How to upload any file on Slack via Slack-App in c#

I need help with uploading files to Slack.

I have a Slack-App that is working with my code(below) so far. But all I can do is post messages. I can not attach images to the messages - because I do not understand how to use the so called "methods" and the syntax Slack is "showing" on their API-page.

This creates my "content" and below its just a Stream for reading a file I could upload:

    public class PostMessage
    {


        public FormUrlEncodedContent Content(string message, string file)
        {
            var values = new Dictionary<string, string>
            {
                {"token", "xoxp-myToken"},
                { "username", "X"},         
                { "channel", "myChannel"},
                { "as_user", "false"},     
                {"text", message},
                { "content", file},
                { "attachments","[{ \"fallback\":\"dummy\", \"text\":\"this is a waste of time\"}]"}
            };

            var content = new FormUrlEncodedContent(values);

            return content;
        }
    }

    public class PostFile
    {
        String path = @"C:\Users\f.held\Desktop\Held-Docs\dagged.jpg";

        public string ReadImageFile()
        {            
            FileInfo fileInfo = new FileInfo(path);
            long imageFileLength = fileInfo.Length;
            FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
            BinaryReader br = new BinaryReader(fs);
            byte[] imageData = br.ReadBytes((int)imageFileLength);
            var str = Encoding.Default.GetString(imageData);
            return str;
        }
    }
}

The client that communicates:

public class SlackClient
{
        private readonly Uri _webhookUrl;
        private readonly HttpClient _httpClient = new HttpClient {};

        public SlackClient(Uri webhookUrl)
        {
            _webhookUrl = webhookUrl;
        }

        public async Task<HttpResponseMessage> SendMessageAsync(FormUrlEncodedContent content)
        {
            var response = await _httpClient.PostAsync(_webhookUrl, content);

            return response;
        }    
     }
}

The Main:

public static void Main(string[] args)
{
    Task.WaitAll(IntegrateWithSlackAsync());
}

private static async Task IntegrateWithSlackAsync()
{
    var webhookUrl = new Uri("https://slack.com/api/files.upload");
    var slackClient = new SlackClient(webhookUrl);
    PostMessage PM = new PostMessage();
    PostFile PF = new PostFile();


    while (true)
    {
        Console.Write("Type a message: ");
        var message = Console.ReadLine();
        var testFile = PF.ReadImageFile();
        FormUrlEncodedContent payload = PM.Content(message, testFile);
        var response = await slackClient.SendMessageAsync(payload);
        var isValid = response.IsSuccessStatusCode ? "valid" : "invalid";
        Console.WriteLine($"Received {isValid} response.");
        Console.WriteLine(response);
        response.Dispose();
    }
}

} }

If somebody has an example on what a upload has to look like. Or even better,

if somebody could really explain the syntax these Slack-Messages have to have.

That would be great! I still do not know where and HOW I should put the so called "Accepted content types: multipart/form-data, application/x-www-form-urlencoded" to my upload. I just can not find examples on this...

Edit:

What confuses me needlesly is that Slack states they have an extra method called file.upload - but we shouldn't use it anymore, we should use just postMessage.

But how would I "pack" a file in a message? My syntax always seems to be off. Especially when it comes to "content"... I just can not figure out what the c#-code has to look like. Where do I declare the aforementioned "content type"?

Another problem is, it always sends my messages through - means I get a 200-response from the server. But it never shows the file (which probably means the syntax is off) Or I get the 200-response but the message never shows in Slack.

Solution

Here is a shorter working example showing how to just upload any file to Slack with C# only. The example will also automatically share the file the given channel.

I have included the logic to convert the API response from JSON, which will always be needed to determine if the API call was successful.

Note: This example requires Newtonsoft.Json

using System;
using System.Net;
using System.Collections.Specialized;
using System.Text;
using Newtonsoft.Json;

public class SlackExample
{
    // classes for converting JSON respones from API method into objects
    // note that only those properties are defind that are needed for this example

    // reponse from file methods
    class SlackFileResponse
    {
        public bool ok { get; set; }
        public String error { get; set; }
        public SlackFile file { get; set; }
    }

    // a slack file
    class SlackFile
    {
        public String id { get; set; }
        public String name { get; set; }        
    }

    // main method with logic
    public static void Main()
    {
        var parameters = new NameValueCollection();

        // put your token here
        parameters["token"] = "xoxp-YOUR-TOKEN";
        parameters["channels"] = "test";

        var client = new WebClient();
        client.QueryString = parameters;
        byte[] responseBytes = client.UploadFile(
                "https://slack.com/api/files.upload",
                "D:\\temp\\Stratios_down.jpg"
        );

        String responseString = Encoding.UTF8.GetString(responseBytes);

        SlackFileResponse fileResponse =
            JsonConvert.DeserializeObject<SlackFileResponse>(responseString);
    }
}

About content types: Those are part of the header of a HTTP request and can be set manually in the WebClient object (see also this answer). However, for our case you can ignore it, because the default content types that WebClient is using for the POST request will work just fine.

Also see this answer on how to upload files with the WebClient class.