我写了一个类使用WebClient方式做多部分的表单上传。

http://ferozedaud.blogspot.com/2010/03/multipart-form-upload-helper.html

/// 
/// MimePart
/// Abstract class for all MimeParts
/// 

abstract class MimePart
{
    public string Name { get; set; }

    public abstract string ContentDisposition { get; }

    public abstract string ContentType { get; }

    public abstract void CopyTo(Stream stream);

    public String Boundary
    {
        get;
        set;
    }
}

class NameValuePart : MimePart
{
    private NameValueCollection nameValues;

    public NameValuePart(NameValueCollection nameValues)
    {
        this.nameValues = nameValues;
    }

    public override void CopyTo(Stream stream)
    {
        string boundary = this.Boundary;
        StringBuilder sb = new StringBuilder();

        foreach (object element in this.nameValues.Keys)
        {
            sb.AppendFormat("--{0}", boundary);
            sb.Append("\r\n");
            sb.AppendFormat("Content-Disposition: form-data; name=\"{0}\";", element);
            sb.Append("\r\n");
            sb.Append("\r\n");
            sb.Append(this.nameValues[element.ToString()]);

            sb.Append("\r\n");

        }

        sb.AppendFormat("--{0}", boundary);
        sb.Append("\r\n");

        //Trace.WriteLine(sb.ToString());
        byte [] data = Encoding.ASCII.GetBytes(sb.ToString());
        stream.Write(data, 0, data.Length);
    }

    public override string ContentDisposition
    {
        get { return "form-data"; }
    }

    public override string ContentType
    {
        get { return String.Empty; }
    }
} 

class FilePart : MimePart

{

    private Stream input;

    private String contentType;



    public FilePart(Stream input, String name, String contentType)

    {

        this.input = input;

        this.contentType = contentType;

        this.Name = name;

    }



    public override void CopyTo(Stream stream)

    {

        StringBuilder sb = new StringBuilder();

        sb.AppendFormat("Content-Disposition: {0}", this.ContentDisposition);

        if (this.Name != null)

            sb.Append("; ").AppendFormat("name=\"{0}\"", this.Name);

        if (this.FileName != null)

            sb.Append("; ").AppendFormat("filename=\"{0}\"", this.FileName);

        sb.Append("\r\n");

        sb.AppendFormat(this.ContentType);

        sb.Append("\r\n");

        sb.Append("\r\n");



    // serialize the header data.

    byte[] buffer = Encoding.ASCII.GetBytes(sb.ToString());

    stream.Write(buffer, 0, buffer.Length);



    // send the stream.

    byte[] readBuffer = new byte[1024];

    int read = input.Read(readBuffer, 0, readBuffer.Length);

    while (read > 0)

    {

        stream.Write(readBuffer, 0, read);

        read = input.Read(readBuffer, 0, readBuffer.Length);

    }



    // write the terminating boundary

    sb.Length = 0;

    sb.Append("\r\n");

    sb.AppendFormat("--{0}", this.Boundary);

    sb.Append("\r\n");

    buffer = Encoding.ASCII.GetBytes(sb.ToString());

    stream.Write(buffer, 0, buffer.Length);



}

 public override string ContentDisposition
 {
      get { return "file"; }
 }



 public override string ContentType
 {
    get { 
       return String.Format("content-type: {0}", this.contentType); 
     }
 }

 public String FileName { get; set; }

}

    /// 
    /// Helper class that encapsulates all file uploads
    /// in a mime part.
    /// 

    class FilesCollection : MimePart
    {
        private List files;

        public FilesCollection()
        {
            this.files = new List();
            this.Boundary = MultipartHelper.GetBoundary();
        }

        public int Count
        {
            get { return this.files.Count; }
        }

        public override string ContentDisposition
        {
            get
            {
                return String.Format("form-data; name=\"{0}\"", this.Name);
            }
        }

        public override string ContentType
        {
            get { return String.Format("multipart/mixed; boundary={0}", this.Boundary); }
        }

        public override void CopyTo(Stream stream)
        {
            // serialize the headers
            StringBuilder sb = new StringBuilder(128);
            sb.Append("Content-Disposition: ").Append(this.ContentDisposition).Append("\r\n");
            sb.Append("Content-Type: ").Append(this.ContentType).Append("\r\n");
            sb.Append("\r\n");
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");

            byte[] headerBytes = Encoding.ASCII.GetBytes(sb.ToString());
            stream.Write(headerBytes, 0, headerBytes.Length);
            foreach (FilePart part in files)
            {
                part.Boundary = this.Boundary;
                part.CopyTo(stream);
            }
        }

        public void Add(FilePart part)
        {
            this.files.Add(part);
        }
    }

