This is a well documented issue, but just in case it helps someone.
If your ListView doesn't show groups, ensure the following (starting with the most obvious, and ending with the probable cause):
- Ensure your ListView has, at least, one column
- Ensure you have created Groups correctly
- Properly assign a group to each Item, through their “Group” property
- Set listView.ShowGroups property = True
- Ensure you have called System.Windows.Forms.Application.EnableVisualStyles() method at the beginning of your application
ListView groups rely on VisualStyles of the application, what is only available on Windows XP systems and above. So, groups won´t work on anything below Windows XP.
Here, you can find some explanations, like the following:
“ListView groups are available only on Windows XP and Windows Server 2003 when your application calls the Application.EnableVisualStyles method. On earlier operating systems, any code relating to groups has no effect and the groups will not appear. As a result, any code that depends on the grouping feature might not work correctly.”