how to set layout in mvc

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MVC

i am working on mvc3. i want to @html.Action("","") in _ViewStart.cshtml can do this on basis of

(Request.Isauthenticated)
{
@html.action("LogOff","Account")
}
else
{
@html.action("LogOn","Account")
}

i want to do above code for layout. how can i do please help