How to Send Files via HTTP POST Using C#: A Comprehensive Guide

Detailed explanation of C# HTTP POST file upload

This article will introduce in detail how to use C# to upload files to a web server. This is a common requirement in Windows application development and requires an understanding of the principles of HTTP form requests.

Implemented using HttpWebRequest (applicable before .NET 4.5)

Prior to .NET 4.5, file uploads typically used the traditional HttpWebRequest object. The steps are as follows:

1. Create an HttpWebRequest object, specifying the URI, method, and other necessary headers.
2. Create an HttpStream object by calling req.GetRequestStream().
3. Read the contents of the file to be uploaded into the stream.

Sample code:

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);
}

Implemented using HttpClient and MultipartFormDataContent (for .NET 4.5 and above)

.NET 4.5 and later (or by using the "Microsoft.Net.Http" NuGet package in .NET 4.0), you can use HttpClient and MultipartFormDataContent to more easily simulate form requests.

Sample code:

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;
}

With the above steps, you can easily upload files to a web server via HTTP POST using a C# application.

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