'system.argumentexception' When uploading multipart

Question

I am trying to post multipart data using System.Net.Http.HttpClient but when I am instanciating my content I am getting this exception:

A first chance exception of type 'System.ArgumentException' occurred in System.Net.Http.DLL
An exception of type 'System.ArgumentException' occurred in System.Net.Http.DLL and wasn't handled before a managed/native boundary
e: System.ArgumentException: The format of value '---###---' is invalid.
Parameter name: boundary
   at System.Net.Http.MultipartContent.ValidateBoundary(String boundary)
   at System.Net.Http.MultipartContent..ctor(String subtype, String boundary)
   at System.Net.Http.MultipartFormDataContent..ctor(String boundary)
   at RestClientUploadFoto.MultipartStackOverflow.<postMultipart>d__2.MoveNext()

Why does it happen?

here is the method in where I am getting the exception:

public async Task postMultipart()
{
    var client = new HttpClient();
    client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "multipart/form-data");

    // boundary      
    String boundary = "---###---"; // should never occur in your data

    // This is the postdata
    MultipartFormDataContent content = new MultipartFormDataContent(boundary);
    content.Add(new StringContent("12", Encoding.UTF8), "userId");
    content.Add(new StringContent("78", Encoding.UTF8), "noOfAttendees");
    content.Add(new StringContent("chennai", Encoding.UTF8), "locationName");
    content.Add(new StringContent("32.56", Encoding.UTF8), "longitude");
    content.Add(new StringContent("32.56", Encoding.UTF8), "latitude");

    Console.Write(content);
    // upload the file sending the form info and ensure a result.
    // it will throw an exception if the service doesn't return a valid successful status code
    await client.PostAsync(fileUploadUrl, content)
        .ContinueWith((postTask) =>
        {
            postTask.Result.EnsureSuccessStatusCode();
        });

}

Well what's the detail of the exception? Please post the complete stack trace including the message. — Jon Skeet
– Jon Skeet, Commented Mar 16, 2015 at 11:16
Well that seems pretty self-explanatory, doesn't it? ---###--- is an invalid boundary. Have you tried using a different boundary string? — Jon Skeet
– Jon Skeet, Commented Mar 16, 2015 at 11:27
thanks, I had no idea what a boundary is, I am going to read it first — eeadev
– eeadev, Commented Mar 16, 2015 at 11:28

Community · Accepted Answer · 2021-10-07 08:58:39Z

4

# is invalid in a MIME boundary.

From RFC 2046:

The only mandatory global parameter for the "multipart" media type is the boundary parameter, which consists of 1 to 70 characters from a set of characters known to be very robust through mail gateways, and NOT ending with white space. (If a boundary delimiter line appears to end with white space, the white space must be presumed to have been added by a gateway, and must be deleted.) It is formally specified by the following BNF:
boundary := 0*69<bchars> bcharsnospace

bchars := bcharsnospace / " "

bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" /
                 "+" / "_" / "," / "-" / "." /
                 "/" / ":" / "=" / "?"

So basically, you should use a boundary string which doesn't contain a #. I'd suggest using one which doesn't contain hyphens either, just because the boundary lines will have hyphens anyway (at the start and end). Having them in the boundary as well is a bit confusing.

Unless you need a specific boundary, I suggest you just call the parameterless constructor, which will use a random GUID as the boundary.

edited Oct 7, 2021 at 8:58

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answered Mar 16, 2015 at 11:29

Jon Skeet

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3 Comments

eeadev Over a year ago

do you know how to get the response body from there?

Jon Skeet Over a year ago

@eeadev: I don't know what you mean by "from there" but this sounds like it should be a new quesiton.

eeadev Over a year ago

yes, right; here is the post: stackoverflow.com/questions/29078917/…

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