I am building a WPF application using C# 3.5 I have a WPF menu on the top of the application.
When clicking on a top level menu item, i need the second level of the menu to show horizontally instead of vertically "and" remain opened ( not disappear like a normal menu).
What I need to have is something similar to the following screen shot
How can I style the WPF menu to change its behavior as above?
Ok after trying this out, I don't think re-templating the Menu is a good idea. Menu is a very complex control. A better idea would be to avoid using a menu entirely and use a control with behavior similar to what you've described (selected items remain open, can display their children horizontally, etc.). The TabControl is a very good fit so I would simply use that instead. Here is some sample XAML to show you how this could work:
<TabControl VerticalAlignment="Top">
<TabItem Header="MAIN ACTIONS"/>
<TabItem Header="GOALS" IsSelected="True">
<StackPanel Orientation="Horizontal">
<ToggleButton Margin="5">Enter Goals</ToggleButton>
<ToggleButton Margin="5">Edit Goals</ToggleButton>
<ToggleButton IsChecked="True" Margin="5">View Work Plan</ToggleButton>
</StackPanel>
</TabItem>
<TabItem Header="ACTIVITIES"/>
</TabControl>
Obviously, you will need to restyle the TabControl with fonts and colors to get the appearance you want. And for the submenus, I would actually use radio-buttons instead of toggle buttons, to ensure that only one can be selected at a time (you will need to re-template the radio-button as well). But looking at even this simple result, you can see that this control is quite well suited for your scenario:
