“no_file_data” error when trying to upload file to slack

Question

when I try to upload any kidn of file through my SlackApp(via c# using HttpClient), I allways get the following response:

{\"ok\":false,\"error\":\"no_file_d         


        

Answer 1

You have two bugs in your code:

• main(): The parameter to specify the channels is called channels, not channel
• UploadFile(): When you add your file content to the multipart you need to include the correct API parameter for the file which is file, not slack. And also want to include a reasonable filename (instead of slack.txt).

Additional comments

• UploadFile(): Its wrong to set the content type to multipart/form-data. The correct type for that content would be image/jpeg. However, the correct type seams to be detected automatically, so just remove the line.
• main(): The Slack API will always return OK (http 200, unless there is a network problem), so you want to also look on the ok and error properties of the JSON response instead.

Here is an update version of your code. I changed your main() method to include a call to `UploadFile()?.

using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;


namespace SlackApp
{
    public class PostFile
    {
        string path = @"C:\Users\Stratios_down.jpg";

        public byte[] ReadImageFile()
        {
            FileInfo fileInfo = new FileInfo(path);
            long imageFileLength = fileInfo.Length;
            FileStream fs = new FileStream(path, FileMode.Open, FileAccess.ReadWrite);
            BinaryReader br = new BinaryReader(fs);
            byte[] imageData = br.ReadBytes((int)imageFileLength);
            return imageData;
        }
    }

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

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

        public async Task UploadFile(byte[] file)
        {
            var requestContent = new MultipartFormDataContent();
            var fileContent = new ByteArrayContent(file);            
            requestContent.Add(fileContent, "file", "stratios.jpg");

            var response = await _httpClient.PostAsync(_webhookUrl, requestContent);
            return response;
        }
    }

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

        private static async Task IntegrateWithSlackAsync()
        {
            var webhookUrl = new Uri(
                "https://slack.com/api/files.upload?token=xoxp-MY-TOKEN&channels=test"
            );
            var slackClient = new SlackClient(webhookUrl);

            PostFile PF = new PostFile();
            var testFile = PF.ReadImageFile();

            var response = await slackClient.UploadFile(testFile);

            string content = await response.Content.ReadAsStringAsync();
            Console.WriteLine(content);
            Console.ReadKey();

        }
    }
}

In addition I would have a couple of suggestions to improve your code.

• Instead of including the additional API parameters in the URL, I would send them in the POST request as recommended by the API documentation.
• Including the file as FileStream instead of loading it yourself into a ByteArray is the better approach and recommended for larger files.
• Not sure why you need an infinite loop in your main. Those are really bad and should be avoided.

Please also take also a look at my new async example for uploading a file to Slack where I applied those two ideas.