c# multipart/form-data submit programmatically

Question

So got an small problem. Im creating an small application to automate an form submission on one website. But the bad thing is that they are using multipart/form-data for tha

Answer 1

posts os multipart/form-data type have a different structure because they are meant to transfer data and not just plain text.

Here's the format:

--[random number, a GUID is good here]
Content-Disposition: form-data; name="[name of variable]"

[actual value]
--[random number, a GUID is good here]--

Using HTTPWebRequest you can create a request that has that format. Here's a sample:

string boundary = Guid.NewGuid().ToString();
string header = string.Format("--{0}", boundary);
string footer = string.Format("--{0}--", boundary);

StringBuilder contents = new StringBuilder();
contents.AppendLine(header);

contents.AppendLine(header);
contents.AppendLine(String.Format("Content-Disposition: form-data; name=\"{0}\"", "username"));
contents.AppendLine();
contents.AppendLine("your_username");

contents.AppendLine(header);
contents.AppendLine(String.Format("Content-Disposition: form-data; name=\"{0}\"", "password"));
contents.AppendLine();
contents.AppendLine("your_password");

contents.AppendLine(footer);

Answer 2

The format of multipart/form-data requests is outlined here: http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.2.

Answer 3

Here is an article on multipart form posts in C# with more detail. This code was eventually merged into RestSharp, which is an excellent library you could use to generate the request.

Answer 4

The best way to send multipart form data in C# is shown in the snippet, here you see that adding different type of content is as easy as adding it to the wrapper multipart content type:

var documentContent = new MultipartFormDataContent();
documentContent.Add(new StringContent("AnalyticsPage.xlsx"), "title");
documentContent.Add(new ByteArrayContent(File.ReadAllBytes("C:\\Users\\awasthi\\Downloads\\AnalyticsPage.xlsx")), "file", "AnalyticsPage.xlsx");

Then just make an api call:

using (var client = new HttpClient(new HttpClientHandler() { UseDefaultCredentials = true, CookieContainer = new CookieContainer() }))
{
  response = client.PostAsync(documentAddApi, documentContent).Result;
  var responseContent = response.Content.ReadAsStringAsync().Result;
 }

Here the expectation is that the rest endpoint you are making a call to is accepting a 'title' field for the file and the byte array of the file named 'file'.