Question

I have a web project that has a page called editemployee.aspx. The code behind is editemployee.aspx.cs. From that code, I call another project, a class library. The method I’m calling in that class library looks like…

        public static Employee GetItem(int employeeid)
        {
            try
            {
                Employee chosenemployee = EmployeeDB.GetItem(employeeid);
                chosenemployee.EmployeeInsuranceRecord = EmployeeInsuranceRecordManager.GetItem(employeeid);
                return chosenemployee;
            }
            catch (Exception ex)
            {
                if (ex is NullReferenceException)
                {
                    //How can I return gracefully from here
                    //without disrupting the user experience?
                }
                else
                {
                    throw ex;  //if it makes it this far, it must be some other unknown error and I need an exception to be thrown for real so I can track down the bug.
                }
            }
        }

Basically this code gets the full employee record from a data access layer based on the passed EmployeeID. If the exception is NullReferenceException, that means that somehow a bogus EmployeeID got through. In that case, what I want to happen is to return to the calling .aspx.cs page, STOP the process of sorting out the Employee right there, and display a JavaScript alert box that says, “The EmployeeID supplied is not valid.”

So I tried saying:

            if (ex is NullReferenceException)
            {
                Response.Write("<script>alert('"The EmployeeID supplied is not valid."')</script>"); 
            }

But this does not work from a separate class library like that, it doesn’t recognize Response.Write.

Basically I just need a way to signal, from this method, the calling page to just stop and display an error gracefully, rather than give the user a huge ugly unrecoverable error message. Any tips?

Answer 1

Why don’t you catch your NullReferenceException from your editemployee.aspx.cs?
Something like:

try
{
    var employee = Employee.GetItem(int employeeid)
}
catch(NullReferenceException ex)
{
    Response.Write("<script>alert('"The EmployeeID supplied is not valid."')</script>"); 
}

Plus, you should not use the is operator to filter exception type but add the correct type in the catch statement.

A catch block can specify an exception type to catch. This type is called an exception filter, and must either be the Exception type, or derived from this type. Application-defined exceptions should derive from ApplicationException.

(source)

Finally, it’s a recommended practice to throw more specified Exception. In your case a custom EmployeeNotFoundException would be better to throw from your business layer.

catch(NullReferenceException ex)
{
     throw new EmployeeNotFoundException(ex);
}