Lesson 57 - Understanding Global Exception Handling

Tutorial Series: Free C# Fundamentals via ASP.NET Web Apps

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If you remember, in the last lesson, we introduced the concept of Exception handling in your code. The reason for this because if an Exception is not handled in any given method we create, it has the effect of “bubbling up” and producing a negative cascading effect for each method that doesn’t handle the Exception. In other words, if the Exception is not handled directly, in the method that causes the Exception, the error cascades to the method that called that method – and if it’s not handled there, to the method that called that method, and so on. Eventually, this will arrive at the point where the cascade of unhandled Exceptions stops within some calling method in the .NET Framework itself, outside of your control. At which point, the ASP.NET Runtime handles the Exception in the most basic way possible, such as a very generic “Yellow Screen of Death” – which is undesirable because it is out of our control and does not help us, nor the end user.

Step 1: Create a New Project

In this case, we can employ a Global Exception Handling “catch all” as a safety net in our code so that we never get to the point where we let the ASP.NET Framework determine how to handle Exceptions we may have missed in our code. To demonstrate this, create a new ASP.NET project called “GlobalExceptionHandling” and in the Default.aspx include the following Server Controls and programmatic IDs:

  1. okButton

  2. argumentOutOfRangeButton


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In the Default.aspx.cs, include the following code for okButton_Click() and argumentOutOfRangeButton_Click(), as well as an external helper method that gets referenced in okButton_Click():


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Step 2: Employ Global Exception Handling via Global.asax

The obvious way to handle these (intentional) Exceptions in our code is wherever the events that are tied to these methods are called. We don’t really have direct access to these events so instead we can impose a Global Exception Handler – that handles all errors caught at the application level – via the Global.asax file in our project:


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We do this by defining a method called Application_Error(), which is tied into an event that fires just before the exception would “bubble up” outside of our control throwing a “Yellow Screen of Death.” Notice how we reference the InnerException property, telling us what the initial Exception was before it turned into an HttpUnhandledException (which is the Exception type that ex will end up referencing):


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We can then write a custom server response message, detailing the error (similar to when you see a server output 404 – Page Not Found, for example). After that we have to call Server.ClearError() to avoid getting the “Yellow Screen of Death”:


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Now, when you run the application you will still see the Exception in Visual Studio, but if you click “Continue” it will output in the browser:


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Step 3: Transferring to an Error Page

What we can also do is look for a particular type of Exception and then serve up a different web page to handle it:


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Add a new .aspx file to the project, calling it “Error,” and set it up with a simple message and resultLabel:


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In Error.aspx.cs write the following for the Page_Load() method, grabbing the error and outputting different messages depending on if the returned Message was null (it would be null if you navigated to the page directly, for example):


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Now when you run Default.aspx and click the “Argument Out of Range” button, you will see an error output for the specific type of error the user encountered:


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Notice also how the URL shows that this is coming from the Default.aspx page, however, we know that it is actually being served up from Error.aspx (via Server.Transfer):


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Related Articles in this Tutorial:

