protected override void Render(HtmlTextWriter writer)
{
      this.LoadFromConfigFile();
      this.master = this.Page.LoadControl(this.MasterPageFile);
      Type type1 = new PlaceHolder().GetType();
      for (int num1 = 0; num1 <= (this.Page.Controls.Count - 1); num1++)
      {
            if (this.Page.Controls[num1].GetType() == type1)
            {
                  this.CheckControl((PlaceHolder) this.Page.Controls[num1], this.Page.Controls[num1].ID);
            }
      }
      this.Page.Controls.Clear();
      this.Page.Controls.AddAt(0, this.master);
      base.Render(writer);
}

<masterPages>
<settings>
<add key="MasterPageFile" value="~/master_blue.ascx" />
<add key="red.aspx" value="~/master_red.ascx" />
</settings>
</masterPages>