Robert 的部落格

程式碼範例：

public class UploadFile
    {
        public UploadFile()
        {
            ContentType = "application/octet-stream";
        }
        public string Name { get; set; }
        public string Filename { get; set; }
        public string ContentType { get; set; }
        public Stream Stream { get; set; }
    }

public byte[] UploadFiles(string address, IEnumerable<UploadFile> files, NameValueCollection values)
    {
        var request = WebRequest.Create(address);
        request.Method = "POST";
        var boundary = "---------------------------" + DateTime.Now.Ticks.ToString("x", NumberFormatInfo.InvariantInfo);
        request.ContentType = "multipart/form-data; boundary=" + boundary;
        boundary = "--" + boundary;

        using (var requestStream = request.GetRequestStream())
        {
            // Write the values
            foreach (string name in values.Keys)
            {
                var buffer = Encoding.ASCII.GetBytes(boundary + Environment.NewLine);
                requestStream.Write(buffer, 0, buffer.Length);
                buffer = Encoding.ASCII.GetBytes(string.Format("Content-Disposition: form-data; name=\"{0}\"{1}{1}", name, Environment.NewLine));
                requestStream.Write(buffer, 0, buffer.Length);
                buffer = Encoding.UTF8.GetBytes(values[name] + Environment.NewLine);
                requestStream.Write(buffer, 0, buffer.Length);
            }

            // Write the files
            foreach (var file in files)
            {
                var buffer = Encoding.ASCII.GetBytes(boundary + Environment.NewLine);
                requestStream.Write(buffer, 0, buffer.Length);
                buffer = Encoding.UTF8.GetBytes(string.Format("Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"{2}", file.Name, file.Filename, Environment.NewLine));
                requestStream.Write(buffer, 0, buffer.Length);
                buffer = Encoding.ASCII.GetBytes(string.Format("Content-Type: {0}{1}{1}", file.ContentType, Environment.NewLine));
                requestStream.Write(buffer, 0, buffer.Length);
                file.Stream.CopyTo(requestStream);
                buffer = Encoding.ASCII.GetBytes(Environment.NewLine);
                requestStream.Write(buffer, 0, buffer.Length);
            }

            var boundaryBuffer = Encoding.ASCII.GetBytes(boundary + "--");
            requestStream.Write(boundaryBuffer, 0, boundaryBuffer.Length);
        }

        using (var response = request.GetResponse())
        using (var responseStream = response.GetResponseStream())
        using (var stream = new MemoryStream())
        {
            responseStream.CopyTo(stream);
            return stream.ToArray();
        }
    }

using (var stream1 = File.Open("test.txt", FileMode.Open))
    using (var stream2 = File.Open("test.xml", FileMode.Open))
    using (var stream3 = File.Open("test.pdf", FileMode.Open))
    {
        var files = new[] 
        {
            new UploadFile
            {
                Name = "file",
                Filename = "test.txt",
                ContentType = "text/plain",
                Stream = stream1
            },
            new UploadFile
            {
                Name = "file",
                Filename = "test.xml",
                ContentType = "text/xml",
                Stream = stream2
            },
            new UploadFile
            {
                Name = "file",
                Filename = "test.pdf",
                ContentType = "application/pdf",
                Stream = stream3
            }
        };

        var values = new NameValueCollection
        {
            { "key1", "value1" },
            { "key2", "value2" },
            { "key3", "value3" },
        };

        byte[] result = UploadFiles("http://localhost:1234/upload", files, values);
    }

請依所需自行調整。

參考：Uploading multiple files with C#
Upload files with POST from a C#-WinForm using WebBrowser-Instance?