Lesson 1 - Series Introduction

Lesson 2 - Installing Visual Studio 2015

Lesson 3 - Building Your First Web App

Lesson 4 - Understanding What You Just Did

Lesson 5 - Working with Projects in Visual Studio

Lesson 6 - Simple Web Page Formatting in Visual Studio

Challenge 1

Solution 1

Lesson 7 - Variables and Data Types

Lesson 8 - Data Type Conversion

Lesson 9 - Arithmetic Operators

Lesson 10 - C# Syntax Basics

Challenge 2 - ChallengeSimpleCalculator

Solution - ChallengeSimpleCalculator

Lesson 11 - Conditional If Statements

Lesson 12 - The Conditional Ternary Operator

Challenge 3 - ChallengeConditionalRadioButton

Solution - Challenge Conditional RadioButton

Lesson 13 - Comparison and Logical Operators

Lesson 13 Challenge - First Papa Bob's Website

Solution - Challenge First Papa Bob's Website

Lesson 14 - Working with Dates and Times

Lesson 15 - Working With Spans of Time

Lesson 16 - Working with the Calendar Server Control

Challenge 4 - Challenge Days Between Dates

Solution - Challenge Days Between Dates

Lesson 17 - Page_Load and Page.IsPostBack

Lesson 18 - Setting a Break Point and Debugging

Lesson 19 - Formatting Strings

Challenge 5 - Challenge Epic Spies Assignment

Solution - Challenge Epic Spies Assignment

Lesson 20 - Maintaining State with ViewState

Lesson 21 - Storing Values in Arrays

Lesson 22 - Understanding Multidimensional Arrays

Lesson 23 - Changing the Length of an Array

Challenge 6 - Challenge Epic Spies Asset Tracker

Solution - Challenge Epic Spies Asset Tracker

Lesson 24 - Understanding Variable Scope

Lesson 25 - Code Blocks and Nested If Statements

Lesson 26 - Looping with the For Iteration Statement

Challenge 7 - Challenge For Xmen Battle Count

Solution - Challenge For Xmen Battle Count

Lesson 27 - Looping with the while() & do...while() Iteration Statements

Lesson 28 - Creating and Calling Simple Helper Methods

Lesson 29 - Creating Methods with Input Parameters

Lesson 30 - Returning Values from Methods

Lesson 31 - Creating Overloaded Methods

Lesson 32 - Creating Optional Parameters

Lesson 33 - Creating Names Parameters

Lesson 34 - Creating Methods with Output Parameters

Challenge 8 - Challenge Postal Calculator Helper Methods

Solution - Challenge Postal Calculator Helper Methods

Mega Challenge Casino

Solution - Mega Challenge Casino

Lesson 35 - Manipulating Strings

Challenge 9 - Phun With Strings

Solution - Challenge Phun With Strings

Lesson 36 - Introduction to Classes and Objects

Challenge - Hero Monster Classes Part 1

Solution - Hero Monster Classes Part 1

Challenge - Hero Monster Classes Part 2

Solution - Challenge Hero Monster Classes Part 2

Lesson 37 - Creating Class Files Creating Cohesive Classes and Code Navigation

Lesson 38 - Understanding Object References and Object Lifetime

Lesson 39 - Understanding the .NET Framework and Compilation

Lesson 40 - Namespaces and Using Directives

Lesson 41 - Creating Class Libraries and Adding References to Assemblies

Lesson 42 - Accessibility Modifiers, Fields and Properties

Lesson 43 - Creating Constructor Methods

Lesson 44 - Naming Conventions for Identifiers

Lesson 45 - Static vs Instance Members

Challenge 10 - Challenge Simple Darts

Solution - Challenge Simple Darts

Lesson 46 - Working with the List Collection

Lesson 47 - Object Initializers

Lesson 48 - Collection Initializers

Lesson 49 - Working with the Dictionary Collection

Lesson 50 - Looping with the foreach Iteration Statement

Lesson 51 - Implicitly-Typed Variables with the var Keyword

Challenge 11 - Challenge Student Courses

Solution - Challenge Student Courses

Mega Challenge War

Solution - Mega Challenge War

Lesson 52 - Creating GUIDs

Lesson 53 - Working with Enumerations

Lesson 54 - Understanding the switch() Statement

Lesson 55 - First Pass at the Separation of Concerns Principle

Lesson 56 - Understanding Exception Handling

Lesson 57 - Understanding Global Exception Handling

Lesson 58 - Understanding Custom Exceptions

Lesson 59 - Creating a Database in Visual Studio

Lesson 60 - Creating an Entity Data Model

Lesson 61 - Displaying the DbSet Result in an ASP.NET GridView

Lesson 62 - Implementing a Button Command in a GridView

Lesson 63 - Using a Tools-Centric Approach to Building a Database Application

Lesson 64 - Using a Maintenance-Driven Approach to Building a Database Application

Lesson 65 - Creating a New Instance of an Entity and Persisting it to the Database

Lesson 66 - Package Management with NuGet

Lesson 67 - NuGet No-Commit Workflow

Lesson 68 - Introduction the Twitter Bootstrap CSS Framework

Lesson 69 - Mapping Enum Types to Entity Properties in the Framework Designer

Lesson 70 - Deploying the App to Microsoft Azure Web Services Web Apps

Papa Bob's Mega Challenge

Papa Bob's Mega Solution Part 1 - Setting up the Solution

Papa Bob's Mega Solution Part 2 - Adding an Order to the Database

Papa Bob's Mega Solution Part 3 - Passing an Order from the Presentation Layer

Papa Bob's Mega Solution Part 4 - Creating the Order Form

Papa Bob's Mega Solution Part 5 - Adding Enums

Papa Bob's Mega Solution Part 6 - Creating an Order with Validation

Papa Bob's Mega Solution Part 7 - Calculating the Order Price

Papa Bob's Mega Solution Part 8 - Displaying the Price to the User

Papa Bob's Mega Solution Part 9 - Creating the Order Management Page


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