ASP. Net hosting uploading file message 500 error in IIS

Version

• Windows Server 2016
• dotnet-hosting-6.0.2-win
• IIS 10.0

symptoms

• Local tests are normal
• Upload ~<50kb The file of is normal
• Upload ~>60kb The file message of is abnormal 500 (Internal Server Error), And the exception is not caught

Climb a pit

Because no exception is caught by the controller , So my first thought was IIS On the level of .

see IIS journal

No abnormal information is found

open stdoutLog（ The key ）

Exception information found ：

fail: Microsoft.AspNetCore.Server.IIS.Core.IISHttpServer[2]
      Connection ID "17726168139235852346", Request ID "8000003d-0001-f600-b63f-84710c7967bb": An unhandled exception was thrown by the application.
      System.UnauthorizedAccessException: Access to the path 'C:\Windows\TEMP\ASPNETCORE_9df0985f-8e73-4710-a2c6-8c15c60b0835.tmp' is denied.
         ···

The truth

When trying to cache files on disk , Because of the permission problem of the folder, a System.UnauthorizedAccessException.

solve

to IIS user c:\windows\temp Read / write permission of .

reason

Why? ~50kb Your files can , A little more is not enough ？ The reason is found in the description document of the file upload ：

https://docs.microsoft.com/en-us/aspnet/core/mvc/models/file-uploads?view=aspnetcore-6.0#file-upload-scenarios
The entire file is read in IFormFile, It is a file C# Representation form , Used to process or save files .
Resources used for file uploading （ disk 、 Memory ） Depends on the number and size of concurrent file uploads . If the app tries to buffer too many Uploads , The site will crash when there is not enough memory or disk space . If the size or frequency of file uploading will consume application resources , Please use streaming .
It will be greater than 64 KB All the individual buffer files are moved from memory to the temporary files on disk .