How to upload file to server with HTTP POST multipart/form-data?

Basic implementation using MultipartFormDataContent :-

HttpClient httpClient = new HttpClient();MultipartFormDataContent form = new MultipartFormDataContent();form.Add(new StringContent(username), "username");form.Add(new StringContent(useremail), "email");form.Add(new StringContent(password), "password");            form.Add(new ByteArrayContent(file_bytes, 0, file_bytes.Length), "profile_pic", "hello1.jpg");HttpResponseMessage response = await httpClient.PostAsync("PostUrl", form);response.EnsureSuccessStatusCode();httpClient.Dispose();string sd = response.Content.ReadAsStringAsync().Result;

Here's my final working code. My web service needed one file (POST parameter name was "file") & a string value (POST parameter name was "userid").

/// <summary>/// Occurs when upload backup application bar button is clicked. Author : Farhan Ghumra /// </summary>private async void btnUploadBackup_Click(object sender, EventArgs e){    var dbFile = await ApplicationData.Current.LocalFolder.GetFileAsync(Util.DBNAME);    var fileBytes = await GetBytesAsync(dbFile);    var Params = new Dictionary<string, string> { { "userid", "9" } };    UploadFilesToServer(new Uri(Util.UPLOAD_BACKUP), Params, Path.GetFileName(dbFile.Path), "application/octet-stream", fileBytes);}/// <summary>/// Creates HTTP POST request & uploads database to server. Author : Farhan Ghumra/// </summary>private void UploadFilesToServer(Uri uri, Dictionary<string, string> data, string fileName, string fileContentType, byte[] fileData){    string boundary = "----------" + DateTime.Now.Ticks.ToString("x");    HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create(uri);    httpWebRequest.ContentType = "multipart/form-data; boundary=" + boundary;    httpWebRequest.Method = "POST";    httpWebRequest.BeginGetRequestStream((result) =>    {        try        {            HttpWebRequest request = (HttpWebRequest)result.AsyncState;            using (Stream requestStream = request.EndGetRequestStream(result))            {                WriteMultipartForm(requestStream, boundary, data, fileName, fileContentType, fileData);            }            request.BeginGetResponse(a =>            {                try                {                    var response = request.EndGetResponse(a);                    var responseStream = response.GetResponseStream();                    using (var sr = new StreamReader(responseStream))                    {                        using (StreamReader streamReader = new StreamReader(response.GetResponseStream()))                        {                            string responseString = streamReader.ReadToEnd();                            //responseString is depend upon your web service.                            if (responseString == "Success")                            {                                MessageBox.Show("Backup stored successfully on server.");                            }                            else                            {                                MessageBox.Show("Error occurred while uploading backup on server.");                            }                         }                    }                }                catch (Exception)                {                }            }, null);        }        catch (Exception)        {        }    }, httpWebRequest);}/// <summary>/// Writes multi part HTTP POST request. Author : Farhan Ghumra/// </summary>private void WriteMultipartForm(Stream s, string boundary, Dictionary<string, string> data, string fileName, string fileContentType, byte[] fileData){    /// The first boundary    byte[] boundarybytes = Encoding.UTF8.GetBytes("--" + boundary + "\r\n");    /// the last boundary.    byte[] trailer = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");    /// the form data, properly formatted    string formdataTemplate = "Content-Dis-data; name=\"{0}\"\r\n\r\n{1}";    /// the form-data file upload, properly formatted    string fileheaderTemplate = "Content-Dis-data; name=\"{0}\"; filename=\"{1}\";\r\nContent-Type: {2}\r\n\r\n";    /// Added to track if we need a CRLF or not.    bool bNeedsCRLF = false;    if (data != null)    {        foreach (string key in data.Keys)        {            /// if we need to drop a CRLF, do that.            if (bNeedsCRLF)                WriteToStream(s, "\r\n");            /// Write the boundary.            WriteToStream(s, boundarybytes);            /// Write the key.            WriteToStream(s, string.Format(formdataTemplate, key, data[key]));            bNeedsCRLF = true;        }    }    /// If we don't have keys, we don't need a crlf.    if (bNeedsCRLF)        WriteToStream(s, "\r\n");    WriteToStream(s, boundarybytes);    WriteToStream(s, string.Format(fileheaderTemplate, "file", fileName, fileContentType));    /// Write the file data to the stream.    WriteToStream(s, fileData);    WriteToStream(s, trailer);}/// <summary>/// Writes string to stream. Author : Farhan Ghumra/// </summary>private void WriteToStream(Stream s, string txt){    byte[] bytes = Encoding.UTF8.GetBytes(txt);    s.Write(bytes, 0, bytes.Length);}/// <summary>/// Writes byte array to stream. Author : Farhan Ghumra/// </summary>private void WriteToStream(Stream s, byte[] bytes){    s.Write(bytes, 0, bytes.Length);}/// <summary>/// Returns byte array from StorageFile. Author : Farhan Ghumra/// </summary>private async Task<byte[]> GetBytesAsync(StorageFile file){    byte[] fileBytes = null;    using (var stream = await file.OpenReadAsync())    {        fileBytes = new byte[stream.Size];        using (var reader = new DataReader(stream))        {            await reader.LoadAsync((uint)stream.Size);            reader.ReadBytes(fileBytes);        }    }    return fileBytes;}

I am very much thankful to Darin Rousseau for helping me.

This simplistic version also works.

public void UploadMultipart(byte[] file, string filename, string contentType, string url){    var webClient = new WebClient();    string boundary = "------------------------" + DateTime.Now.Ticks.ToString("x");    webClient.Headers.Add("Content-Type", "multipart/form-data; boundary=" + boundary);    var fileData = webClient.Encoding.GetString(file);    var package = string.Format("--{0}\r\nContent-Disposition: form-data; name=\"file\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n{3}\r\n--{0}--\r\n", boundary, filename, contentType, fileData);    var nfile = webClient.Encoding.GetBytes(package);    byte[] resp = webClient.UploadData(url, "POST", nfile);}

Add any extra required headers if needed.