ASP.NET Error Handling
Overview
ASP.NET applications must be able to handle errors that occur during execution in a consistent manner. ASP.NET uses the common language runtime (CLR), which provides a way of notifying applications of errors in a uniform way. When an error occurs, an exception is thrown. An exception is any error, condition, or unexpected behavior that an application encounters.
In the .NET Framework, an exception is an object that inherits from the System.Exception
class. An exception is thrown from an area of code where a problem has occurred. The exception is passed up the call stack to a place where the application provides code to handle the exception. If the application does not handle the exception, the browser is forced to display the error details.
As a best practice, handle errors in at the code level in Try
/Catch
/Finally
blocks within your code. Try to place these blocks so that the user can correct problems in the context in which they occur. If the error handling blocks are too far away from where the error occurred, it becomes more difficult to provide users with the information they need to fix the problem.
Exception Class
The Exception class is the base class from which exceptions inherit. Most exception objects are instances of some derived class of the Exception class, such as the SystemException
class, the IndexOutOfRangeException
class, or the ArgumentNullException
class. The Exception class has properties, such as the StackTrace
property, the InnerException
property, and the Message
property, that provide specific information about the error that has occurred.
Exception Inheritance Hierarchy
The runtime has a base set of exceptions deriving from the SystemException
class that the runtime throws when an exception is encountered. Most of the classes that inherit from the Exception class, such as the IndexOutOfRangeException
class and the ArgumentNullException
class, do not implement additional members. Therefore, the most important information for an exception can be found in the hierarchy of exceptions, the exception name, and the information contained in the exception.
Exception Handling Hierarchy
In an ASP.NET Web Forms application, exceptions can be handled based on a specific handling hierarchy. An exception can be handled at the following levels:
- Application level
- Page level
- Code level
When an application handles exceptions, additional information about the exception that is inherited from the Exception class can often be retrieved and displayed to the user. In addition to application, page, and code level, you can also handle exceptions at the HTTP module level and by using an IIS custom handler.
Application Level Error Handling
You can handle default errors at the application level either by modifying your application's configuration or by adding an Application_Error
handler in the Global.asax file of your application.
You can handle default errors and HTTP errors by adding a customErrors
section to the Web.config file. The customErrors
section allows you to specify a default page that users will be redirected to when an error occurs. It also allows you to specify individual pages for specific status code errors.
<configuration>
<system.web>
<customErrors mode="On" defaultRedirect="ErrorPage.aspx?handler=customErrors%20section%20-%20Web.config">
<error statusCode="404" redirect="ErrorPage.aspx?msg=404&handler=customErrors%20section%20-%20Web.config"/>
</customErrors>
</system.web>
</configuration>
Unfortunately, when you use the configuration to redirect the user to a different page, you do not have the details of the error that occurred.
通过web.config中的配置,跳转页面,是不带错误的detail的。
However, you can trap errors that occur anywhere in your application by adding code to the Application_Error
handler in the Global.asax file.
void Application_Error(object sender, EventArgs e)
{
Exception exc = Server.GetLastError();
if (exc is HttpUnhandledException)
{
// Pass the error on to the error page.
Server.Transfer("ErrorPage.aspx?handler=Application_Error%20-%20Global.asax", true);
}
}
Page Level Error Event Handling
A page-level handler returns the user to the page where the error occurred, but because instances of controls are not maintained, there will no longer be anything on the page. To provide the error details to the user of the application, you must specifically write the error details to the page.
You would typically use a page-level error handler to log unhandled errors or to take the user to a page that can display helpful information.
This code example shows a handler for the Error event in an ASP.NET Web page. This handler catches all exceptions that are not already handled within try
/catch
blocks in the page.
private void Page_Error(object sender, EventArgs e)
{
Exception exc = Server.GetLastError();
// Handle specific exception.
if (exc is HttpUnhandledException)
{
ErrorMsgTextBox.Text = "An error occurred on this page. Please verify your " +
"information to resolve the issue."
}
// Clear the error from the server.
Server.ClearError();
}
After you handle an error, you must clear it by calling the ClearError
method of the Server object (HttpServerUtility
class), otherwise you will see an error that has previously occurred.
Application_Error() not firing
If you have <customErrors mode="Off" />
while debugging, the code won't reach to Application_Error
event because exception is displayed inside browser right away.
If you want to reach Application_Error while debugging, you want to enable custom error -
<customErrors mode="On" defaultRedirect="~/Error.aspx" />
ASP.NET Global.asax Application_Error works but not when using the Error event
I'll up vote this for the first comment, but I don't believe the second part is true, I'm currently running my application with out the CustomErrors element in my webconfig and it's hitting the Error handler fine.
没有customErrors这个配置,应该也能触发application_error的
Page_Error not firing
Page_PreInit & OnPreInit - whats the difference?
make sure the AutoEventWireup is set to true
ASP.NET OnError not catching ConfigurationErrorsException
I peeked into the source code for ProcessRequestMain
. There is a try...catch
block in this method which catches any ConfigurationException
and throws it again (look into line 4680). All other Exceptions are handed over to HandleError
method which will call OnError
. (Also ThreadAbortException
is handled separately which is not your concern here.)
This is why you can't capture ConfigurationException
(and ConfigurationErrorsException
which is inherited from ConfigurationException
) using either OnError
method or Error
event in an ASP.NET page.
可以在application_error中捕获这种异常。
Using ELMAH
ELMAH (Error Logging Modules and Handlers) is an error logging facility that you plug into your ASP.NET application as a NuGet package. ELMAH provides the following capabilities:
- Logging of unhandled exceptions.
- A web page to view the entire log of recoded unhandled exceptions.
- A web page to view the full details of each logged exception.
- An email notification of each error at the time it occurs.
- An RSS feed of the last 15 errors from the log.
Before you can work with the ELMAH, you must install it. This is easy using the NuGet package installer. As mentioned earlier in this tutorial series, NuGet is a Visual Studio extension that makes it easy to install and update open source libraries and tools in Visual Studio.
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