.net - 提交请求后无法执行此操作

Question

以下代码使用了 FSharp.Data 的 Http 模块，当 Http. RequestString 被调用。当在不同的网站上使用 query 参数而不是 body 时，类似的代码有效。什么会导致问题？ 顺便说一句，它在使用修改后的(用于分配代理)以前版本的 Http 模块时有效。

let headers = [ UserAgent ConfigurationManager.AppSettings.["UserAgent"] ]
let setProxy (req: HttpWebRequest) = 
    req.Proxy <- WebProxy(ConfigurationManager.AppSettings.["Proxy"], true)
    req // Error here
let cc = CookieContainer()

let body = HttpRequestBody.FormValues [
             "username", user; 
             "password", password; 
             "nothing.x", "21"; "nothing.y", "34"]
let html =
    Http.RequestString("https://..../CheckLoginServlet", 
        httpMethod = "POST", 
        body = body, cookieContainer = cc, headers = headers, 
        customizeHttpRequest = setProxy )

异常(exception):

An exception of type 'System.InvalidOperationException' occurred in System.dll but was not handled in user code

Additional information: This operation cannot be performed after the request has been submitted.

Answer 1

我认为这是 FSharp.Data 库中的一个错误。翻阅其source code on GitHub ，我们可以将我们感兴趣的代码提取为:

let values = [ "id", "2"; ]
let req = WebRequest.Create("https://..../CheckLoginServlet") :?> HttpWebRequest
req.Method <- HttpMethod.Post
req.UserAgent <- "Magic"
req.AutomaticDecompression <- DecompressionMethods.GZip ||| DecompressionMethods.Deflate
let cookieContainer = new CookieContainer()
req.CookieContainer <- cookieContainer
req.ContentType <- "application/x-www-form-urlencoded"
let bytes = [ for k, v in values -> Uri.EscapeDataString k + "=" + Uri.EscapeDataString v ]
                    |> String.concat "&"
                    |> HttpEncodings.PostDefaultEncoding.GetBytes
let f = fun () -> async {
    do! writeBody req bytes
    let req = customizeHttpRequest req
    let! resp = Async.FromBeginEnd(req.BeginGetResponse , req.EndGetResponse)
    let stream = resp.GetResponseStream()
    return! toHttpResponse
}
f()

问题出在 writeBody 调用 before 调用 customizeHttpRequest，而它的代码是:

let writeBody (req:HttpWebRequest) (postBytes:byte[]) = async { 
    req.ContentLength <- int64 postBytes.Length
    use! output = Async.FromBeginEnd(req.BeginGetRequestStream, req.EndGetRequestStream)
    do! output.AsyncWrite(postBytes, 0, postBytes.Length)
    output.Flush()
}

其中 Async.FromBeginEnd(req.BeginGetRequestStream, req.EndGetRequestStream) 更改了 HttpWebRequest.m_RequestSubmitted 标志，然后由(与其他许多一样)属性 setter 检查HttpWebRequest.Proxy。我的建议:

1. 直接使用HttpWebRequest编写自己的方法，
2. 编译您自己的 FSharp.Data 版本，更改 的顺序! writeBody req bytes 和 let req = customizeHttpRequest req 调用
3. 通知库的贡献者对此进行修复

注意:这可以很容易地在 C# 示例程序中重现:

private static async void GetRequestStreamCallback(IAsyncResult ar)
{
    HttpWebRequest req = (HttpWebRequest)ar.AsyncState; 
    Stream postStream = req.EndGetRequestStream(ar); // Here flag req.m_RequestSubmitted is true already
    byte[] byteArray = Encoding.UTF8.GetBytes("id=1");
    await postStream.WriteAsync(byteArray, 0, byteArray.Length);
    postStream.Close();
    req.Proxy = new WebProxy("localhost:8888", true);
    allDone.WaitOne();
}
private static ManualResetEvent allDone = new ManualResetEvent(false);
static void Main(string[] args)
{
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://localhost:64343/api/products");
    req.Method = "POST";
    req.UserAgent = "Magic";
    req.AutomaticDecompression = DecompressionMethods.GZip;
    req.CookieContainer = new CookieContainer();
    req.ContentType = "application/x-www-form-urlencoded";
    var bytes = Encoding.UTF8.GetBytes("id=2");
    req.ContentLength = bytes.Length;
    req.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), req);
    allDone.WaitOne();
}