- allow use of separators in context menu

Lib.NotifyIcon/Windows/NotifyIcon.cs

@@ -185,14 +185,18 @@ namespace Lib.NotifyIcon.Windows
             uint i = 1;
             foreach (var item in ContextMenu.Items)
             {
-                MenuItem menuItem = (MenuItem)item;
+                if (item is MenuItem menuItem)
+                {
+                    // Add items to the native context menu by simply reusing
+                    // the information provided within the Avalonia ContextMenu.
+                    WindowApi.AppendMenu(popupMenu, MenuFlags.MfString, i, (string) menuItem.Header);
 
-                // Add items to the native context menu by simply reusing
-                // the information provided within the Avalonia ContextMenu.
-                WindowApi.AppendMenu(popupMenu, MenuFlags.MfString, i, (string) menuItem.Header);
+                    // Add the mapping so that we can find the selected item later
+                    contextItemLookup.Add(i, menuItem);
+                }
+                else if (item is Separator)
+                    WindowApi.AppendMenu(popupMenu, MenuFlags.MfSeparator, i, null);
 
-                // Add the mapping so that we can find the selected item later
-                contextItemLookup.Add(i, menuItem);
                 i++;
             }