Master pages can be changed dynamically or not?

protected void Page_PreInit(object sender, EventArgs e)
{
   Page.MasterPageFile = "~/MasterPage.master";
}

this code write before page_Load event

protected void Page_PreInit(object sender, EventArgs e)
{
   this.MasterPageFile = "~/MasterPage.master";
}

Master pages are a feature in ASP.NET 2.0. They allow you to easily create a consistent feel throughout our website. They also provide an easy way to set/change the master pages at runtime. This is achieved by setting the MasterPageFile property of a page in the PreInit event.

protected void Page_PreInit(object sender, EventArgs e)
{
   this.MasterPageFile = "~/MasterPage.master";
}