如何捕获 System.NullReferenceException？

Question

When a session variable reaches timeout a NullReferenceException is thrown. I know you can change how long it takes for the session variable to timeout. I'm trying to remove all possible ways for anything to crash. Is there any reason why the exception is not being caught here?

protected void Page_Load(object sender, EventArgs e)
{
     try
     {
        // Get session variables. 
        String strParticipantID = Session["ParticipantID"].ToString();
     }
     catch (NullReferenceException)
     {
            Response.Redirect("Login.aspx");
     }    
}

Answer 1

You should never attempt to catch a NullReferenceException , nor should you manually throw it.
What you should do is write null-safe code - and that's pretty easy using the null conditional operator ( ?. ) -

// This will never throw a null reference exception
var participantID = Session["ParticipantID"]?.ToString(); 

If you want an empty string instead of null , you can combine that with the null coalescing operator ( ?? ) :

// This will never throw a null reference exception - 
// participantID will be an empty string if Session["ParticipantID"] is null.
var participantID = Session["ParticipantID"]?.ToString() ?? "";