Httplistener and file upload

Question

I am trying to retrieve an uploaded file from my webserver. As the client sends its files through a webform (random files), I need to parse the request to get the file out and to process it further on. Basically, the code goes as:

HttpListenerContext context = listener.GetContext();
HttpListenerRequest request = context.Request;
StreamReader r = new StreamReader(request.InputStream, System.Text.Encoding.Default);
// this is the retrieved file from streamreader
string file = null;

while ((line = r.ReadLine()) != null){
     // i read the stream till i retrieve the filename
     // get the file data out and break the loop 
}
// A byststream is created by converting the string,
Byte[] bytes = request.ContentEncoding.GetBytes(file);
MemoryStream mstream = new MemoryStream(bytes);

// do the rest

As a result, i am able to retrieve textfiles, but for all other files, they are corrupted. Could someone tell me how to parse these HttplistnerRequests properly (or providing a lightweighted alternative)?

Answer 1

I think you are making things harder on yourself than necessary by doing this with an HttpListener rather than using the built in facilities of ASP.Net. But if you must do it this way here is some sample code. Note: 1) I'm assuming you're using enctype="multipart/form-data" on your <form>. 2) This code is designed to be used with a form containing only your <input type="file" /> if you want to post other fields or multiple files you'll have to change the code. 3) This is meant to be a proof of concept/example, it may have bugs, and is not particularly flexible.

static void Main(string[] args)
{
    HttpListener listener = new HttpListener();
    listener.Prefixes.Add("http://localhost:8080/ListenerTest/");
    listener.Start();

    HttpListenerContext context = listener.GetContext();

    SaveFile(context.Request.ContentEncoding, GetBoundary(context.Request.ContentType), context.Request.InputStream);

    context.Response.StatusCode = 200;
    context.Response.ContentType = "text/html";
    using (StreamWriter writer = new StreamWriter(context.Response.OutputStream, Encoding.UTF8))
        writer.WriteLine("File Uploaded");

    context.Response.Close();

    listener.Stop();

}

private static String GetBoundary(String ctype)
{
    return "--" + ctype.Split(';')[1].Split('=')[1];
}

private static void SaveFile(Encoding enc, String boundary, Stream input)
{
    Byte[] boundaryBytes = enc.GetBytes(boundary);
    Int32 boundaryLen = boundaryBytes.Length;

    using (FileStream output = new FileStream("data", FileMode.Create, FileAccess.Write))
    {
        Byte[] buffer = new Byte[1024];
        Int32 len = input.Read(buffer, 0, 1024);
        Int32 startPos = -1;

        // Find start boundary
        while (true)
        {
            if (len == 0)
            {
                throw new Exception("Start Boundaray Not Found");
            }

            startPos = IndexOf(buffer, len, boundaryBytes);
            if (startPos >= 0)
            {
                break;
            }
            else
            {
                Array.Copy(buffer, len - boundaryLen, buffer, 0, boundaryLen);
                len = input.Read(buffer, boundaryLen, 1024 - boundaryLen);
            }
        }

        // Skip four lines (Boundary, Content-Disposition, Content-Type, and a blank)
        for (Int32 i = 0; i < 4; i++)
        {
            while (true)
            {
                if (len == 0)
                {
                    throw new Exception("Preamble not Found.");
                }

                startPos = Array.IndexOf(buffer, enc.GetBytes("\n")[0], startPos);
                if (startPos >= 0)
                {
                    startPos++;
                    break;
                }
                else
                {
                    len = input.Read(buffer, 0, 1024);
                }
            }
        }