/// 
/// Helper class to aid in uploading multipart
/// entities to HTTP web endpoints.
/// 

class MultipartHelper
{
    private static Random random = new Random(Environment.TickCount);

    private List formData = new List();
    private FilesCollection files = null;
    private MemoryStream bufferStream = new MemoryStream();
    private string boundary;

    public String Boundary { get { return boundary; } }

    public static String GetBoundary()
    {
        return Environment.TickCount.ToString("X");
    }

    public MultipartHelper()
    {
        this.boundary = MultipartHelper.GetBoundary();
    }

    public void Add(NameValuePart part)
    {
        this.formData.Add(part);
        part.Boundary = boundary;
    }

    public void Add(FilePart part)
    {
        if (files == null)
        {
            files = new FilesCollection();
        }
        this.files.Add(part);
    }

    public void Upload(WebClient client, string address, string method)
    {
        // set header
        client.Headers.Add(HttpRequestHeader.ContentType, "multipart/form-data; boundary=" + this.boundary);
        Trace.WriteLine("Content-Type: multipart/form-data; boundary=" + this.boundary + "\r\n");

        // first, serialize the form data
        foreach (NameValuePart part in this.formData)
        {
            part.CopyTo(bufferStream);
        }

        // serialize the files.
        this.files.CopyTo(bufferStream);

        if (this.files.Count > 0)
        {
            // add the terminating boundary.
            StringBuilder sb = new StringBuilder();
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");
            byte [] buffer = Encoding.ASCII.GetBytes(sb.ToString());
            bufferStream.Write(buffer, 0, buffer.Length);
        }

        bufferStream.Seek(0, SeekOrigin.Begin);

        Trace.WriteLine(Encoding.ASCII.GetString(bufferStream.ToArray()));
        byte [] response = client.UploadData(address, method, bufferStream.ToArray());
        Trace.WriteLine("----- RESPONSE ------");
        Trace.WriteLine(Encoding.ASCII.GetString(response));
    }

    /// 
    /// Helper class that encapsulates all file uploads
    /// in a mime part.
    /// 

    class FilesCollection : MimePart
    {
        private List files;

        public FilesCollection()
        {
            this.files = new List();
            this.Boundary = MultipartHelper.GetBoundary();
        }

        public int Count
        {
            get { return this.files.Count; }
        }

        public override string ContentDisposition
        {
            get
            {
                return String.Format("form-data; name=\"{0}\"", this.Name);
            }
        }

        public override string ContentType
        {
            get { return String.Format("multipart/mixed; boundary={0}", this.Boundary); }
        }

        public override void CopyTo(Stream stream)
        {
            // serialize the headers
            StringBuilder sb = new StringBuilder(128);
            sb.Append("Content-Disposition: ").Append(this.ContentDisposition).Append("\r\n");
            sb.Append("Content-Type: ").Append(this.ContentType).Append("\r\n");
            sb.Append("\r\n");
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");

            byte[] headerBytes = Encoding.ASCII.GetBytes(sb.ToString());
            stream.Write(headerBytes, 0, headerBytes.Length);
            foreach (FilePart part in files)
            {
                part.Boundary = this.Boundary;
                part.CopyTo(stream);
            }
        }

        public void Add(FilePart part)
        {
            this.files.Add(part);
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        Trace.Listeners.Add(new ConsoleTraceListener());
        try
        {
            using (StreamWriter sw = new StreamWriter("testfile.txt", false))
            {
                sw.Write("Hello there!");
            }

            using (Stream iniStream = File.OpenRead(@"c:\platform.ini"))
            using (Stream fileStream = File.OpenRead("testfile.txt"))
            using (WebClient client = new WebClient())
            {
                MultipartHelper helper = new MultipartHelper();

                NameValueCollection props = new NameValueCollection();
                props.Add("fname", "john");
                props.Add("id", "acme");
                helper.Add(new NameValuePart(props));

