Setting an Outlook mailitem's category programmatically?

Question

There doesn't seem to much information or any good code samples for setting an Outlook 2007 MailItem's categories programmatically.

MSDN has a limited page, and mentions using VB's Split function, saying more or less "you're on your own from here onwards, so sort it out yourself".

So far as I can tell we manipulate the Categories as a comma delimited string property of the mailitem. It seems a bit primitive, is that all there is to it?

Does everyone just dig out their library of string functions and parse the Categories property, trusting not to get in a muddle when multiple categories are set for a single mailitem and (heaven forbid) categories are renamed?

score 23 · Accepted Answer · answered Feb 06 '12 at 23:24

You can choose to manipulate the comma-delimited string of Categories any way you choose. To insert a category, I usually check if the current string is null and then just assign it. If the category is not null then I append it if it doesn't already exist. To remove an item, I just replace the category name with an empty string.

Adding Category

 var customCat = "Custom Category";
 if (mailItem.Categories == null) // no current categories assigned
   mailItem.Categories = customCat;
 else if (!mailItem.Categories.Contains(customCat)) // insert as first assigned category
   mailItem.Categories = string.Format("{0}, {1}", customCat, mailItem.Categories);

Removing Category

var customCat = "Custom Category";
if (mailItem.Categories.Contains(customCat))
  mailItem.Categories = mailItem.Categories.Replace(string.Format("{0}, ", customCat), "").Replace(string.Format("{0}", customCat), "");

There are multitudes of ways to manipulate strings - they just chose to keep the serialized data structure simple underneath.

I tend to create my own Categories during Add-in startup to verify they exist. Certainly - category renaming is a concern, but if you are ensuring that your categories exist each time your add-in loads, you can at least ensure some level of validity.

To manage Outlook Categories, you can use ThisAddIn.Application.Session.Categories.

var customCat = "Custom Category";
if (Application.Session.Categories[customCat] == null)  
  Application.Session.Categories.Add(customCat, Outlook.OlCategoryColor.olCategoryColorDarkRed);

Hi, I just tried doing this, however the category doesn't update. It runs the instruction `mailItem.Categories = customCat;` with no problems, but it doesn't show in Outlook. Should I save or something? — Soph, Oct 01 '12 at 15:40
You need to issue a [`MailItem.Save()`](http://msdn.microsoft.com/en-us/library/office/ff866979.aspx) to persist any metadata changes with [`MailItems`](http://msdn.microsoft.com/en-us/library/office/ff861332.aspx) — SliverNinja - MSFT, Oct 01 '12 at 16:16
FYI according to this: http://msdn.microsoft.com/en-us/library/office/ff860423(v=office.15).aspx the delimiter is localized (which is a dumb idea) `This property uses the character specified in the value name, sList, under HKEY_CURRENT_USER\Control Panel\International in the Windows registry, as the delimiter for multiple categories.` — The Muffin Man, May 28 '14 at 18:29
Note you can ensure you are using the current culture "List dilimiter" with mailItem.Categories = $"{customCat}{System.Globalization.CultureInfo.CurrentCulture.TextInfo.ListSeparator} {mailItem.Categories}"; — Jows, Sep 01 '17 at 01:46

score 0 · Answer 2 · answered May 15 '20 at 22:26

Just to include one last information inside of the @SilverNija - MSFT post. After do something like this:

 var customCat = "Custom Category";
 if (mailItem.Categories == null) // no current categories assigned
   mailItem.Categories = customCat;
 else if (!mailItem.Categories.Contains(customCat)) // insert as first assigned category
   mailItem.Categories = string.Format("{0}, {1}", customCat, mailItem.Categories);

Don't forget and it is the most important, to update the instance of the mailItem this way:

mailItem.Save();

Because, there is an issue that sometimes a bunch of mail items are not updating when they are inside of a loop, so it worked for me to solve this problem.

Setting an Outlook mailitem's category programmatically?

2 Answers2

Adding Category

Removing Category

To manage Outlook Categories, you can use ThisAddIn.Application.Session.Categories.

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