        Array.Copy(buffer, startPos, buffer, 0, len - startPos);
        len = len - startPos;

        while (true)
        {
            Int32 endPos = IndexOf(buffer, len, boundaryBytes);
            if (endPos >= 0)
            {
                if (endPos > 0) output.Write(buffer, 0, endPos-2);
                break;
            }
            else if (len <= boundaryLen)
            {
                throw new Exception("End Boundaray Not Found");
            }
            else
            {
                output.Write(buffer, 0, len - boundaryLen);
                Array.Copy(buffer, len - boundaryLen, buffer, 0, boundaryLen);
                len = input.Read(buffer, boundaryLen, 1024 - boundaryLen) + boundaryLen;
            }
        }
    }
}

private static Int32 IndexOf(Byte[] buffer, Int32 len, Byte[] boundaryBytes)
{
    for (Int32 i = 0; i <= len - boundaryBytes.Length; i++)
    {
        Boolean match = true;
        for (Int32 j = 0; j < boundaryBytes.Length && match; j++)
        {
            match = buffer[i + j] == boundaryBytes[j];
        }

        if (match)
        {
            return i;
        }
    }

    return -1;
}

To help you better understand what the code above is doing, here is what the body of the HTTP POST looks like:

Content-Type: multipart/form-data; boundary=----WebKitFormBoundary9lcB0OZVXSqZLbmv

------WebKitFormBoundary9lcB0OZVXSqZLbmv
Content-Disposition: form-data; name="my_file"; filename="Test.txt"
Content-Type: text/plain

Test
------WebKitFormBoundary9lcB0OZVXSqZLbmv--

I've left out the irrelevant headers. As you can see, you need to parse the body by scanning through to find the beginning and ending boundary sequences, and drop the sub headers that come before the content of your file. Unfortunately you cannot use StreamReader because of the potential for binary data. Also unfortunate is the fact that there is no per file Content-Length (the Content-Length header for the request specifies the total length of the body including boundaries, sub-headers, and spacing.

Answer 2

You can't use StreamReader because it is meant to read streams in which the bytes are in the UTF8. Instead, you want to read the contents of the stream to a receive buffer, remove all the stuff you don't need, get the file extension of the uploaded file, extract the contents of the uploaded file, then save the file contents to a new file. The code I show in this post assumes your form looks like this:

<form enctype="multipart/form-data" method="POST" action="/uploader">
    <input type="file" name="file">
    <input type="submit">
</form>

As you can see, the code is only meant to handle a form that only has the file. Since there is no way to extract the contents of a file on the server from a application/x-www-form-urlencoded form, so you have to include the "multipart/form-data".

First, for this method of handling uploaded files, you will first need this little bit of code:

using System;
using System.IO;
using System.Text;
using System.Net;
using System.Collections.Generic;

Second, you need to read the contents of the request.InputStream to a receive buffer, or a byte[]. We do this by making a byte[] buffer with the length of the Content-Length header sent by the browser. Then, we read the contents of the request.InputStream to the buffer. The code would look like this:

int len = int.Parse(request.Headers["Content-Length"]);
byte[] buffer = new byte[len];
request.InputStream.Read(buffer, 0, len);

The stream will look somewhat like this:

------WebKitFormBoundary9lcB0OZVXSqZLbmv
Content-Disposition: form-data; name="file"; filename="example-file.txt"
Content-Type: text/plain

file contents here
------WebKitFormBoundary9lcB0OZVXSqZLbmv--

Next, you need to get the file extension of the uploaded file. We can do this using this code:

string fileExtension = Encoding.UTF8.GetString(bytes).Split("\r\n")[1].Split("filename=\"")[1].Replace("\"", "").Split('.')[^1];

Then, we need to get the contents of the file. We do this by removing the stuff at the beginning (the -----WebKitFormBoundary, the Content-Disposition, the Content-Type, and a blank line), then removing the last line of the request body, plus an extra \r\n at the end. Here is the code that does just that:

