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You can make any RadialMenuItem checkable by setting its IsCheckable property to True. In RadialMenu, a checked item is marked similarly to a highlighted item instead of with a typical check mark. You can see the Insert Above option checked in the following image.
Note that the check is thinner than the highlight you can see in the RadialMenu Elements topic:
To create a checkable RadialMenuItem as in the previous image, follow these steps:
<c1:C1RadialMenuItem Header="C1RadialMenuItem" IsCheckable="True"/>
In Source view, locate the <c1:C1RadialMenuItem> tag for the item you wish to make checkable and add Name="CheckableRadialMenuItem" to it. This will give the item a unique identifier that you can use in code.
Enter Code View and add the following code beneath the InitializeComponent() method:
CheckableRadialMenuItem.IsCheckable = true;
Run the program.