Loading Master Pages Dynamically for Multiple Content Pages : Master page « Development « ASP.NET Tutorial

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After registering the DynamicMasterPage class, every page in your application automatically inherits from the new base class. 
Every page inherits the new OnPreInit() method and every page loads a Master Page dynamically.


File: DynamicMasterPage.cs

using System;
using System.Web.UI;
using System.Web.Profile;

public class DynamicMasterPage : Page
{
    protected override void OnPreInit(EventArgs e)
    {
        this.MasterPageFile = (string)Context.Profile["MasterPageFile"];
        base.OnPreInit(e);
    }

}

After you create a new base Page class, you need to register it in the web configuration file. 

<configuration>
  <system.web>
    <pages pageBaseType="DynamicMasterPage" />
    <profile>
      <properties>
        <add
          name="MasterPageFile"
          defaultValue="Dynamic1.master" />
      </properties>
    </profile>
  </system.web>
</configuration>