请求提交后无法执行此操作答案

【问题标题】：This operation cannot be performed after the request has been submitted请求提交后无法执行此操作
【发布时间】：2014-07-25 19:51:41
【问题描述】：

以下代码使用 FSharp.Data 的 Http 模块，在调用 Http.RequestString 时在函数 setProxy 中引发异常。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，它在使用经过修改（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 )

例外：

System.dll 中出现“System.InvalidOperationException”类型的异常，但未在用户代码中处理

补充信息：请求提交后无法执行此操作。

【问题讨论】：

为什么这个标签是C#？

标签： .net http f# f#-data

【解决方案1】：

我相信这是 FSharp.Data 库中的一个错误。查看它的source code on GitHub，我们可以提取我们感兴趣的代码，例如：

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()

问题在于writeBody调用之前调用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HttpWebRequest.m_RequestSubmitted 标志，然后由HttpWebRequest.Proxy 的属性设置器（如许多其他）检查。我的建议：

直接使用HttpWebRequest 编写您自己的方法，
编译您自己的 FSharp.Data 版本，更改 do! writeBody req bytes 和 let req = customizeHttpRequest req 调用的顺序
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();
}

【讨论】：

【解决方案2】：

此问题已在最新版本的 F# Data 中得到修复

【讨论】：