Posting Multipart Form Data Issue

Question

I'm using HttpWebRequest to POST to an MMS API. The body of the post contains XML data about the delivery and the MMS message as a MIME-multipart attachment that needs to be Base64 encoded.

The post is successful, but I receive only the text, not the image.

When looking at my code, it seems to build the form data okay, but when I convert it back to a string, the file data is missing.

CONTENTS OF str VARIABLE:

------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="username"

trilogy ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="password"

ZBo8KE6m ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="number"

61402720898 ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="subject"

Test Message Subject

------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="message"

Test message body.

------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="type0"

image/jpeg ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="type1"

image/jpeg ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="name0"

Voucher.png ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="name1"

QRCode.png ------------f2de17263b724d5a919b14a6834c489f Content-Disposition: form-data; name="attachment0"; filename="Voucher.png" Content-Type: image/jpeg

�PNG

    private static readonly Encoding encoding = Encoding.UTF8;
    public static HttpWebResponse MultipartFormDataPost(string postUrl, string userAgent, Dictionary<string, object> postParameters)
    {
        string formDataBoundary = String.Format("----------{0:N}", Guid.NewGuid());
        string contentType = "multipart/form-data; boundary=" + formDataBoundary;

        byte[] formData = GetMultipartFormData(postParameters, formDataBoundary);
        var str = Encoding.UTF8.GetString(formData);

        return PostForm(postUrl, userAgent, contentType, formData);
    }

    private static byte[] GetMultipartFormData(Dictionary<string, object> postParameters, string boundary)
    {
        Stream formDataStream = new System.IO.MemoryStream();
        byte[] formData = new byte[0];
        bool needsCLRF = false;
        try
        {
            foreach (var param in postParameters)
            {
                // add a CRLF to allow multiple parameters to be added (skip it on the 1st parameter)
                if (needsCLRF)
                    formDataStream.Write(encoding.GetBytes("\r\n"), 0, encoding.GetByteCount("\r\n"));

                needsCLRF = true;

                if (param.Value is FileParameter)
                {
                    FileParameter fileToUpload = (FileParameter)param.Value;

                    // add just the first part of this param, since we will write the file data directly to the stream
                    string header = string.Format("--{0}\r\nContent-Disposition: form-data; name=\"{1}\"; filename=\"{2}\"\r\nContent-Type: {3}\r\n\r\n",
                        boundary,
                        param.Key,
                        fileToUpload.FileName ?? param.Key,
                        fileToUpload.ContentType ?? "application/octet-stream");

                    formDataStream.Write(encoding.GetBytes(header), 0, encoding.GetByteCount(header));

                    // write the file to the stream
                    string str = Convert.ToBase64String(fileToUpload.File);
                    byte[] myBytes = Convert.FromBase64String(str);
                    formDataStream.Write(myBytes, 0, myBytes.Length);
                                        }
                else
                {
                    string postData = string.Format("--{0}\r\nContent-Disposition: form-data; name=\"{1}\"\r\n\r\n{2}",
                        boundary,
                        param.Key,
                        param.Value);
                    formDataStream.Write(encoding.GetBytes(postData), 0, encoding.GetByteCount(postData));
                }
            }

            // add the end of the request. Start with a newline
            string footer = "\r\n--" + boundary + "--\r\n";
            formDataStream.Write(encoding.GetBytes(footer), 0, encoding.GetByteCount(footer));

            // dump stream into byte array
            formDataStream.Position = 0;
            formData = new byte[formDataStream.Length];
            formDataStream.Read(formData, 0, formData.Length);
            formDataStream.Close();
        }
        catch (Exception ex)
        {
            gFunc.ProcessError(true, ex.ToString(), "Post Data");
        }
        return formData;
    }

I managed to fix part of the problem by changing byte[] myBytes = Convert.FromBase64String(str); to byte[] myBytes = encoding.GetBytes(str); but when I receive the MMS I still get only the text and no picture. — Ross Kelly

Ross Kelly Ross Kelly · Accepted Answer · 2019-07-10T07:45:08

It turns out that the problem was with the post data.

After removing the additional fields in the "Content-Distribution" for the File parameter, it worked.

PREVIOUSLY: Content-Disposition: form-data; name="attachment0"; filename="Voucher.png" Content-Type: image/jpeg

NOW: Content-Disposition: form-data; name="attachment0"

Posting Multipart Form Data Issue

1 Answers