这样做容易得多：

var mydata = HttpContext.Current.Request.Form["mydata"];

比这个：

string root = HttpContext.Current.Server.MapPath("~/somedir");
var provider = new MultipartFormDataStreamProvider(root);

this.Request.Content.ReadAsMultipartAsync(provider).ContinueWith(t =>
{
    var mydata = provider.Contents.First(c => c.Headers.ContentDisposition.Name == "\"mydata\"").ReadAsStringAsync().Result;
});

PS – 我正在尝试构建一个ApiController来接受文件上传。我读过这篇文章http://www.asp.net/web-api/overview/working-with-http/sending-html-form-data,-part-2。