How to parse XHR multipart/form-data request with multiple files

Question

I have seen multiple examples online of multipart/form-data parsers but none of them work on my site. I am using a Kendo Upload control set to async mode and with batch upload enabled, the generated request looks something like this:

BOUNDARY
Content-Disposition: form-data; name="Param_1"

Param_1 Value
BOUNDARY
Content-Disposition: form-data; name="Param_2"

Param_2 Value
BOUNDARY
Content-Disposition: form-data; name="files[]"; filename="A.docx"
Content-Type: application/octet-stream

[Binary Data Here]
BOUNDARY
Content-Disposition: form-data; name="files[]"; filename="B.docx"
Content-Type: application/octet-stream

[Binary Data Here]
BOUNDARY--

Every library I have found online successfully retrieves the parameters and the first file, but some never see the second and the ones that do save it incorrectly. There is a similar question on SO WCF multipart/form data with multiple files but that solution only works for text files not binary files.

Answer 1

The problem other solutions had with binary files was that they converted the response into a string to parse it and then converted that string back into the file, this does not work with binary data. I came up with a solution where instead of turning the response into a string to parse it, I left it as a byte[] and split it by the delimiter as a byte[] using the code found here. Once that's done convert each piece to a string to see if it is a parameter or a file, if it is a parameter then read it otherwise write it as a file. Here is the working code that assumes you have the Stream as a byte[] and the delimiter as a string:

// given byte[] streamByte, String delimiterString, and Encoding encoding
Regex regQuery;
Match regMatch;
string propertyType;

byte[] delimiterBytes = encoding.GetBytes(delimiterString);
byte[] delimiterWithNewLineBytes = encoding.GetBytes(delimiterString + "\r\n");
// the request ends DELIMITER--\r\n
byte[] delimiterEndBytes = encoding.GetBytes("\r\n" + delimiterString + "--\r\n");
int lengthDifferenceWithEndBytes = (delimiterString + "--\r\n").Length;

// seperate by delimiter + newline
// ByteArraySplit code found at https://stackoverflow.com/a/9755250/4244411
byte[][] separatedStream = ByteArraySplit(streamBytes, delimiterWithNewLineBytes);
streamBytes = null;
for (int i = 0; i < separatedStream.Length; i++)
{
    // parse out whether this is a parameter or a file
    // get the first line of the byte[] as a string
    string thisPieceAsString = encoding.GetString(separatedStream[i]);

    if (string.IsNullOrWhiteSpace(thisPieceAsString)) { continue; }

    string firstLine = thisPieceAsString.Substring(0, thisPieceAsString.IndexOf("\r\n"));

    // Check the item to see what it is
    regQuery = new Regex(@"(?<=name\=\"")(.*?)(?=\"")");
    regMatch = regQuery.Match(firstLine);
    propertyType = regMatch.Value.Trim();

    // get the index of the start of the content and the end of the content
    int indexOfStartOfContent = thisPieceAsString.IndexOf("\r\n\r\n") + "\r\n\r\n".Length;

    // this line compares the name to the name of the html input control, 
    // this can be smarter by instead looking for the filename property
    if (propertyType != "files")
    {
        // this is a parameter!
        // if this is the last piece, chop off the final delimiter
        int lengthToRemove = (i == separatedStream.Length - 1) ? lengthDifferenceWithEndBytes : 0;
        string value = thisPieceAsString.Substring(indexOfStartOfContent, thisPieceAsString.Length - "\r\n".Length - indexOfStartOfContent - lengthToRemove);
        // do something with the parameter
    }
    else
    {
        // this is a file!
        regQuery = new Regex(@"(?<=filename\=\"")(.*?)(?=\"")");
        regMatch = regQuery.Match(firstLine);
        string fileName = regMatch.Value.Trim();

        // get the content byte[]
        // if this is the last piece, chop off the final delimiter
        int lengthToRemove = (i == separatedStream.Length - 1) ? delimiterEndBytes.Length : 0;
        int contentByteArrayStartIndex = encoding.GetBytes(thisPieceAsString.Substring(0, indexOfStartOfContent)).Length;
        byte[] fileData = new byte[separatedStream[i].Length - contentByteArrayStartIndex - lengthToRemove];
        Array.Copy(separatedStream[i], contentByteArrayStartIndex, fileData, 0, separatedStream[i].Length - contentByteArrayStartIndex - lengthToRemove);
        // save the fileData byte[] as the file
    }

}