                FilePart filepart = new FilePart(fileStream, "pics1", "text/plain");
                filepart.FileName = "1.jpg";
                helper.Add(filepart);

                FilePart ini = new FilePart(iniStream, "pics2", "text/plain");
                ini.FileName = "inifile.ini";
                helper.Add(ini);

                helper.Upload(client, "http://localhost/form.aspx", "POST");
            }
        }
        catch (Exception e)
        {
            Trace.WriteLine(e);
        }
    }
}

这将适用于所有版本的. net框架。

我写了一个类使用WebClient方式做多部分的表单上传。

http://ferozedaud.blogspot.com/2010/03/multipart-form-upload-helper.html

/// 
/// MimePart
/// Abstract class for all MimeParts
/// 

abstract class MimePart
{
    public string Name { get; set; }

    public abstract string ContentDisposition { get; }

    public abstract string ContentType { get; }

    public abstract void CopyTo(Stream stream);

    public String Boundary
    {
        get;
        set;
    }
}

class NameValuePart : MimePart
{
    private NameValueCollection nameValues;

    public NameValuePart(NameValueCollection nameValues)
    {
        this.nameValues = nameValues;
    }

    public override void CopyTo(Stream stream)
    {
        string boundary = this.Boundary;
        StringBuilder sb = new StringBuilder();

        foreach (object element in this.nameValues.Keys)
        {
            sb.AppendFormat("--{0}", boundary);
            sb.Append("\r\n");
            sb.AppendFormat("Content-Disposition: form-data; name=\"{0}\";", element);
            sb.Append("\r\n");
            sb.Append("\r\n");
            sb.Append(this.nameValues[element.ToString()]);

            sb.Append("\r\n");

        }

        sb.AppendFormat("--{0}", boundary);
        sb.Append("\r\n");

        //Trace.WriteLine(sb.ToString());
        byte [] data = Encoding.ASCII.GetBytes(sb.ToString());
        stream.Write(data, 0, data.Length);
    }

    public override string ContentDisposition
    {
        get { return "form-data"; }
    }

    public override string ContentType
    {
        get { return String.Empty; }
    }
} 

class FilePart : MimePart

{

    private Stream input;

    private String contentType;



    public FilePart(Stream input, String name, String contentType)

    {

        this.input = input;

        this.contentType = contentType;

        this.Name = name;

    }



    public override void CopyTo(Stream stream)

    {

        StringBuilder sb = new StringBuilder();

        sb.AppendFormat("Content-Disposition: {0}", this.ContentDisposition);

        if (this.Name != null)

            sb.Append("; ").AppendFormat("name=\"{0}\"", this.Name);

        if (this.FileName != null)

            sb.Append("; ").AppendFormat("filename=\"{0}\"", this.FileName);

        sb.Append("\r\n");

        sb.AppendFormat(this.ContentType);

        sb.Append("\r\n");

        sb.Append("\r\n");



    // serialize the header data.

    byte[] buffer = Encoding.ASCII.GetBytes(sb.ToString());

    stream.Write(buffer, 0, buffer.Length);



    // send the stream.

    byte[] readBuffer = new byte[1024];

    int read = input.Read(readBuffer, 0, readBuffer.Length);

    while (read > 0)

    {

        stream.Write(readBuffer, 0, read);

        read = input.Read(readBuffer, 0, readBuffer.Length);

    }



    // write the terminating boundary

    sb.Length = 0;

    sb.Append("\r\n");

    sb.AppendFormat("--{0}", this.Boundary);

    sb.Append("\r\n");

    buffer = Encoding.ASCII.GetBytes(sb.ToString());

    stream.Write(buffer, 0, buffer.Length);



}

 public override string ContentDisposition
 {
      get { return "file"; }
 }



 public override string ContentType
 {
    get { 
       return String.Format("content-type: {0}", this.contentType); 
     }
 }

 public String FileName { get; set; }

}

    /// 
    /// Helper class that encapsulates all file uploads
    /// in a mime part.
    /// 

    class FilesCollection : MimePart
    {
        private List files;

        public FilesCollection()
        {
            this.files = new List();
            this.Boundary = MultipartHelper.GetBoundary();
        }

        public int Count
        {
            get { return this.files.Count; }
        }

        public override string ContentDisposition
        {
            get
            {
                return String.Format("form-data; name=\"{0}\"", this.Name);
            }
        }

