Exception handling is vital for ensuring that ASP.NET MVC applications are reliable and provide a smooth user experience. By using robust exception-handling methods, we can prevent runtime errors from disrupting the application’s flow and inform users of issues in a controlled way. Let’s explore the basics of exception handling in ASP.NET MVC, including common approaches, pitfalls, and recommended improvements.
1. Understanding Exceptions in ASP.NET MVC
An exception is an event that disrupts the normal flow of a program’s execution. In ASP.NET MVC, exceptions might arise due to multiple reasons: database connection issues, invalid user inputs, unhandled null references, and more.
Example Scenario: Null Reference Exception
Let’s consider a case where a null reference exception might occur:
public ActionResult GetUserDetails(int userId)
{
var user = dbContext.Users.SingleOrDefault(u => u.UserId == userId);
return View(user.Name); // Potential null reference exception
}
Why Does This Exception Occur?
In the above code, if the userId doesn’t exist in the database, the user will be null, and attempt to access user.Name will cause a NullReferenceException. This is because we’re trying to access a property of a null object.
Solution: Null Check Before Access
A simple way to prevent this issue is by checking for null:
public ActionResult GetUserDetails(int userId)
{
var user = dbContext.Users.SingleOrDefault(u => u.UserId == userId);
if (user == null)
{
// Log the error or return a friendly error page
return RedirectToAction("Error", new { message = "User not found." });
}
return View(user.Name);
}
2. Centralized Exception Handling with Custom Error Pages
Instead of handling errors within each controller action, it’s more efficient to manage errors globally. Centralized exception handling ensures that unhandled exceptions are captured and managed consistently across the application.
Default ASP.NET MVC Error Handling with HandleError
ASP.NET MVC includes a HandleError attribute to catch unhandled exceptions in controller actions. Applying this attribute to a controller or globally to all controllers catches exceptions and redirects users to a default error page.
[HandleError]
public class HomeController : Controller
{
// Action methods
}
Additionally, we can configure a custom error page in the Web.config file:
Limitations of HandleError:
- Only works with synchronous controller actions.
- Does not capture exceptions in asynchronous methods, middleware, or background processes.
- Limited flexibility for customizing responses based on exception types.
Improvement: Using a Custom ExceptionFilterAttribute
Creating a custom exception filter offers more control over error handling, allowing us to customize responses and handle errors from various sources, including asynchronous methods.
Sample Code for Custom Exception Filter
1. Define the filter:
public class CustomExceptionFilter : ExceptionFilterAttribute
{
public override void OnException(ExceptionContext context)
{
// Log the exception
Logger.LogError(context.Exception);
// Redirect to error page or provide a custom response
context.Result = new RedirectToRouteResult(
new RouteValueDictionary(new { controller = “Home”, action = “Error” })
);
context.ExceptionHandled = true;
}
}
2. Register the filter globally:
In Global.asax.cs, add this filter to the global filter collection:
public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
filters.Add(new CustomExceptionFilter());
}
Benefits of Using a Custom Exception Filter:
- Enhanced Flexibility: Allows specific handling for different exception types.
- Consistent Error Handling: Works for both synchronous and asynchronous methods, making it suitable for a wide range of error sources.
- Extensible: Easy to add additional functionality, like sending notifications for critical errors.
3. Using try-catch Blocks for Specific Error Scenarios
While centralized error handling is ideal for global exceptions, certain actions may require specific handling for predictable exceptions, such as SqlException for database operations.
Example Scenario: Database Exception Handling
In this example, we handle a potential database error with a try-catch block:
public ActionResult SaveUser(User user)
{
try
{
dbContext.Users.Add(user);
dbContext.SaveChanges();
return RedirectToAction("Index");
}
catch (SqlException ex)
{
// Handle database-specific exception
return RedirectToAction("Error", new { message = "Database error occurred. Please try again later." });
}
catch (Exception ex)
{
// Handle general exception
return RedirectToAction("Error", new { message = "An unexpected error occurred." });
}
}
Advantages of try-catch Blocks:
- Granular Control: Allows handling of specific exceptions, providing more detailed responses for predictable errors.
- Localized Error Management: Keeps the error-handling logic close to the potential source, which can be helpful for more controlled error responses.