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

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With  NET 4 5 you currently could use System Net Http namespace  Below the example for uploading single file using multipart form data   using System  using System IO  using System Net Http   namespace HttpClientTest       class Program               static void Main string   args                        var client   new HttpClient                var content   new MultipartFormDataContent                content Add new StreamContent File Open        Image1 png   FileMode Open     Image    Image png                content Add new StringContent  Place string content here     Content-Id in the HTTP                 var result   client PostAsync  https   hostname api Account UploadAvatar   content               Console WriteLine result Result ToString

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Building on dnolans example  this is the version I could actually get to work  there were some errors with the boundary  encoding wasn t set   -   To send the data   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create  http   you url here    oRequest ContentType    multipart form-data  boundary     PostData boundary  oRequest Method    POST   PostData pData   new PostData    Encoding encoding   Encoding UTF8  Stream oStream   null          set the parameters  read files  etc  IE     pData Params Add new PostDataParam  email    example example com   PostDataParamType Field       pData Params Add new PostDataParam  fileupload    filename txt    filecontents  PostDataParamType File        byte   buffer   encoding GetBytes pData GetPostData      oRequest ContentLength   buffer Length   oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close     HttpWebResponse oResponse    HttpWebResponse oRequest GetResponse      The PostData class should look like   public class PostData          Change this if you need to  not necessary     public static string boundary    AaB03x        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                 StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine  --    boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  application octet-stream                    sb AppendLine                    sb AppendLine p Value                             else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine  --    boundary    --             return sb ToString             public enum PostDataParamType       Field      File    public class PostDataParam       public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public PostDataParam string name  string filename  string value  PostDataParamType type                Name   name          Value   value          FileName   filename          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type

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Below is the code which I m using         This URL not exist  it s only an example      string url    http   myBox s3 amazonaws com          Instantiate new CustomWebRequest class     CustomWebRequest wr   new CustomWebRequest url         Set values for parameters     wr ParamsCollection Add new ParamsStruct  key      filename          wr ParamsCollection Add new ParamsStruct  acl    public-read         wr ParamsCollection Add new ParamsStruct  success action redirect    http   www yahoo com         wr ParamsCollection Add new ParamsStruct  x-amz-meta-uuid    14365123651274         wr ParamsCollection Add new ParamsStruct  x-amz-meta-tag             wr ParamsCollection Add new ParamsStruct  AWSAccessKeyId    zzzz                     wr ParamsCollection Add new ParamsStruct  Policy    adsfadsf         wr ParamsCollection Add new ParamsStruct  Signature    hH6lK6cA            For file type  send the inputstream of selected file     StreamReader sr   new StreamReader   file txt        wr ParamsCollection Add new ParamsStruct  file   sr  ParamsStruct ParamType File   file txt          wr PostData      from the following link I ve downloaded the same code http   www codeproject com KB cs multipart request C  aspx  Any Help

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Thanks for the code  it saved me a lot of time  including the Except100 error     Anyway  I found a bug in the code  here   formDataStream Write encoding GetBytes postData   0  postData Length     In case your POST data is utf-16  postData Length  will return the number of characters and not the number of bytes  This will truncate the data being posted  for example  if you have 2 chars that are encoded as utf-16  they take 4 bytes  but postData Length will say it takes 2 bytes  and you loose the 2 final bytes of the posted data    Solution - replace that line with   byte   aPostData encoding GetBytes postData   formDataStream Write aPostData  0  aPostData Length     Using this  the length is calculated by the size of the byte    not the string size