        public override string ContentType
        {
            get { return String.Format("multipart/mixed; boundary={0}", this.Boundary); }
        }

        public override void CopyTo(Stream stream)
        {
            // serialize the headers
            StringBuilder sb = new StringBuilder(128);
            sb.Append("Content-Disposition: ").Append(this.ContentDisposition).Append("\r\n");
            sb.Append("Content-Type: ").Append(this.ContentType).Append("\r\n");
            sb.Append("\r\n");
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");

            byte[] headerBytes = Encoding.ASCII.GetBytes(sb.ToString());
            stream.Write(headerBytes, 0, headerBytes.Length);
            foreach (FilePart part in files)
            {
                part.Boundary = this.Boundary;
                part.CopyTo(stream);
            }
        }

        public void Add(FilePart part)
        {
            this.files.Add(part);
        }
    }

/// 
/// Helper class to aid in uploading multipart
/// entities to HTTP web endpoints.
/// 

class MultipartHelper
{
    private static Random random = new Random(Environment.TickCount);

    private List formData = new List();
    private FilesCollection files = null;
    private MemoryStream bufferStream = new MemoryStream();
    private string boundary;

    public String Boundary { get { return boundary; } }

    public static String GetBoundary()
    {
        return Environment.TickCount.ToString("X");
    }

    public MultipartHelper()
    {
        this.boundary = MultipartHelper.GetBoundary();
    }

    public void Add(NameValuePart part)
    {
        this.formData.Add(part);
        part.Boundary = boundary;
    }

    public void Add(FilePart part)
    {
        if (files == null)
        {
            files = new FilesCollection();
        }
        this.files.Add(part);
    }

    public void Upload(WebClient client, string address, string method)
    {
        // set header
        client.Headers.Add(HttpRequestHeader.ContentType, "multipart/form-data; boundary=" + this.boundary);
        Trace.WriteLine("Content-Type: multipart/form-data; boundary=" + this.boundary + "\r\n");

        // first, serialize the form data
        foreach (NameValuePart part in this.formData)
        {
            part.CopyTo(bufferStream);
        }

        // serialize the files.
        this.files.CopyTo(bufferStream);

        if (this.files.Count > 0)
        {
            // add the terminating boundary.
            StringBuilder sb = new StringBuilder();
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");
            byte [] buffer = Encoding.ASCII.GetBytes(sb.ToString());
            bufferStream.Write(buffer, 0, buffer.Length);
        }

        bufferStream.Seek(0, SeekOrigin.Begin);

        Trace.WriteLine(Encoding.ASCII.GetString(bufferStream.ToArray()));
        byte [] response = client.UploadData(address, method, bufferStream.ToArray());
        Trace.WriteLine("----- RESPONSE ------");
        Trace.WriteLine(Encoding.ASCII.GetString(response));
    }

    /// 
    /// Helper class that encapsulates all file uploads
    /// in a mime part.
    /// 

    class FilesCollection : MimePart
    {
        private List files;

        public FilesCollection()
        {
            this.files = new List();
            this.Boundary = MultipartHelper.GetBoundary();
        }

        public int Count
        {
            get { return this.files.Count; }
        }

        public override string ContentDisposition
        {
            get
            {
                return String.Format("form-data; name=\"{0}\"", this.Name);
            }
        }

        public override string ContentType
        {
            get { return String.Format("multipart/mixed; boundary={0}", this.Boundary); }
        }

        public override void CopyTo(Stream stream)
        {
            // serialize the headers
            StringBuilder sb = new StringBuilder(128);
            sb.Append("Content-Disposition: ").Append(this.ContentDisposition).Append("\r\n");
            sb.Append("Content-Type: ").Append(this.ContentType).Append("\r\n");
            sb.Append("\r\n");
            sb.AppendFormat("--{0}", this.Boundary).Append("\r\n");

            byte[] headerBytes = Encoding.ASCII.GetBytes(sb.ToString());
            stream.Write(headerBytes, 0, headerBytes.Length);
            foreach (FilePart part in files)
            {
                part.Boundary = this.Boundary;
                part.CopyTo(stream);
            }
        }

        public void Add(FilePart part)
        {
            this.files.Add(part);
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        Trace.Listeners.Add(new ConsoleTraceListener());
        try
        {
            using (StreamWriter sw = new StreamWriter("testfile.txt", false))
            {
                sw.Write("Hello there!");
            }

            using (Stream iniStream = File.OpenRead(@"c:\platform.ini"))
            using (Stream fileStream = File.OpenRead("testfile.txt"))
            using (WebClient client = new WebClient())
            {
                MultipartHelper helper = new MultipartHelper();

