Send a file via HTTP POST with C#

Using .NET 4.5 (or .NET 4.0 by adding the Microsoft.Net.Http package from NuGet) there is an easier way to simulate form requests. Here is an example:

private async Task<System.IO.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 null;
        }
        return await response.Content.ReadAsStreamAsync();
    }
}

To send the raw file only:

using(WebClient client = new WebClient()) {
    client.UploadFile(address, filePath);
}

If you want to emulate a browser form with an <input type="file"/>, then that is harder. See this answer for a multipart/form-data answer.

For me client.UploadFile still wrapped the content in a multipart request so I had to do it like this:

using (WebClient client = new WebClient())
{
    client.Headers.Add("Content-Type", "application/octet-stream");
    using (Stream fileStream = File.OpenRead(filePath))
    using (Stream requestStream = client.OpenWrite(new Uri(fileUploadUrl), "POST"))
    {
        fileStream.CopyTo(requestStream);
    }
}