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My implementation       lt summary gt      Sending file via multipart form-data      lt  summary gt       lt param name  url  gt URL for send lt  param gt       lt param name  file  gt Local file path lt  param gt       lt param name  paramName  gt Request file param lt  param gt       lt param name  contentType  gt Content-Type file headr lt  param gt       lt param name  nvc  gt Additional post params lt  param gt  private static string httpUploadFile string url  string file  string paramName  string contentType  NameValueCollection nvc          delimeter     var boundary    ---------------------------    DateTime Now Ticks ToString  x           creating request     var wr    HttpWebRequest WebRequest Create url       wr ContentType    multipart form-data  boundary     boundary      wr Method    POST       wr KeepAlive   true         sending request     using var requestStream   wr GetRequestStream                  using  var requestWriter   new StreamWriter requestStream  Encoding UTF8                           params             const string formdataTemplate    Content-Disposition  form-data  name    0    r n r n 1                foreach  string key in nvc Keys                                requestWriter Write boundary                   requestWriter Write String Format formdataTemplate  key  nvc key                               requestWriter Write boundary                  file header             const string headerTemplate    Content-Disposition  form-data  name    0     filename    1    r nContent-Type   2  r n r n               requestWriter Write String Format headerTemplate  paramName  file  contentType                   file content             using  var fileStream   new FileStream file  FileMode Open  FileAccess Read                                 fileStream CopyTo requestStream                              requestWriter Write   r n--    boundary    -- r n                           reading response     try               using  var wresp    HttpWebResponse wr GetResponse                          if  wresp StatusCode    HttpStatusCode OK                                using  var responseStream   wresp GetResponseStream                                          if  responseStream    null                          return null                      using  var responseReader   new StreamReader responseStream                                                 return responseReader ReadToEnd                                                                       throw new ApplicationException  Error while upload files  Server status code      wresp StatusCode ToString                         catch  Exception ex                throw new ApplicationException  Error while uploading file   ex

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I needed to simulate a browser login to a website to get a login cookie  and the login form was multipart form-data   I took some clues from the other answers here  and then tried to get my own scenario working  It took a bit of frustrating trial and error before it worked right  but here is the code       public static class WebHelpers                    lt summary gt              Post the data as a multipart form              lt  summary gt         public static HttpWebResponse MultipartFormDataPost string postUrl  string userAgent  Dictionary lt string  string gt  values                      string formDataBoundary    ---------------------------    WebHelpers RandomHexDigits 12              string contentType    multipart form-data  boundary     formDataBoundary              string formData   WebHelpers MakeMultipartForm values  formDataBoundary              return WebHelpers PostForm postUrl  userAgent  contentType  formData                          lt summary gt              Post a form              lt  summary gt          public static HttpWebResponse PostForm string postUrl  string userAgent  string contentType  string formData                        HttpWebRequest request   WebRequest Create postUrl  as HttpWebRequest               if  request    null                                throw new NullReferenceException  request is not a http request                                  Add these  as we re doing a POST             request Method    POST               request ContentType   contentType              request UserAgent   userAgent              request CookieContainer   new CookieContainer                    We need to count how many bytes we re sending               byte   postBytes   Encoding UTF8 GetBytes formData               request ContentLength   postBytes Length               using  Stream requestStream   request GetRequestStream                                     Push it out there                 requestStream Write postBytes  0  postBytes Length                   requestStream Close                               return request GetResponse   as HttpWebResponse                          lt summary gt              Generate random hex digits               lt  summary gt          public static string RandomHexDigits int count                        Random random   new Random                StringBuilder result   new StringBuilder                for  int i   0  i  lt  count  i                                  int digit   random Next 16                   result AppendFormat   0 x    digit                              return result ToString                            lt summary gt              Turn the key and value pairs into a multipart form              lt  summary gt          private static string MakeMultipartForm Dictionary lt string  string gt  values  string boundary                        StringBuilder sb   new StringBuilder                 foreach  var pair in values                                sb AppendFormat  -- 0  r nContent-Disposition  form-data  name    1    r n r n 2  r n   boundary  pair Key  pair Value                              sb AppendFormat  -- 0 -- r n   boundary                return sb ToString                            It doesn t handle file data  just form since that s all that I needed  I called like this       try               using  HttpWebResponse response   WebHelpers MultipartFormDataPost postUrl  UserAgentString  this loginForm                          if  response    null                                Cookie loginCookie   response Cookies  logincookie

