1. Exception Handling .NET MVC
MIS Professor Sandvig
11/10/2018
Exception Handling .NET MVC
MIS 424
Professor Sandvig

2. MIS 324 -- Professor Sandvig
11/10/2018
Today
Exception Handling
Objective
IIS handlers
.NET
Exception Class
Techniques
Try Catch block
HandleError Attribute
Application-level trapping
Global.asax

3. MIS 324 -- Professor Sandvig
11/10/2018
Error Handling
Goal:
User NEVER sees yellow screen!!
Handle errors gracefully
Example: WWU, ThinkGeek, Reddit (refresh)

4. Error Handling Implementation: Robust programming Trap all errors
Validate all user inputs
Handle foreseeable problems
Trap all errors
Handle gracefully
Display message
Log errors

5. MIS 324 -- Professor Sandvig
11/10/2018
Error Handling
A good programmer is someone who always looks both ways before crossing a one-way street.
Doug Linder
systems administrator

6. Error Handling Exceptions should be exceptional Expensive to handle
Avoid them
Expensive to handle
Resource intensive
Do not use for:
Data Validation
Manage buggy code

7. Error Handling Where: IIS .NET Errors that do not reach .NET
404, 500, …
.NET
Controller
Try Catch – line of code
OnException – entire controller
Application
Global.aspx

8. MIS 324 -- Professor Sandvig
11/10/2018
Error Handling - IIS
Incorrect URLs may not reach .NET
Examples:
404 error handled by IIS
IIS has dozens of error pages
May edit messages

9. MIS 324 -- Professor Sandvig
11/10/2018
IIS

10. MIS 324 -- Professor Sandvig
11/10/2018
.NET
Request reaches .NET
At least six options for handling
All exceptions throw .NET exception object
Contains details about exception
See message in Yellow screen

11. MIS 324 -- Professor Sandvig
11/10/2018
Exception Class
Thrown when exception occurs
Contains details
Documentation:
Exception base class
Specific: sqlException class
Contains information specific to exception type

12. MIS 324 -- Professor Sandvig
11/10/2018
Exception Class
Can throw custom exceptions
Provide specific information
Example: Dog class (from 324)

13. .NET Exception Handling
Six options (at least) for handling
Three easy options:
Try Catch
line of code
HandleError Attribute –
single action method
Application_Exception handler
Global.asax
Entire application

14. Line-level Error Handling
MIS Professor Sandvig
11/10/2018
Line-level Error Handling
Try Catch Block
Wrap high-risk code
Database, web services
Any code that interacts with external resources
Benefits
Provide handlers
Graceful response
Log error
Continue code execution
Easy to implement

15. MIS 324 -- Professor Sandvig
11/10/2018
Try Catch Block Syntax
try {
// problematic code }
catch (Exception e)
// error handler code

16. MIS 324 -- Professor Sandvig
11/10/2018
Try Catch
Can trap by type of exception
Trap specific first, general last
Syntax:
Catch (FormatException ex)
{ handler }
Catch (OleDbException ex)
{ Handler }
Catch (Exception ex)

17. MIS 324 -- Professor Sandvig
11/10/2018
Try Catch Block
Where:
Catch in controller
Errors will bubble up
Provide user with nice message
Log error
Try Catch
Example (see handout)

18. HandleError Attribute
Decorate ActionMethods
Redirect to error page
Exception sent to error page

19. MIS 324 -- Professor Sandvig
11/10/2018
Application Level
Global.asax
Application_Error() handler
Catches all unhandled exceptions
Including 404s
Log exception
Redirect to error page

20. MIS 324 -- Professor Sandvig
11/10/2018
Application Level
Global.asax
Handles application events
Application start
Application end
Session start
Session end
Application_Error()
Example (see handout)

21. MIS 324 -- Professor Sandvig
11/10/2018
Other Options
OnException method
Handles all exceptions in:
Controller or
Application
Web.config
Redirect to error page
No logging
More information:
Exception handling in ASP.NET MVC

22. MIS 324 -- Professor Sandvig
11/10/2018
Summary
Error Trapping
Objectives
Handle exceptions gracefully
Record details in log file
3 Levels
Line - Try Catch Block
Controller action method
HandleError Attribute
Application
Global.asax Application_Error