Example of uploading a file using the FileUpload control in Asp.net

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/// <Summary> use the FileUpload control to upload a file </summary> /// <Param name = "page"> this </param> /// <Param name = "path"> File storage directory. Example of relative path :"~ /UploadFile ", absolute path example:" E: UploadFile ". Tip: Use a slash in the web path and a backslash in the file system </param> /// <Param name = "fu"> FileUpload </param> /// <Param name = "checkFileName"> whether to check the file name. "true" indicates that duplicate file names are not allowed. "false" indicates renaming the file name with time. </param> /// <Param name = "allowTypes"> specifies the file type and extension that can be uploaded, for example, "xls". If the file type is not limited, pass null. </param> /// <Returns> absolute file path </returns>          Public static string UploadFileToServer (Page page, String path, FileUpload fu, Boolean checkFileName, params String [] allowTypes)         {  // Record file name String fileName = fu. FileName; // Select a file If (string. IsNullOrEmpty (fileName ))             {  MsgBox. Alert (page, "select a file first! "); Return null;             }  // Record file extension String fileType = fileName. Substring (fileName. LastIndexOf ('.') + 1 ); // Determine whether the extension is allowed Bool typeRight = false; // Record the types of files that can be uploaded String allowType = ""; // Specify the file type If (allowTypes! = Null)             {  // Traverse the array of allowed file types For (int I = 0; I <allowTypes. Length; I ++)                 {  // If it has been judged as permitted, it will not be judged If (! TypeRight)                     {  // Determine whether the extension size conversion meets the requirements If (fileType = allowTypes [I] | fileType. ToLowerInvariant () = allowTypes [I] | fileType. ToUpperInvariant () = allowTypes [I])                         {  // Set to allow TypeRight = true;                         }                      }  // Record the types of files that can be uploaded AllowType + = "." + allowTypes [I] + "| ";                 }  // Delete the last separator AllowType = allowType. Remove (allowType. LastIndexOf ("| "));             }  // When the file type is not specified Else             {  // Directly set to allow TypeRight = true;             }  // Incorrect extension If (! TypeRight)             {  // Indicates the file type that can be uploaded. MsgBox. Alert (page, "the file format is incorrect. Select a file with the extension [" + allowType +! "); Return null;             }  // Whether the file can be obtained normally If (fu. PostedFile. ContentLength = 0)             {  MsgBox. Alert (page, "the selected file cannot be found. Please reselect it! "); Return null;             }    // Absolute directory path String dirRootPath = ""; // File path exception handling Try             {  // If the path is relative If (! Path. IsPathRooted (path ))                 {  // Convert relative directory path to absolute path DirRootPath = HttpContext. Current. Server. MapPath (@ "" + path + "/"). Trim ();                 }  Else                 {  // Save path DirRootPath = path;                 }  // Whether the file upload directory exists DirectoryInfo dirInfo = new DirectoryInfo (dirRootPath ); If (! DirInfo. Exists)                 {  // This directory is created if it does not exist. DirInfo. Create ();                 }              }  Catch (Exception pathError)             {  // Exception dialog box prompt MsgBox. Alert (page, "error:" + pathError. Message ); Return null;             }  // The absolute path of the record file String fileRootPath = ""; // When you need to check whether the file name is repeated If (checkFileName)             {  // Absolute file path FileRootPath = Path. Combine (dirRootPath, fileName ); // The file name already exists. If (File. Exists (fileRootPath ))                 {  // Rename MsgBox. Alert (page, "a file with the same name already exists on the server. Modify the file name and try again! "); Return null;                 }              }  Else             {  // Rename the selected file by time String newFileName = fileName. remove (fileName. lastIndexOf (". ") + DateTime. now. toString ("yyMMddHHmmss") + DateTime. now. millisecond. toString () + ". "+ fileType; // Absolute file path FileRootPath = Path. Combine (dirRootPath, newFileName );             }  // Upload to the server Fu. SaveAs (fileRootPath );   // Returns the absolute path of the object. Return fileRootPath;

Pay attention to the following three aspects::

1. Check whether the file is included

Before calling the SaveAs method to save the file to the server, use the HasFile attribute to verify that the FileUpload control does contain the file. If HasFile returns true, the SaveAs method is called. If false is returned, a message is displayed to the user indicating that the control does not contain files. Do not check the PostedFile attribute to determine whether the file to be uploaded exists, because this attribute contains 0 bytes by default. Therefore, even if the FileUpload control is empty, the PostedFile attribute returns a non-null value.

2. File upload size limit

By default, the size of the uploaded file is limited to 4096 KB (4 MB ). You can set the maxRequestLength attribute of the httpRuntime element to allow uploading larger files. To increase the maximum file size allowed by the entire application, set the maxRequestLength attribute in the Web. config file. To increase the maximum file size allowed by the specified page, set the maxRequestLength attribute in the location element in Web. config.
When uploading a large file, you may receive the following error message:
Aspnet_wp.exe (PID: 1520) was recycled because memory consumption exceeded 460 MB (60 percent of available RAM ).
The above information indicates that the size of the uploaded file cannot exceed 60% of the server memory size. Here, 60% is Web. The default configuration of the config file is the default value of the memoryLimit attribute in the <processModel> configuration section. It can be modified, but the larger the file to be uploaded, the smaller the chances of success. It is not recommended.

3. Write permission for the upload folder

Applications can obtain write access permissions in two ways. You can explicitly grant the write access permission to the directory where the uploaded files will be saved to the account used to run the application. You can also increase the level of trust granted to ASP. NET applications. To grant the application write access permission to the execution directory, you must grant the application the AspNetHostingPermission object and set its trust level to the value of AspNetHostingPermissionLevel. Medium. Increasing the level of trust can improve the application's access to server resources. Please note that this method is not safe, because if a malicious user controls the application, he or she can run the application with a higher level of trust. The best practice is to run ASP. NET applications in user context that only has the minimum privilege required to run the application.