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I needed to simulate a browser login to a website to get a login cookie  and the login form was multipart form-data   I took some clues from the other answers here  and then tried to get my own scenario working  It took a bit of frustrating trial and error before it worked right  but here is the code       public static class WebHelpers                    lt summary gt              Post the data as a multipart form              lt  summary gt         public static HttpWebResponse MultipartFormDataPost string postUrl  string userAgent  Dictionary lt string  string gt  values                      string formDataBoundary    ---------------------------    WebHelpers RandomHexDigits 12              string contentType    multipart form-data  boundary     formDataBoundary              string formData   WebHelpers MakeMultipartForm values  formDataBoundary              return WebHelpers PostForm postUrl  userAgent  contentType  formData                          lt summary gt              Post a form              lt  summary gt          public static HttpWebResponse PostForm string postUrl  string userAgent  string contentType  string formData                        HttpWebRequest request   WebRequest Create postUrl  as HttpWebRequest               if  request    null                                throw new NullReferenceException  request is not a http request                                  Add these  as we re doing a POST             request Method    POST               request ContentType   contentType              request UserAgent   userAgent              request CookieContainer   new CookieContainer                    We need to count how many bytes we re sending               byte   postBytes   Encoding UTF8 GetBytes formData               request ContentLength   postBytes Length               using  Stream requestStream   request GetRequestStream                                     Push it out there                 requestStream Write postBytes  0  postBytes Length                   requestStream Close                               return request GetResponse   as HttpWebResponse                          lt summary gt              Generate random hex digits               lt  summary gt          public static string RandomHexDigits int count                        Random random   new Random                StringBuilder result   new StringBuilder                for  int i   0  i  lt  count  i                                  int digit   random Next 16                   result AppendFormat   0 x    digit                              return result ToString                            lt summary gt              Turn the key and value pairs into a multipart form              lt  summary gt          private static string MakeMultipartForm Dictionary lt string  string gt  values  string boundary                        StringBuilder sb   new StringBuilder                 foreach  var pair in values                                sb AppendFormat  -- 0  r nContent-Disposition  form-data  name    1    r n r n 2  r n   boundary  pair Key  pair Value                              sb AppendFormat  -- 0 -- r n   boundary                return sb ToString                            It doesn t handle file data  just form since that s all that I needed  I called like this       try               using  HttpWebResponse response   WebHelpers MultipartFormDataPost postUrl  UserAgentString  this loginForm                          if  response    null                                Cookie loginCookie   response Cookies  logincookie

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A little optimization of the class before  In this version the files are not totally loaded into memory   Security advice  a check for the boundary is missing  if the file contains the bounday it will crash   namespace WindowsFormsApplication1       public static class FormUpload               private static string NewDataBoundary                         Random rnd   new Random                string formDataBoundary                   while  formDataBoundary Length  lt  15                                formDataBoundary   formDataBoundary   rnd Next                              formDataBoundary   formDataBoundary Substring 0  15               formDataBoundary    -----------------------------    formDataBoundary              return formDataBoundary                     public static HttpWebResponse MultipartFormDataPost string postUrl  IEnumerable lt Cookie gt  cookies  Dictionary lt string  string gt  postParameters                        string boundary   NewDataBoundary                 HttpWebRequest request    HttpWebRequest WebRequest Create postUrl                   Set up the request properties             request Method    POST               request ContentType    multipart form-data  boundary     boundary              request UserAgent    PhasDocAgent 1 0               request CookieContainer   new CookieContainer                 foreach  var cookie in cookies                                request CookieContainer Add cookie                               region WRITING STREAM             using  Stream formDataStream   request GetRequestStream                                  foreach  var param in postParameters                                        if  param Value StartsWith  file                                                     string filepath   param Value Substring 7                               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                              Path GetFileName filepath     param Key                              MimeTypes GetMime filepath                             formDataStream Write Encoding UTF8 GetBytes header   0  header Length                               Write the file data directly to the Stream  rather than serializing it to a string                           byte   buffer   new byte 2048                            FileStream fs   new FileStream filepath  FileMode Open                            for  int i   0  i  lt  fs Length                                                          int k   fs Read buffer  0  buffer Length                               if  k  gt  0                                                                formDataStream Write buffer  0  k                                                             i   i   k                                                    fs Close                                              else                                               string postData   string Format  -- 0  r nContent-Disposition  form-data  name    1    r n r n 2  r n                               boundary                              param Key                              param Value                           formDataStream Write Encoding UTF8 GetBytes postData   0  postData Length                                                              Add the end of the request                 byte   footer   Encoding UTF8 GetBytes   r n--    boundary    -- r n                    formDataStream Write footer  0  footer Length                   request ContentLength   formDataStream Length                  formDataStream Close                               endregion              return request GetResponse   as HttpWebResponse

