Nested Master Page and Master Page

I have a parent MasterPage (Main.master) and a child NestedMasterPage
In the NestedMasterPage i have assigned the MasterPageFile property in the html code like below
MasterPageFile="~/Themes/ABC/Main.Master" Now in the code behind of the NestedMasterPage i am trying to change the MasterPageFile property to some other MasterPage but its not working.
protected void Page_PreInit(object sender, EventArgs e) { MasterPageFile = "~/Themes/XYZ/Main.Master"; } No code error, just the HTML assigned path is working and not the code behind. What am i missing???