使用 MultipartContent - 它工作正常，但工作正常吗？

Question

I've got a little app that generates an HttpMessage with Multipart content...

using (var client = new HttpClient())
{
    using (var content = new MultipartContent("mixed", "----123"))
    {
        content.Headers.ContentType = MediaTypeHeaderValue.Parse("multipart/mixed; boundary=----123");
        // repeated calls to content.Add(...)

        var result = client.PostAsync(url, content). Result;

        Console.WriteLine(result);
    }
}

and I have a little HttpServer that listens for POST calls and does this when it gets one...

var streamContent = new StreamContent(inputStream);
streamContent.Headers.ContentType = MediaTypeHeaderValue.Parse("multipart/mixed; boundary=----123");
var provider = streamContent.ReadAsMultipartAsync().Result;

foreach (var httpContent in provider.Contents)
{
    var t = httpContent.Headers.ContentType;
    var c = httpContent.ReadAsStringAsync().Result;
}

And it all works.

But if, in my receiver code, I do not include the line streamContent.Headers.ContentType... the receiver crashes on the var provider... line with the error Invalid 'HttpContent' instance provided. It does not have a content-type header value. 'HttpContent' instances must have a content-type header starting with 'multipart/'. Invalid 'HttpContent' instance provided. It does not have a content-type header value. 'HttpContent' instances must have a content-type header starting with 'multipart/'. .

So, whilst I have code that works, it will only work if I know, in advance, what the boundary is going to be.

This can't be right.

I've looked through, and tried, dozens of permutations based on questions here in SO and elsewhere but I can't find anything that works without me setting the ContentType header in the receiver and, therefore, knowing what the boundary value is.

What should I be doing?

UPDATE

If I remove the boundary part of the ContentType header in the receiver, it still crashes, but with a different error... Invalid 'HttpContent' instance provided. It does not have a 'multipart' content-type header with a 'boundary' parameter. Invalid 'HttpContent' instance provided. It does not have a 'multipart' content-type header with a 'boundary' parameter.

Answer 1

I don't think your server is doing what you think it is doing. new StreamContent(Stream) is used when you create your own stream content with the intent to return it from controller action. And the stream you pass to it should contain the raw data (entity, response body) that will be returned. It doesn't try to interpret the data from the stream in any way. Even if you pass valid http stream in the parameter, it won't try to parse content-type headers from it - you have to supply it. And that's a big IF, you didn't show where you get the inputStream , it's not standard part of MVC.

Actual content you received from client is accessible in Request.Content , along with the proper headers like content-type or boundary. ReadAsMultipartAsync should work on that too, but I never tried that extension in practice.

As a side note, using Task.Result should be last resort. Make your controller action async and await that task.

Edit: for illustration, I think this would work and doesn't require knowing the boundary in advance. Still, it's very suboptimal solution when you can just call Request.Content.ReadAsMultipartAsync() :

var streamContent = new StreamContent(inputStream);
streamContent.Headers.ContentType = Request.Content.Headers.ContentType;
                                 // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
var provider = streamContent.ReadAsMultipartAsync().Result;

foreach (var httpContent in provider.Contents)
{
    var t = httpContent.Headers.ContentType;
    var c = httpContent.ReadAsStringAsync().Result;
}