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My implementation       lt summary gt      Sending file via multipart form-data      lt  summary gt       lt param name  url  gt URL for send lt  param gt       lt param name  file  gt Local file path lt  param gt       lt param name  paramName  gt Request file param lt  param gt       lt param name  contentType  gt Content-Type file headr lt  param gt       lt param name  nvc  gt Additional post params lt  param gt  private static string httpUploadFile string url  string file  string paramName  string contentType  NameValueCollection nvc          delimeter     var boundary    ---------------------------    DateTime Now Ticks ToString  x           creating request     var wr    HttpWebRequest WebRequest Create url       wr ContentType    multipart form-data  boundary     boundary      wr Method    POST       wr KeepAlive   true         sending request     using var requestStream   wr GetRequestStream                  using  var requestWriter   new StreamWriter requestStream  Encoding UTF8                           params             const string formdataTemplate    Content-Disposition  form-data  name    0    r n r n 1                foreach  string key in nvc Keys                                requestWriter Write boundary                   requestWriter Write String Format formdataTemplate  key  nvc key                               requestWriter Write boundary                  file header             const string headerTemplate    Content-Disposition  form-data  name    0     filename    1    r nContent-Type   2  r n r n               requestWriter Write String Format headerTemplate  paramName  file  contentType                   file content             using  var fileStream   new FileStream file  FileMode Open  FileAccess Read                                 fileStream CopyTo requestStream                              requestWriter Write   r n--    boundary    -- r n                           reading response     try               using  var wresp    HttpWebResponse wr GetResponse                          if  wresp StatusCode    HttpStatusCode OK                                using  var responseStream   wresp GetResponseStream                                          if  responseStream    null                          return null                      using  var responseReader   new StreamReader responseStream                                                 return responseReader ReadToEnd                                                                       throw new ApplicationException  Error while upload files  Server status code      wresp StatusCode ToString                         catch  Exception ex                throw new ApplicationException  Error while uploading file   ex

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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 lt string  object gt  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 lt string  object gt  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 lt string  object gt  postParameters   new Dictionary lt string  object gt     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

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This is cut and pasted from some sample code I wrote  hopefully it should give the basics  It only supports File data and form-data at the moment   public class PostData        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                 Add sample param         m Params Add new PostDataParam  email    MyEmail   PostDataParamType Field                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                    Get boundary  default is --AaB03x         string boundary   ConfigurationManager AppSettings  ContentBoundary   ToString             StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  text plain                    sb AppendLine                    sb AppendLine p Value                                              else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine boundary            return sb ToString                        public enum PostDataParamType       Field      File    public class PostDataParam         public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type      To send the data you then need to   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create oURL URL   oRequest ContentType    multipart form-data                          oRequest Method    POST   PostData pData   new PostData     byte   buffer   encoding GetBytes pData GetPostData         Set content length of our data oRequest ContentLength   buffer Length      Dump our buffered postdata to the stream  booyah oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close        get the response oResponse    HttpWebResponse oRequest GetResponse      Hope thats clear  i ve cut and pasted from a few sources to get that tidier

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Building on dnolans example  this is the version I could actually get to work  there were some errors with the boundary  encoding wasn t set   -   To send the data   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create  http   you url here    oRequest ContentType    multipart form-data  boundary     PostData boundary  oRequest Method    POST   PostData pData   new PostData    Encoding encoding   Encoding UTF8  Stream oStream   null          set the parameters  read files  etc  IE     pData Params Add new PostDataParam  email    example example com   PostDataParamType Field       pData Params Add new PostDataParam  fileupload    filename txt    filecontents  PostDataParamType File        byte   buffer   encoding GetBytes pData GetPostData      oRequest ContentLength   buffer Length   oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close     HttpWebResponse oResponse    HttpWebResponse oRequest GetResponse      The PostData class should look like   public class PostData          Change this if you need to  not necessary     public static string boundary    AaB03x        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                 StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine  --    boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  application octet-stream                    sb AppendLine                    sb AppendLine p Value                             else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine  --    boundary    --             return sb ToString             public enum PostDataParamType       Field      File    public class PostDataParam       public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public PostDataParam string name  string filename  string value  PostDataParamType type                Name   name          Value   value          FileName   filename          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type

