Post to asp Net core length limitation and solution when transferring data

Use HTTP The standard practice for uploading files under the agreement is ： Use  multipart/form-data . But sometimes the files to be uploaded are not too large for simplicity , Will still apply  application/x-www-form-urlencoded  Upload files , This requires encoding binary files before uploading , For example, use Base64 .

public async Task<IActionResult> SubmitImage(int id, [FromForm(Name = "image")] string image)
{
    if (string.IsNullOrWhiteSpace(image)) throw new UserFriendlyException(" Picture cannot be empty ");
    var data = Convert.FromBase64String(image);
    //TODO: Processing images 
}

The above code has been working well , After a long time, I received feedback from customers that several pictures failed to be uploaded , Clearly selected the picture but still prompted “ Picture cannot be empty ”. This explanation image Parameters are not properly bound . The size of the picture is 6MB about , There is obviously no breakthrough IIS The limitation of （ The default is 30MB）, There was no breakthrough ASP.NET Ccor Limits on the size of a single request （ The default is 30MB）. I went specially to check Windows journal , No error message is seen .

Simulate a user's request , It is HTTP I saw the picture in the packet capture , But in the debugger image It's really empty . After several searches , I found one named  FormOptions  Configuration class ：

https://docs.microsoft.com/zh-cn/dotnet/api/microsoft.aspnetcore.http.features.formoptions?view=aspnetcore-6.0

Among them  ValueLengthLimit  Attributes are defined as follows ：

A limit on the length of individual form values. Forms containing values that exceed this limit will throw an InvalidDataException when parsed. Defaults to 4,194,304 bytes‬, which is approximately 4MB.

in other words ： By default ,ASP.NET Core Limits every one of them POST The length of the data value is 4 MB size , If it exceeds, it will be thrown  InvalidDataException  abnormal . I probably encountered this limitation , But I'm not sure because I didn't throw an exception .

services.Configure<FormOptions>(options=>{
  options.ValueLengthLimit = 209715200;//200MB
});

The above code raises the limit to 200MB, The tested problem is solved .

Use Base64 Although it will be simplified after encoding the file , But at the same time, the network transmission content will increase , Or encounter the exception shown in this article （ By default ,multipart/form-data  The limit on a single upload item is 128MB）. If you're using MVC, It can also be used directly  RequestFormLimitsAttribute  Solve the problem ：

https://docs.microsoft.com/zh-cn/dotnet/api/microsoft.aspnetcore.mvc.requestformlimitsattribute?view=aspnetcore-6.0