// note that the variable buffer is the byte[], and the variable bytes is the List<byte>
string stringBuffer = Encoding.UTF8.GetString(buffer);
List<byte> bytes = new List<byte>(buffer);
string[] splitString = stringBuffer.Split('\n');
int lengthOfFourLines = splitString[0].Length + splitString[1].Length + 
splitString[2].Length + splitString[3].Length + 4;
bytes.RemoveRange(0, lengthOfFourLines);
int lengthOfLastLine = splitString[^2].Length+2;
bytes.RemoveRange(bytes.Count - lengthOfLastLine, lengthOfLastLine);
buffer = bytes.ToArray();

Finally, we need to save the contents to a file. The code below generates a random file name with the user-specified file extension, and saves the files contents to it.

string fname = "";
string[] chars = "q w e r t y u i o p a s d f g h j k l z x c v b n m Q W E R T Y U I O P A S D F G H J K L Z X C V B N M 1 2 3 4 5 6 7 8 9 0".Split(" ");
for (int i = 0; i < 10; i++)
{
    fname += chars[new Random().Next(chars.Length)];
}
fname += fileExtension;
FileStream file = File.Create(fname);
file.Write(buffer);
file.Close();

Here is the whole code put together:

public static void Main()
{
    var listener = new HttpListener();
    listener.Prefixes.Add("http://localhost:8080/");
    listener.Start();
    while(true)
    {
        HttpListenerContext context = listener.GetContext();
        HttpListenerRequest request = context.Request;
        HttpListenerResponse response = context.Response;
        if(request.HttpMethod=="POST") SaveFile(request);
        response.OutputStream.Write(Encoding.UTF8.GetBytes("file successfully uploaded"));
        response.OutputStream.Close();
    }
}
void SaveFile(HttpListenerRequest request)
{
    int len = (int)request.ContentLength64;
    Console.WriteLine(len);
    byte[] buffer = new byte[len];
    request.InputStream.Read(buffer, 0, len);
    string stringBuffer = Encoding.UTF8.GetString(buffer);
    Console.WriteLine(stringBuffer.Replace("\r\n","\\r\\n\n"));
    string fileExtension = stringBuffer.Split("\r\n")[1]
        .Split("filename=\"")[1]
        .Replace("\"", "")
        .Split('.')[^1]
    ;
    List<byte> bytes = new List<byte>(buffer);
    string[] splitString = stringBuffer.Split('\n');
    int lengthOfFourLines = splitString[0].Length + splitString[1].Length + splitString[2].Length + splitString[3].Length + 4;
    bytes.RemoveRange(0, lengthOfFourLines);
    int lengthOfLastLine = splitString[^2].Length+2;
    bytes.RemoveRange(bytes.Count - lengthOfLastLine, lengthOfLastLine);
    buffer = bytes.ToArray();
    
    string fname = "";
    string[] chars = "q w e r t y u i o p a s d f g h j k l z x c v b n m Q W E R T Y U I O P A S D F G H J K L Z X C V B N M 1 2 3 4 5 6 7 8 9 0".Split(" ");
    for (int i = 0; i < 10; i++)
    {
        fname += chars[new Random().Next(chars.Length)];
    }
    fname += "." + fileExtension;
    FileStream file = File.Create(fname);
    file.Write(buffer);
    file.Close();
}

Also, if you want to send an uploaded file to the client, here is a useful function that sends the file to the client.

// Make sure you are using System.IO, and System.Net when making this function.
// Also make sure you set the content type of the response before calling this function.
// fileName is the name of the file you want to send to the client, and output is the response.OutputStream.
public static void SendFile(string fileName, Stream output)
{
    FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
    fs.CopyTo(output);
    fs.Close();
    output.Close();
}

Answer 3

The problem is you are reading the file as text.