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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 lt string  object gt  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 lt string  object gt  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 lt string  object gt  postParameters   new Dictionary lt string  object gt     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

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In the version of  NET I am using you also have to do this   System Net ServicePointManager Expect100Continue   false    If you don t  the HttpWebRequest class will automatically add the Expect 100-continue request header which fouls everything up   Also I learned the hard way that you have to have the right number of dashes   whatever you say is the  boundary  in the Content-Type header has to be preceded by two dashes  --THEBOUNDARY   and at the end  --THEBOUNDARY--   exactly as it does in the example code   If your boundary is a lot of dashes followed by a number then this mistake won t be obvious by looking at the http request in a proxy server

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This is cut and pasted from some sample code I wrote  hopefully it should give the basics  It only supports File data and form-data at the moment   public class PostData        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                 Add sample param         m Params Add new PostDataParam  email    MyEmail   PostDataParamType Field                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                    Get boundary  default is --AaB03x         string boundary   ConfigurationManager AppSettings  ContentBoundary   ToString             StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  text plain                    sb AppendLine                    sb AppendLine p Value                                              else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine boundary            return sb ToString                        public enum PostDataParamType       Field      File    public class PostDataParam         public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type      To send the data you then need to   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create oURL URL   oRequest ContentType    multipart form-data                          oRequest Method    POST   PostData pData   new PostData     byte   buffer   encoding GetBytes pData GetPostData         Set content length of our data oRequest ContentLength   buffer Length      Dump our buffered postdata to the stream  booyah oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close        get the response oResponse    HttpWebResponse oRequest GetResponse      Hope thats clear  i ve cut and pasted from a few sources to get that tidier

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In the version of  NET I am using you also have to do this   System Net ServicePointManager Expect100Continue   false    If you don t  the HttpWebRequest class will automatically add the Expect 100-continue request header which fouls everything up   Also I learned the hard way that you have to have the right number of dashes   whatever you say is the  boundary  in the Content-Type header has to be preceded by two dashes  --THEBOUNDARY   and at the end  --THEBOUNDARY--   exactly as it does in the example code   If your boundary is a lot of dashes followed by a number then this mistake won t be obvious by looking at the http request in a proxy server

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Building on dnolans example  this is the version I could actually get to work  there were some errors with the boundary  encoding wasn t set   -   To send the data   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create  http   you url here    oRequest ContentType    multipart form-data  boundary     PostData boundary  oRequest Method    POST   PostData pData   new PostData    Encoding encoding   Encoding UTF8  Stream oStream   null          set the parameters  read files  etc  IE     pData Params Add new PostDataParam  email    example example com   PostDataParamType Field       pData Params Add new PostDataParam  fileupload    filename txt    filecontents  PostDataParamType File        byte   buffer   encoding GetBytes pData GetPostData      oRequest ContentLength   buffer Length   oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close     HttpWebResponse oResponse    HttpWebResponse oRequest GetResponse      The PostData class should look like   public class PostData          Change this if you need to  not necessary     public static string boundary    AaB03x        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                 StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine  --    boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  application octet-stream                    sb AppendLine                    sb AppendLine p Value                             else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine  --    boundary    --             return sb ToString             public enum PostDataParamType       Field      File    public class PostDataParam       public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public PostDataParam string name  string filename  string value  PostDataParamType type                Name   name          Value   value          FileName   filename          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type

