.NET Framework
Télécharger le fichier et les données POST sur le serveur Web

Télécharger le fichier avec WebRequest

Pour envoyer un fichier et des données de formulaire en une seule requête, le contenu doit avoir le type multipart / form-data .

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Threading.Tasks;

public async Task<string> UploadFile(string url, string filename, 
    Dictionary<string, object> postData)
{
    var request = WebRequest.CreateHttp(url);
    var boundary = $"{Guid.NewGuid():N}"; // boundary will separate each parameter
    request.ContentType = $"multipart/form-data; {nameof(boundary)}={boundary}";
    request.Method = "POST";

    using (var requestStream = request.GetRequestStream())
    using (var writer = new StreamWriter(requestStream))
    {
        foreach (var data in postData)
            await writer.WriteAsync( // put all POST data into request
                $"\r\n--{boundary}\r\nContent-Disposition: " +
                $"form-data; name=\"{data.Key}\"\r\n\r\n{data.Value}");

        await writer.WriteAsync( // file header
            $"\r\n--{boundary}\r\nContent-Disposition: " +
            $"form-data; name=\"File\"; filename=\"{Path.GetFileName(filename)}\"\r\n" +
            "Content-Type: application/octet-stream\r\n\r\n");

        await writer.FlushAsync();
        using (var fileStream = File.OpenRead(filename))
            await fileStream.CopyToAsync(requestStream);

        await writer.WriteAsync($"\r\n--{boundary}--\r\n");
    }

    using (var response = (HttpWebResponse) await request.GetResponseAsync())
    using (var responseStream = response.GetResponseStream())
    {
        if (responseStream == null)
            return string.Empty;
        using (var reader = new StreamReader(responseStream))
            return await reader.ReadToEndAsync();
    }
}

var response = await uploader.UploadFile("< YOUR URL >", "< PATH TO YOUR FILE >",
    new Dictionary<string, object>
    {
        {"Comment", "test"},
        {"Modified", DateTime.Now }
    });