                NameValueCollection props = new NameValueCollection();
                props.Add("fname", "john");
                props.Add("id", "acme");
                helper.Add(new NameValuePart(props));

                FilePart filepart = new FilePart(fileStream, "pics1", "text/plain");
                filepart.FileName = "1.jpg";
                helper.Add(filepart);

                FilePart ini = new FilePart(iniStream, "pics2", "text/plain");
                ini.FileName = "inifile.ini";
                helper.Add(ini);

                helper.Upload(client, "http://localhost/form.aspx", "POST");
            }
        }
        catch (Exception e)
        {
            Trace.WriteLine(e);
        }
    }
}

这将适用于所有版本的. net框架。

修改了@CristianRomanescu代码，以使用内存流，接受文件作为字节数组，允许空nvc，返回请求响应和使用授权头。使用Web Api 2测试代码。

private string HttpUploadFile(string url, byte[] file, string fileName, string paramName, string contentType, NameValueCollection nvc, string authorizationHeader)
{
    string boundary = "---------------------------" + DateTime.Now.Ticks.ToString("x");
    byte[] boundarybytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");

    HttpWebRequest wr = (HttpWebRequest)WebRequest.Create(url);
    wr.ContentType = "multipart/form-data; boundary=" + boundary;
    wr.Method = "POST";
    wr.Headers.Add("Authorization", authorizationHeader);
    wr.KeepAlive = true;

    Stream rs = wr.GetRequestStream();

    string formdataTemplate = "Content-Disposition: form-data; name=\"{0}\"\r\n\r\n{1}";
    if (nvc != null)
    {
        foreach (string key in nvc.Keys)
        {
            rs.Write(boundarybytes, 0, boundarybytes.Length);
            string formitem = string.Format(formdataTemplate, key, nvc[key]);
            byte[] formitembytes = System.Text.Encoding.UTF8.GetBytes(formitem);
            rs.Write(formitembytes, 0, formitembytes.Length);
        }
    }

    rs.Write(boundarybytes, 0, boundarybytes.Length);

    string headerTemplate = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";
    string header = string.Format(headerTemplate, paramName, fileName, contentType);
    byte[] headerbytes = System.Text.Encoding.UTF8.GetBytes(header);
    rs.Write(headerbytes, 0, headerbytes.Length);

    rs.Write(file, 0, file.Length);

    byte[] trailer = System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "--\r\n");
    rs.Write(trailer, 0, trailer.Length);
    rs.Close();

    WebResponse wresp = null;
    try
    {
        wresp = wr.GetResponse();
        Stream stream2 = wresp.GetResponseStream();
        StreamReader reader2 = new StreamReader(stream2);
        var response = reader2.ReadToEnd();
        return response;
    }
    catch (Exception ex)
    {
        if (wresp != null)
        {
            wresp.Close();
            wresp = null;
        }
        return null;
    }
    finally
    {
        wr = null;
    }
}

Testcode:

[HttpPost]
[Route("postformdata")]
public IHttpActionResult PostFormData()
{
    // Check if the request contains multipart/form-data.
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    var provider = new MultipartMemoryStreamProvider();

    try
    {
        // Read the form data.
        var result = Request.Content.ReadAsMultipartAsync(provider).Result;
        string response = "";
        // This illustrates how to get the file names.
        foreach (var file in provider.Contents)
        {
            var fileName = file.Headers.ContentDisposition.FileName.Trim('\"');
            var buffer =  file.ReadAsByteArrayAsync().Result;
            response = HttpUploadFile("https://localhost/api/v1/createfromfile", buffer, fileName, "file", "application/pdf", null, "AuthorizationKey");
        }
        return Ok(response);
    }
    catch (System.Exception e)
    {
        return InternalServerError();
    }
}