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Building on dnolans example  this is the version I could actually get to work  there were some errors with the boundary  encoding wasn t set   -   To send the data   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create  http   you url here    oRequest ContentType    multipart form-data  boundary     PostData boundary  oRequest Method    POST   PostData pData   new PostData    Encoding encoding   Encoding UTF8  Stream oStream   null          set the parameters  read files  etc  IE     pData Params Add new PostDataParam  email    example example com   PostDataParamType Field       pData Params Add new PostDataParam  fileupload    filename txt    filecontents  PostDataParamType File        byte   buffer   encoding GetBytes pData GetPostData      oRequest ContentLength   buffer Length   oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close     HttpWebResponse oResponse    HttpWebResponse oRequest GetResponse      The PostData class should look like   public class PostData          Change this if you need to  not necessary     public static string boundary    AaB03x        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                 StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine  --    boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  application octet-stream                    sb AppendLine                    sb AppendLine p Value                             else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine  --    boundary    --             return sb ToString             public enum PostDataParamType       Field      File    public class PostDataParam       public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public PostDataParam string name  string filename  string value  PostDataParamType type                Name   name          Value   value          FileName   filename          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type

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Below is the code which I m using         This URL not exist  it s only an example      string url    http   myBox s3 amazonaws com          Instantiate new CustomWebRequest class     CustomWebRequest wr   new CustomWebRequest url         Set values for parameters     wr ParamsCollection Add new ParamsStruct  key      filename          wr ParamsCollection Add new ParamsStruct  acl    public-read         wr ParamsCollection Add new ParamsStruct  success action redirect    http   www yahoo com         wr ParamsCollection Add new ParamsStruct  x-amz-meta-uuid    14365123651274         wr ParamsCollection Add new ParamsStruct  x-amz-meta-tag             wr ParamsCollection Add new ParamsStruct  AWSAccessKeyId    zzzz                     wr ParamsCollection Add new ParamsStruct  Policy    adsfadsf         wr ParamsCollection Add new ParamsStruct  Signature    hH6lK6cA            For file type  send the inputstream of selected file     StreamReader sr   new StreamReader   file txt        wr ParamsCollection Add new ParamsStruct  file   sr  ParamsStruct ParamType File   file txt          wr PostData      from the following link I ve downloaded the same code http   www codeproject com KB cs multipart request C  aspx  Any Help

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This is cut and pasted from some sample code I wrote  hopefully it should give the basics  It only supports File data and form-data at the moment   public class PostData        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                 Add sample param         m Params Add new PostDataParam  email    MyEmail   PostDataParamType Field                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                    Get boundary  default is --AaB03x         string boundary   ConfigurationManager AppSettings  ContentBoundary   ToString             StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  text plain                    sb AppendLine                    sb AppendLine p Value                                              else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine boundary            return sb ToString                        public enum PostDataParamType       Field      File    public class PostDataParam         public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type      To send the data you then need to   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create oURL URL   oRequest ContentType    multipart form-data                          oRequest Method    POST   PostData pData   new PostData     byte   buffer   encoding GetBytes pData GetPostData         Set content length of our data oRequest ContentLength   buffer Length      Dump our buffered postdata to the stream  booyah oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close        get the response oResponse    HttpWebResponse oRequest GetResponse      Hope thats clear  i ve cut and pasted from a few sources to get that tidier

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With  NET 4 5 you currently could use System Net Http namespace  Below the example for uploading single file using multipart form data   using System  using System IO  using System Net Http   namespace HttpClientTest       class Program               static void Main string   args                        var client   new HttpClient                var content   new MultipartFormDataContent                content Add new StreamContent File Open        Image1 png   FileMode Open     Image    Image png                content Add new StringContent  Place string content here     Content-Id in the HTTP                 var result   client PostAsync  https   hostname api Account UploadAvatar   content               Console WriteLine result Result ToString

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Thanks for the code  it saved me a lot of time  including the Except100 error     Anyway  I found a bug in the code  here   formDataStream Write encoding GetBytes postData   0  postData Length     In case your POST data is utf-16  postData Length  will return the number of characters and not the number of bytes  This will truncate the data being posted  for example  if you have 2 chars that are encoded as utf-16  they take 4 bytes  but postData Length will say it takes 2 bytes  and you loose the 2 final bytes of the posted data    Solution - replace that line with   byte   aPostData encoding GetBytes postData   formDataStream Write aPostData  0  aPostData Length     Using this  the length is calculated by the size of the byte    not the string size

