如何以编程方式更改MasterPageFile

this.MasterPageFile = "~/MasterPages/MyMasterPage.master";

protected void Page_Load(object sender, EventArgs e)
{
    if (someCondition)
    {
        this.MasterPageFile = "~/MasterPages/MyMasterPage1.master";
    }
    else
    {
        this.MasterPageFile = "~/MasterPages/MyMasterPage2.master";
    }
}

protected void Page_PreInit(object sender, EventArgs e)
{
    if (someCondition)
    {
        this.MasterPageFile = "~/MasterPages/MyMasterPage1.master";
    }
    else
    {
        this.MasterPageFile = "~/MasterPages/MyMasterPage2.master";
    }
}