我知道这可能有点晚了，但我一直在寻找同样的解决方案。我从一位微软代表那里找到了以下回复

private void UploadFilesToRemoteUrl(string url, string[] files, string logpath, NameValueCollection nvc)
{

    long length = 0;
    string boundary = "----------------------------" +
    DateTime.Now.Ticks.ToString("x");


    HttpWebRequest httpWebRequest2 = (HttpWebRequest)WebRequest.Create(url);
    httpWebRequest2.ContentType = "multipart/form-data; boundary=" +
    boundary;
    httpWebRequest2.Method = "POST";
    httpWebRequest2.KeepAlive = true;
    httpWebRequest2.Credentials = System.Net.CredentialCache.DefaultCredentials;



    Stream memStream = new System.IO.MemoryStream();
    byte[] boundarybytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");


    string formdataTemplate = "\r\n--" + boundary + "\r\nContent-Disposition: form-data; name=\"{0}\";\r\n\r\n{1}";

    foreach(string key in nvc.Keys)
    {
        string formitem = string.Format(formdataTemplate, key, nvc[key]);
        byte[] formitembytes = System.Text.Encoding.UTF8.GetBytes(formitem);
        memStream.Write(formitembytes, 0, formitembytes.Length);
    }


    memStream.Write(boundarybytes,0,boundarybytes.Length);

    string headerTemplate = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\n Content-Type: application/octet-stream\r\n\r\n";

    for(int i=0;i<files.Length;i++)
    {

        string header = string.Format(headerTemplate,"file"+i,files[i]);
        byte[] headerbytes = System.Text.Encoding.UTF8.GetBytes(header);
        memStream.Write(headerbytes,0,headerbytes.Length);


        FileStream fileStream = new FileStream(files[i], FileMode.Open,
        FileAccess.Read);
        byte[] buffer = new byte[1024];

        int bytesRead = 0;

        while ( (bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0 )
        {
            memStream.Write(buffer, 0, bytesRead);
        }


        memStream.Write(boundarybytes,0,boundarybytes.Length);


        fileStream.Close();
    }

    httpWebRequest2.ContentLength = memStream.Length;
    Stream requestStream = httpWebRequest2.GetRequestStream();

    memStream.Position = 0;
    byte[] tempBuffer = new byte[memStream.Length];
    memStream.Read(tempBuffer,0,tempBuffer.Length);
    memStream.Close();
    requestStream.Write(tempBuffer,0,tempBuffer.Length );
    requestStream.Close();


    WebResponse webResponse2 = httpWebRequest2.GetResponse();

    Stream stream2 = webResponse2.GetResponseStream();
    StreamReader reader2 = new StreamReader(stream2);

    webResponse2.Close();
    httpWebRequest2 = null;
    webResponse2 = null;

}

我想在VB中做文件上传和添加一些参数到multipart/form-data请求。NET而不是通过正规的表单发布。 感谢@JoshCodes的回答，我找到了我一直在寻找的方向。 我发布我的解决方案是为了帮助其他人找到一种方法来使用文件和参数来执行帖子 html等价于我试图实现的是: 超文本标记语言

<form action="your-api-endpoint" enctype="multipart/form-data" method="post"> 
<input type="hidden" name="action" value="api-method-name"/> 
<input type="hidden" name="apiKey" value="gs1xxxxxxxxxxxxxex"/> 
<input type="hidden" name="access" value="protected"/> 
<input type="hidden" name="name" value="test"/> 
<input type="hidden" name="title" value="test"/> 
<input type="hidden" name="signature" value="cf1d4xxxxxxxxcd5"/> 
<input type="file" name="file"/> 
<input type="submit" name="_upload" value="Upload"/> 
</form>

Due to the fact that I have to provide the apiKey and the signature (which is a calculated checksum of the request parameters and api key concatenated string), I needed to do it server side. The other reason I needed to do it server side is the fact that the post of the file can be performed at any time by pointing to a file already on the server (providing the path), so there would be no manually selected file during form post thus form data file would not contain the file stream.Otherwise I could have calculated the checksum via an ajax callback and submitted the file through the html post using JQuery. I am using .net version 4.0 and cannot upgrade to 4.5 in the actual solution. So I had to install the Microsoft.Net.Http using nuget cmd