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This is cut and pasted from some sample code I wrote  hopefully it should give the basics  It only supports File data and form-data at the moment   public class PostData        private List lt PostDataParam gt  m Params       public List lt PostDataParam gt  Params               get   return m Params            set   m Params   value               public PostData                 m Params   new List lt PostDataParam gt                 Add sample param         m Params Add new PostDataParam  email    MyEmail   PostDataParamType Field                     lt summary gt          Returns the parameters array formatted for multi-part form data          lt  summary gt           lt returns gt  lt  returns gt      public string GetPostData                    Get boundary  default is --AaB03x         string boundary   ConfigurationManager AppSettings  ContentBoundary   ToString             StringBuilder sb   new StringBuilder            foreach  PostDataParam p in m Params                        sb AppendLine boundary                if  p Type    PostDataParamType File                                sb AppendLine string Format  Content-Disposition  file  name    0     filename    1      p Name  p FileName                    sb AppendLine  Content-Type  text plain                    sb AppendLine                    sb AppendLine p Value                                              else                               sb AppendLine string Format  Content-Disposition  form-data  name    0      p Name                    sb AppendLine                    sb AppendLine p Value                                    sb AppendLine boundary            return sb ToString                        public enum PostDataParamType       Field      File    public class PostDataParam         public PostDataParam string name  string value  PostDataParamType type                Name   name          Value   value          Type   type             public string Name      public string FileName      public string Value      public PostDataParamType Type      To send the data you then need to   HttpWebRequest oRequest   null  oRequest    HttpWebRequest HttpWebRequest Create oURL URL   oRequest ContentType    multipart form-data                          oRequest Method    POST   PostData pData   new PostData     byte   buffer   encoding GetBytes pData GetPostData         Set content length of our data oRequest ContentLength   buffer Length      Dump our buffered postdata to the stream  booyah oStream   oRequest GetRequestStream    oStream Write buffer  0  buffer Length   oStream Close        get the response oResponse    HttpWebResponse oRequest GetResponse      Hope thats clear  i ve cut and pasted from a few sources to get that tidier

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A little optimization of the class before  In this version the files are not totally loaded into memory   Security advice  a check for the boundary is missing  if the file contains the bounday it will crash   namespace WindowsFormsApplication1       public static class FormUpload               private static string NewDataBoundary                         Random rnd   new Random                string formDataBoundary                   while  formDataBoundary Length  lt  15                                formDataBoundary   formDataBoundary   rnd Next                              formDataBoundary   formDataBoundary Substring 0  15               formDataBoundary    -----------------------------    formDataBoundary              return formDataBoundary                     public static HttpWebResponse MultipartFormDataPost string postUrl  IEnumerable lt Cookie gt  cookies  Dictionary lt string  string gt  postParameters                        string boundary   NewDataBoundary                 HttpWebRequest request    HttpWebRequest WebRequest Create postUrl                   Set up the request properties             request Method    POST               request ContentType    multipart form-data  boundary     boundary              request UserAgent    PhasDocAgent 1 0               request CookieContainer   new CookieContainer                 foreach  var cookie in cookies                                request CookieContainer Add cookie                               region WRITING STREAM             using  Stream formDataStream   request GetRequestStream                                  foreach  var param in postParameters                                        if  param Value StartsWith  file                                                     string filepath   param Value Substring 7                               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                              Path GetFileName filepath     param Key                              MimeTypes GetMime filepath                             formDataStream Write Encoding UTF8 GetBytes header   0  header Length                               Write the file data directly to the Stream  rather than serializing it to a string                           byte   buffer   new byte 2048                            FileStream fs   new FileStream filepath  FileMode Open                            for  int i   0  i  lt  fs Length                                                          int k   fs Read buffer  0  buffer Length                               if  k  gt  0                                                                formDataStream Write buffer  0  k                                                             i   i   k                                                    fs Close                                              else                                               string postData   string Format  -- 0  r nContent-Disposition  form-data  name    1    r n r n 2  r n                               boundary                              param Key                              param Value                           formDataStream Write Encoding UTF8 GetBytes postData   0  postData Length                                                              Add the end of the request                 byte   footer   Encoding UTF8 GetBytes   r n--    boundary    -- r n                    formDataStream Write footer  0  footer Length                   request ContentLength   formDataStream Length                  formDataStream Close                               endregion              return request GetResponse   as HttpWebResponse

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