Change Label MasterPage จะเปลี่ยนข้อความ Label ใน MasterPage เมื่อทำการ Login

Label mpLabel = (Label) Master.FindControl("masterPageLabel");
if(mpLabel != null)
{
    Label1.Text = "Master page label = " + mpLabel.Text;
}

You can access the Masterpage as a property on your current page. However, the controls on your master page are protected so you can't access them directly. But you can access them by using FindControl(string name). The code you need to use depends on if the control is inside or outside a content place holder.
// Gets a reference to a TextBox control inside a ContentPlaceHolder
ContentPlaceHolder mpContentPlaceHolder;
TextBox mpTextBox;
mpContentPlaceHolder = 
    (ContentPlaceHolder)Master.FindControl("ContentPlaceHolder1");
if(mpContentPlaceHolder != null)
{
    mpTextBox = (TextBox) mpContentPlaceHolder.FindControl("TextBox1");
    if(mpTextBox != null)
    {
        mpTextBox.Text = "TextBox found!";
    }
}

// Gets a reference to a Label control that is not in a 
// ContentPlaceHolder control
Label mpLabel = (Label) Master.FindControl("masterPageLabel");
if(mpLabel != null)
{
    Label1.Text = "Master page label = " + mpLabel.Text;
}

                <asp:LoginView ID="HeadLoginView" runat="server" EnableViewState="false">
                    <AnonymousTemplate>
                        [ <a href="~/Account/Login.aspx" ID="HeadLoginStatus" runat="server">Log In</a> ]
                    </AnonymousTemplate>
                    <LoggedInTemplate>
                        Welcome <span class="bold"><asp:LoginName ID="HeadLoginName" runat="server" /></span>!
                        [ <asp:LoginStatus ID="HeadLoginStatus" runat="server" LogoutAction="Redirect" LogoutText="Log Out" LogoutPageUrl="~/"/> ]
                    </LoggedInTemplate>
                </asp:LoginView>

<?xml version="1.0"?>
<configuration>
  <system.web>
    <compilation debug="false" strict="false" explicit="true" targetFramework="4.0" /> <!--4.5-->
    <authentication mode="Forms">
      <forms loginUrl="~/Login.aspx" name=".ASPXFORMSAUTH" timeout="3600" />
    </authentication>
    <authorization>
      <deny users="?" />
    </authorization>
  </system.web>
  
  <location path="Styles">
    <system.web>
      <authorization>
        <allow users="?"/>
      </authorization>
    </system.web>
  </location>
  <location path="Images">
    <system.web>
      <authorization>
        <allow users="?"/>
      </authorization>
    </system.web>
  </location>
  
</configuration>

    protected void Page_Load(object sender, EventArgs e)
    {
        MasterPageName master = this.Master as MasterPageName;
        if (master != null)
        {
            Label lblTitle = master.FindControl("lblTitle") as Label;
            lblTitle.Text = "Application Settings";
            Label lblFunction = master.FindControl("lblFunction") as Label;
            lblFunction.Text = "Master";
        }