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Custom attributes are convenient way to store custom data for menu items.
You can set custom attributes in the ASPX/ASCX file:

<radm:RadMenuItem  State="unchecked" Text="Bold" ></radm:RadMenuItem>

You can also set custom attributes from codebehind via the Attributes collection exposed by the RadMenuItem class.
C#

RadMenuItem item = new RadMenuItem();
item.Attributes["State"] = "unchecked";					

VB.NET

Dim item As RadMenuItem = New RadMenuItem()
item.Attributes("State") = "unchecked"				

To get the value of a custom attribute on the client-side you should use the GetAttribute method.

<script type="text/javascript">
function ToggleCheck (sender, args)
{
	var item = args.Item;
	var state = item.GetAttribute("State");
}
</script>

To set the value of a custom attribute on the client-side you should use the SetAttribute method.

<script type="text/javascript">
function ToggleCheck (sender, args)
{
	var item = args.Item;
	item.SetAttribute("State", "checked");
}
</script>