constant, the code sets the tooLarge variable to True, and exits the loop. After the loop, if the tooLarge variable is True, the code alerts the user and sets the Cancel parameter to the event handler (which was passed by reference) to True, causing Outlook to cancel the sending of the item.

The following managed code samples are written in C# and Visual Basic. To run a .NET Framework managed code sample that needs to call into a Component Object Model (COM), you must use an interop assembly that defines and maps managed interfaces to the COM objects in the object model type library. For Outlook, you can use Visual Studio and the Outlook Primary Interop Assembly (PIA). Before you run managed code samples for Outlook 2013, ensure that you have installed the Outlook 2013 PIA and have added a reference to the Microsoft Outlook 15.0 Object Library component in Visual Studio. You should use the following code samples in the ThisAddIn class of an Outlook add-in (using Office Developer Tools for Visual Studio). The Application object in the code must be a trusted Outlook Application object provided by ThisAddIn.Globals. For more information about using the Outlook PIA to develop managed Outlook solutions, see the Welcome to the Outlook 2013 Primary Interop Assembly Reference on MSDN.

The following code shows how to cancel sending an email if the total attachment size is greater than the specified limit. To demonstrate this functionality, in Visual Studio, create a new managed Outlook add-in named LimitAttachmentSizeAddIn. Replace the code in ThisAddIn.cs or ThisAddIn.vb with the example code shown here.

using Outlook = Microsoft.Office.Interop.Outlook;
namespace LimitAttachmentSizeAddIn
    public partial class ThisAddIn
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
          Application.ItemSend +=new Outlook.ApplicationEvents_11_ItemSendEventHandler(Application_ItemSend);
        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        void Application_ItemSend(object Item, ref bool Cancel)
            // Specify the maximum size for the attachments. For this example,
            // the maximum size is 2 MB.
            const int maxSize = 2 * 1024 * 1000;
            bool tooLarge = false;
            Outlook.MailItem mailItem = Item as Outlook.MailItem;
            if (mailItem != null)
                var attachments = mailItem.Attachments;
                double totalSize = 0;
                foreach (Outlook.Attachment attachment in attachments)
                    totalSize += attachment.Size;
                    if (totalSize > maxSize)
                        tooLarge = true;
            if (tooLarge)
                // If the sum of the attachment sizes is too large, alert the user
                // and cancel the send.
                    "The total attachment size is too large. Sending canceled.", 
                    "Outlook Add-In");
                Cancel = true;
        #region VSTO generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
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