What is the error handling mechanism of .net

.NET's error handling mechanism is: 1. [Page_Error] event; 2. ErrorPage attribute; 3. [Application_Error] event; 4. [] configuration item. The above four processing mechanisms are mainly used for exception handling and page-level error handling.

.NET provides four error handling mechanisms:

1.Page_Error event

The Page_Error event provides a way to capture errors that occur at the page level. You can simply display an error message (as shown in the sample code below), log an event, or perform some other action.

private void Page_Load(object sender, System.EventArgs e)
   { // Put user code to initialize the page here
        throw new Exception("Page Error!");
   }
protected void Page_Error(object sender, EventArgs e) 
   {
      Exception objErr = Server.GetLastError().GetBaseException();
    Response.Write("Error:" + objErr.Message);
       Server.ClearError(); //同样要注意这句代码的使用
  }

Note: This example displays a detailed error message in the browser and is provided for illustration only. Be careful when displaying detailed information to the end users of your application. It is more appropriate to display a message to the user that an error has occurred and then log the specific error details.

2. ErrorPage property

You can set the ErrorPage property almost at any time on the page to determine which page it will redirect to when an error occurs on the page. For the ErrorPage attribute to work, the mode attribute in the configuration item must be set to "On".

this.ErrorPage = "~/ErrorHandling/PageError.html";

If Page_Error and ErrorPage both exist, what is the order of page execution when an Exception is thrown? The page will first execute the Page_Error event processing function. If the function Server.ClearError() is called in the Page_Error() event to clear the exception information, it will not jump to the page specified by the ErrorPage attribute; if Server.ClearError() is not called, the Exception information will continue. Throw it upward and the page will jump to the page specified by ErrorPage. This also proves the order of priority: Page_Error event > ErrorPage attribute.

3.Application_Error event

Similar to the Page_Error event, you can use the Application_Error event to capture errors that occur in the application. Because events occur application-wide, you can log application error information or handle other application-level errors that may occur. Add the following code to the Global.asax file and it will be OK.

protected void Application_Error(object sender, EventArgs e)
      {
         Exception ex = Server.GetLastError().GetBaseException();
         //实际应用中这里可以将Exception信息记Log或是保存到数据库中
         //还可以将错误发邮件给网站维护人员
          Response.Write("Error:" + ex.Message);
         //清除Exception,避免继续传递给上一级处理
         //这里上级就是<CustomerErrors>配置节了
          Server.ClearError(); 
     }

4. Configuration item

The configuration section in the configuration file web.config can specify the redirect page as the default Error page defaultRedirect or specify a specific page based on the HTTP error code raised. This custom page is displayed if an error occurs that was not caught at any previous level of the application.

<customErrors mode="On" defaultRedirect="~/ErrorHandling/ApplicationError.html"> <error statusCode="404" redirect="~/ErrorHandling/404.html" /> </customErrors>

Similarly, if Application_Error and exist at the same time, there is also a problem of execution order. Because the priority Application_Error event> configuration item, when an application-level error occurs, the code in the Application_Error event is executed first. If the Server.ClearError() function is called in the Application_Error event, the configuration section The defaultRedirect does not work because the Exception has been cleared; if the Server.ClearError() function is not called in the Application_Error event, the error page will be relocated to the URL page specified by defaultRedict to display a friendly error message to the user.

Through the analysis of the above four error handling mechanisms provided by .NET, we can classify them from different perspectives to facilitate our understanding and use.

1. Functionally classified: used for exception handling (Handling exceptions) are the Page_Error event and Application_Error event; for user error page redirection (Redirecting the user to an error page) are the ErrorPage attribute and

2. Classification from the scope of error handling: Page_Error event and ErrorPage attribute are used for page level error handling; Application_Error event and < are used for application level error handling. ;customErrors>Configuration item.

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