PM> install-package Microsoft.Net.Http

Private Function UploadFile(req As ApiRequest, filePath As String, fileName As String) As String
    Dim result = String.empty
    Try
        ''//Get file stream
        Dim paramFileStream As Stream = File.OpenRead(filePath)
        Dim fileStreamContent As HttpContent = New  StreamContent(paramFileStream)
        Using client = New HttpClient()
            Using formData = New MultipartFormDataContent()
                ''// This adds parameter name ("action")
                ''// parameter value (req.Action) to form data
                formData.Add(New StringContent(req.Action), "action")
                formData.Add(New StringContent(req.ApiKey), "apiKey")
                For Each param In req.Parameters
                    formData.Add(New StringContent(param.Value), param.Key)
                Next
                formData.Add(New StringContent(req.getRequestSignature.Qualifier), "signature")
                ''//This adds the file stream and file info to form data
                formData.Add(fileStreamContent, "file", fileName)
                ''//We are now sending the request
                Dim response = client.PostAsync(GetAPIEndpoint(), formData).Result
                ''//We are here reading the response
                Dim readR = New StreamReader(response.Content.ReadAsStreamAsync().Result, Encoding.UTF8)
                Dim respContent = readR.ReadToEnd()

                If Not response.IsSuccessStatusCode Then
                    result =  "Request Failed : Code = " & response.StatusCode & "Reason = " & response.ReasonPhrase & "Message = " & respContent
                End If
                result.Value = respContent
            End Using
        End Using
    Catch ex As Exception
        result = "An error occurred : " & ex.Message
    End Try

    Return result
End Function

采用上面的代码并修复，因为它抛出内部服务器错误500。\r\n的位置和空格等存在一些问题。应用内存流重构，直接写入请求流。结果如下:

    public static void HttpUploadFile(string url, string file, string paramName, string contentType, NameValueCollection nvc) {
        log.Debug(string.Format("Uploading {0} to {1}", file, url));
        string boundary = "---------------------------" + DateTime.Now.Ticks.ToString("x");
        byte[] boundarybytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");

        HttpWebRequest wr = (HttpWebRequest)WebRequest.Create(url);
        wr.ContentType = "multipart/form-data; boundary=" + boundary;
        wr.Method = "POST";
        wr.KeepAlive = true;
        wr.Credentials = System.Net.CredentialCache.DefaultCredentials;

        Stream rs = wr.GetRequestStream();

        string formdataTemplate = "Content-Disposition: form-data; name=\"{0}\"\r\n\r\n{1}";
        foreach (string key in nvc.Keys)
        {
            rs.Write(boundarybytes, 0, boundarybytes.Length);
            string formitem = string.Format(formdataTemplate, key, nvc[key]);
            byte[] formitembytes = System.Text.Encoding.UTF8.GetBytes(formitem);
            rs.Write(formitembytes, 0, formitembytes.Length);
        }
        rs.Write(boundarybytes, 0, boundarybytes.Length);

        string headerTemplate = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";
        string header = string.Format(headerTemplate, paramName, file, contentType);
        byte[] headerbytes = System.Text.Encoding.UTF8.GetBytes(header);
        rs.Write(headerbytes, 0, headerbytes.Length);

        FileStream fileStream = new FileStream(file, FileMode.Open, FileAccess.Read);
        byte[] buffer = new byte[4096];
        int bytesRead = 0;
        while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0) {
            rs.Write(buffer, 0, bytesRead);
        }
        fileStream.Close();

        byte[] trailer = System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "--\r\n");
        rs.Write(trailer, 0, trailer.Length);
        rs.Close();

        WebResponse wresp = null;
        try {
            wresp = wr.GetResponse();
            Stream stream2 = wresp.GetResponseStream();
            StreamReader reader2 = new StreamReader(stream2);
            log.Debug(string.Format("File uploaded, server response is: {0}", reader2.ReadToEnd()));
        } catch(Exception ex) {
            log.Error("Error uploading file", ex);
            if(wresp != null) {
                wresp.Close();
                wresp = null;
            }
        } finally {
            wr = null;
        }
    }

以及示例用法:

    NameValueCollection nvc = new NameValueCollection();
    nvc.Add("id", "TTR");
    nvc.Add("btn-submit-photo", "Upload");
    HttpUploadFile("http://your.server.com/upload", 
         @"C:\test\test.jpg", "file", "image/jpeg", nvc);

可以扩展它来处理多个文件，或者对每个文件多次调用它。但它适合你的需要。