如何等待响应并对其进行操作？

Question

我有一个新问题。我想对响应进行一些操作，但我得到一个 NullReferenceException，因为它尚未到达...这是我的代码：

public partial class MainPage : PhoneApplicationPage
{
    public static string res = null;

    // Constructor
    public MainPage()
    {
        InitializeComponent();
        string Url = "http://twitter.com";
        WebRequest req = WebRequest.Create(Url);
        req.BeginGetResponse(new AsyncCallback(request_CallBack), req);
        int i = MainPage.res.Length; // NullReferenceException
    }

    void request_CallBack(IAsyncResult result)
    {
        WebRequest webRequest = result.AsyncState as WebRequest;
        WebResponse response = (WebResponse)webRequest.EndGetResponse(result);
        Stream baseStream = response.GetResponseStream();
        using (StreamReader reader = new StreamReader(baseStream))
        {
            res = reader.ReadToEnd();
            Dispatcher.BeginInvoke(() => { MessageBox.Show("The response is arrived."); });
            Dispatcher.BeginInvoke(() => { tbResponse.Text = res; });
        }
    }
}

但是当我使用 ManualResetEvent 类时，我的应用程序只是挂起，因为 if(dataReady.WaitOne()) 行。以下是 ManualResetEvent 类的完整代码：

public partial class MainPage : PhoneApplicationPage
{
    public static string res = null;
    ManualResetEvent dataReady;

    // Constructor
    public MainPage()
    {
        InitializeComponent();
        string Url = "http://twitter.com";
        dataReady = new ManualResetEvent(false);
        WebRequest req = WebRequest.Create(Url);
        req.BeginGetResponse(new AsyncCallback(request_CallBack), req);
        if (dataReady.WaitOne())
        {
            int i = MainPage.res.Length;
        }
    }

    void request_CallBack(IAsyncResult result)
    {
        WebRequest webRequest = result.AsyncState as WebRequest;
        WebResponse response = (WebResponse)webRequest.EndGetResponse(result);
        Stream baseStream = response.GetResponseStream();
        using (StreamReader reader = new StreamReader(baseStream))
        {
            res = reader.ReadToEnd();
            Dispatcher.BeginInvoke(() => { MessageBox.Show("The response is arrived."); });
            Dispatcher.BeginInvoke(() => { tbResponse.Text = res; });
        }
        dataReady.Set();
    }
}

那么，我的问题是：如何等待响应并对其进行操作？ （我尝试使用Application.DoEvent方法，但它在WP7中不存在......）

Answer 1

您不能将该代码放在

int i = MainPage.res.Length; // NullReferenceException

request_CallBack 函数中吗？

喜欢

void request_CallBack(IAsyncResult result)
    {
        WebRequest webRequest = result.AsyncState as WebRequest;
        WebResponse response = (WebResponse)webRequest.EndGetResponse(result);
        Stream baseStream = response.GetResponseStream();
        using (StreamReader reader = new StreamReader(baseStream))
        {
            res = reader.ReadToEnd();
            Dispatcher.BeginInvoke(() => { MessageBox.Show("The response is arrived."); });
            Dispatcher.BeginInvoke(() => { tbResponse.Text = res; });
        }
    int i = res.Length; //here
    }

Answer 2

当前您正在使用 WebRequest 的异步方法之一。如果您想等待响应，只需使用等效的同步方法 GetResponse 即可。

http://msdn.microsoft.com/en-us /library/system.net.webrequest.getresponse.aspx

Answer 3

public partial class MainPage : PhoneApplicationPage

{
公共静态字符串res = null;

// Constructor
public MainPage()
{
    InitializeComponent();
    string Url = "http://twitter.com";
    WebRequest req = WebRequest.Create(Url);
    req.BeginGetResponse(new AsyncCallback(request_CallBack), req);

}

void request_CallBack(IAsyncResult result)
{
    WebRequest webRequest = result.AsyncState as WebRequest;
    WebResponse response = (WebResponse)webRequest.EndGetResponse(result);
    Stream baseStream = response.GetResponseStream();
    using (StreamReader reader = new StreamReader(baseStream))
    {
        res = reader.ReadToEnd();
        Dispatcher.BeginInvoke(() => { MessageBox.Show("The response is arrived."); });
        Dispatcher.BeginInvoke(() => { tbResponse.Text = res; });
    }
    int i = MainPage.res.Length; //No  NullReferenceException
}

}