Multipart forms from C# client

Question

I am trying to fill a form in a php application from a C# client (Outlook addin). I used Fiddler to see the original request from within the php application and the form is transmitted as a multipart/form. Unfortunately .Net does not come with native support for this type of forms (WebClient has only a method for uploading a file). Does anybody know a library or has some code to achieve this? I want to post different values and additionally (but only sometimes) a file.

Thanks for your help, Sebastian

Answer 1

Thanks for the answers, everybody! I recently had to get this to work, and used your suggestions heavily. However, there were a couple of tricky parts that did not work as expected, mostly having to do with actually including the file (which was an important part of the question). There are a lot of answers here already, but I think this may be useful to someone in the future (I could not find many clear examples of this online). I wrote a blog post that explains it a little more.

Basically, I first tried to pass in the file data as a UTF8 encoded string, but I was having problems with encoding files (it worked fine for a plain text file, but when uploading a Word Document, for example, if I tried to save the file that was passed through to the posted form using Request.Files[0].SaveAs(), opening the file in Word did not work properly. I found that if you write the file data directly using a Stream (rather than a StringBuilder), it worked as expected. Also, I made a couple of modifications that made it easier for me to understand.

By the way, the Multipart Forms Request for Comments and the W3C Recommendation for mulitpart/form-data are a couple of useful resources in case anyone needs a reference for the specification.

I changed the WebHelpers class to be a bit smaller and have simpler interfaces, it is now called FormUpload. If you pass a FormUpload.FileParameter you can pass the byte[] contents along with a file name and content type, and if you pass a string, it will treat it as a standard name/value combination.

Here is the FormUpload class:

// Implements multipart/form-data POST in C# http://www.ietf.org/rfc/rfc2388.txt // http://www.briangrinstead.com/blog/multipart-form-post-in-c public static class FormUpload {     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);          return PostForm(postUrl, userAgent, contentType, formData);     }     private static HttpWebResponse PostForm(string postUrl, string userAgent, string contentType, byte[] formData)     {         HttpWebRequest request = WebRequest.Create(postUrl) as HttpWebRequest;          if (request == null)         {             throw new NullReferenceException("request is not a http request");         }          // Set up the request properties.         request.Method = "POST";         request.ContentType = contentType;         request.UserAgent = userAgent;         request.CookieContainer = new CookieContainer();         request.ContentLength = formData.Length;          // You could add authentication here as well if needed:         // request.PreAuthenticate = true;         // request.AuthenticationLevel = System.Net.Security.AuthenticationLevel.MutualAuthRequested;         // request.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(System.Text.Encoding.Default.GetBytes("username" + ":" + "password")));          // Send the form data to the request.         using (Stream requestStream = request.GetRequestStream())         {             requestStream.Write(formData, 0, formData.Length);             requestStream.Close();         }          return request.GetResponse() as HttpWebResponse;     }      private static byte[] GetMultipartFormData(Dictionary<string, object> postParameters, string boundary)     {         Stream formDataStream = new System.IO.MemoryStream();         bool needsCLRF = false;          foreach (var param in postParameters)         {             // Thanks to feedback from commenters, add a CRLF to allow multiple parameters to be added.             // Skip it on the first parameter, add it to subsequent parameters.             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 data directly to the Stream, rather than serializing it to a string.                 formDataStream.Write(fileToUpload.File, 0, fileToUpload.File.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 the Stream into a byte[]         formDataStream.Position = 0;         byte[] formData = new byte[formDataStream.Length];         formDataStream.Read(formData, 0, formData.Length);         formDataStream.Close();          return formData;     }      public class FileParameter     {         public byte[] File { get; set; }         public string FileName { get; set; }         public string ContentType { get; set; }         public FileParameter(byte[] file) : this(file, null) { }         public FileParameter(byte[] file, string filename) : this(file, filename, null) { }         public FileParameter(byte[] file, string filename, string contenttype)         {             File = file;             FileName = filename;             ContentType = contenttype;         }     } } 

Here is the calling code, which uploads a file and a few normal post parameters:

// Read file data FileStream fs = new FileStream("c:\\people.doc", FileMode.Open, FileAccess.Read); byte[] data = new byte[fs.Length]; fs.Read(data, 0, data.Length); fs.Close();  // Generate post objects Dictionary<string, object> postParameters = new Dictionary<string, object>(); postParameters.Add("filename", "People.doc"); postParameters.Add("fileformat", "doc"); postParameters.Add("file", new FormUpload.FileParameter(data, "People.doc", "application/msword"));  // Create request and receive response string postURL = "http://localhost"; string userAgent = "Someone"; HttpWebResponse webResponse = FormUpload.MultipartFormDataPost(postURL, userAgent, postParameters);  // Process response StreamReader responseReader = new StreamReader(webResponse.GetResponseStream()); string fullResponse = responseReader.ReadToEnd(); webResponse.Close(); Response.Write(fullResponse);