UnauthorizedAccessException() class error - asp.net

Question

I have an application that uses its own authorization to determine if a user has access to pages. I want to display a more friendly "access denied" page in the event that access denied. In the MasterPage...
if (!authorize)
{
throw new UnauthorizedAccessException(); //error occurs here, looks like I'm not allowed to use this class
}
In the web.config
<customErrors mode="Off" defaultRedirect="~/ErrorPages/ErrorPage.aspx">
<error statusCode="403" redirect="AccessDeniedPage.aspx" />
</customErrors>I get the error below.
It appears that I get the error just as a result of just trying to instantiate/use the UnauthorizedAccessException() class. I'd like to do it this way, is there a way to use this?
/**************************************************************************************************************************
Attempted to perform an unauthorized operation.
Exception Details: System.UnauthorizedAccessException: Attempted to perform an unauthorized operation.
ASP.NET is not authorized to access the requested resource. Consider granting access rights to the resource to the ASP.NET request identity. ASP.NET has a base process identity (typically {MACHINE}\ASPNET on IIS 5 or Network Service on IIS 6 and IIS 7, and the configured application pool identity on IIS 7.5) that is used if the application is not impersonating. If the application is impersonating via <identity impersonate="true"/>, the identity will be the anonymous user (typically IUSR_MACHINENAME) or the authenticated request user.
*************************************************************************************************************************/

Answer 1

As Fredrik says, you are throwing an error, so you get an error. If you want to just instantiate the exception don't use throw.
UnauthorizedAccessException uae = new UnauthorizedAccessException("some message");
But again, this just creates an exception; soon as you throw it, you will get the message you are already getting.
Why not just redirect? Response.Redirect("~/AccessDeniedPage.aspx", False);
If you really wanted to use the exception, you could continue to throw the exception as you are but also handle the exception in the Application_Error event of the Global.asax file. In the Application_Error event, test if the exception is an UnauthorizedAccessException, and if so, redirect the user to AccessDeniedPage.aspx. Basic use of Application_Error: MSDN

Answer 2

Well, you are throwing an UnauthorizedAccessException. If there is no try-catch that catches it, the code will crash there. I think that the exception that you see, is your exception.