C# を使用して HTTP POST 経由でファイルを送信する方法: 包括的なガイド

<code class="language-csharp">HttpWebRequest req = WebRequest.Create(uri) as HttpWebRequest;
req.KeepAlive = false;
req.Method = "POST";
req.ContentType = file.ContentType;
req.ContentLength = file.Length;
using (Stream requestStream = req.GetRequestStream())
using (Stream fileStream = File.OpenRead(file.FileName))
{
    fileStream.CopyTo(requestStream);
}</code>

<code class="language-csharp">private async Task<Stream> Upload(string actionUrl, string paramString, Stream paramFileStream, byte[] paramFileBytes)
{
    HttpContent stringContent = new StringContent(paramString);
    HttpContent fileStreamContent = new StreamContent(paramFileStream);
    HttpContent bytesContent = new ByteArrayContent(paramFileBytes);
    using (var client = new HttpClient())
    using (var formData = new MultipartFormDataContent())
    {
        formData.Add(stringContent, "param1", "param1");
        formData.Add(fileStreamContent, "file1", "file1");
        formData.Add(bytesContent, "file2", "file2");
        var response = await client.PostAsync(actionUrl, formData);
        if (response.IsSuccessStatusCode)
        {
            return await response.Content.ReadAsStreamAsync();
        }
    }
    return null;
}</code>