If you have public properties defined in a Master Page that you'd like to access in a strongly-typed manner you can add the MasterType directive into a page as shown next:
<% MasterType VirtualPath="MasterPage.master" %>
You can then access properties in the target master page from a content page by writing code like the following:
this.Master.HeaderText = "Label updated using MasterType directive with VirtualPath attribute.";