Setting the User property requires the ControlPrincipal flag to be set.

The User property provides programmatic access to the properties and methods of the IPrincipal interface. Because ASP.NET pages contain a default reference to the System.Web namespace (which contains the HttpContext class), you can reference the members of HttpContext on an .aspx page without using the fully qualified class reference to HttpContext. For example, you can use User.Identity.Name to get the name of the user on whose behalf the current process is running. However, if you want to use the members of IPrincipal from an ASP.NET code-behind module, you must include a reference to the System.Web namespace in the module and a fully qualified reference to both the currently active request/response context and the class in System.Web that you want to use. For example, in a code-behind page you must specify the fully qualified name HttpContext.Current.User.Identity.Name.

The following example shows how to access properties about the current user through the User property. Those properties are used to set the title of the Web page.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If (User.Identity.IsAuthenticated) Then
        Page.Title = "Home page for " + User.Identity.Name
    Else
        Page.Title = "Home page for guest user."
    End If
End Sub

protected void Page_Load(object sender, EventArgs e)
{
    if (User.Identity.IsAuthenticated)
    {
        Page.Title = "Home page for " + User.Identity.Name;
    }
    else
    {
        Page.Title = "Home page for guest user.";
    }
}

• SecurityPermission 

  for setting the User property. Demand value: Demand. Permission value: ControlPrincipal

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

.NET Framework

Reference