You need to read the file as a bytearray instead and using the BinaryReader is better and easier to use than StreamReader:

Byte[] bytes;
using (System.IO.BinaryReader r = new System.IO.BinaryReader(request.InputStream))
{
    // Read the data from the stream into the byte array
    bytes = r.ReadBytes(Convert.ToInt32(request.InputStream.Length));
}
MemoryStream mstream = new MemoryStream(bytes);

Answer 4

May have bugs, test thoroughly. This one gets all post, get, and files.

using System.Collections.Generic;
using System.Collections.Specialized;
using System.IO;
using System.Net;
using System.Text;
using System.Web;

namespace DUSTLauncher
{
    class HttpNameValueCollection
    {
        public class File
        {
            private string _fileName;
            public string FileName { get { return _fileName ?? (_fileName = ""); } set { _fileName = value; } }

            private string _fileData;
            public string FileData { get { return _fileData ?? (_fileName = ""); } set { _fileData = value; } }

            private string _contentType;
            public string ContentType { get { return _contentType ?? (_contentType = ""); } set { _contentType = value; } }
        }

        private NameValueCollection _get;
        private Dictionary<string, File> _files;
        private readonly HttpListenerContext _ctx;

        public NameValueCollection Get { get { return _get ?? (_get = new NameValueCollection()); } set { _get = value; } }
        public NameValueCollection Post { get { return _ctx.Request.QueryString; } }
        public Dictionary<string, File> Files { get { return _files ?? (_files = new Dictionary<string, File>()); } set { _files = value; } }

        private void PopulatePostMultiPart(string post_string)
        {
            var boundary_index = _ctx.Request.ContentType.IndexOf("boundary=") + 9;
            var boundary = _ctx.Request.ContentType.Substring(boundary_index, _ctx.Request.ContentType.Length - boundary_index);

            var upper_bound = post_string.Length - 4;

            if (post_string.Substring(2, boundary.Length) != boundary)
                throw (new InvalidDataException());

            var raw_post_strings = new List<string>();
            var current_string = new StringBuilder();

            for (var x = 4 + boundary.Length; x < upper_bound; ++x)
            {
                if (post_string.Substring(x, boundary.Length) == boundary)
                {
                    x += boundary.Length + 1;
                    raw_post_strings.Add(current_string.ToString().Remove(current_string.Length - 3, 3));
                    current_string.Clear();
                    continue;
                }

                current_string.Append(post_string[x]);

                var post_variable_string = current_string.ToString();

                var end_of_header = post_variable_string.IndexOf("\r\n\r\n");

                if (end_of_header == -1) throw (new InvalidDataException());

                var filename_index = post_variable_string.IndexOf("filename=\"", 0, end_of_header);
                var filename_starts = filename_index + 10;
                var content_type_starts = post_variable_string.IndexOf("Content-Type: ", 0, end_of_header) + 14;
                var name_starts = post_variable_string.IndexOf("name=\"") + 6;
                var data_starts = end_of_header + 4;

                if (filename_index == -1) continue;

                var filename = post_variable_string.Substring(filename_starts, post_variable_string.IndexOf("\"", filename_starts) - filename_starts);
                var content_type = post_variable_string.Substring(content_type_starts, post_variable_string.IndexOf("\r\n", content_type_starts) - content_type_starts);
                var file_data = post_variable_string.Substring(data_starts, post_variable_string.Length - data_starts);
                var name = post_variable_string.Substring(name_starts, post_variable_string.IndexOf("\"", name_starts) - name_starts);
                Files.Add(name, new File() { FileName = filename, ContentType = content_type, FileData = file_data });
                continue;

            }
        }

        private void PopulatePost()
        {
            if (_ctx.Request.HttpMethod != "POST" || _ctx.Request.ContentType == null) return;

            var post_string = new StreamReader(_ctx.Request.InputStream, _ctx.Request.ContentEncoding).ReadToEnd();

            if (_ctx.Request.ContentType.StartsWith("multipart/form-data"))
                PopulatePostMultiPart(post_string);
            else
                Get = HttpUtility.ParseQueryString(post_string);

        }

        public HttpNameValueCollection(ref HttpListenerContext ctx)
        {
            _ctx = ctx;
            PopulatePost();
        }


    }
}