我怎样才能抓到404？

我有以下代码：

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "HEAD";
request.Credentials = MyCredentialCache;

try
{
    request.GetResponse();
}
catch
{
}

如何捕获特定的404错误？WebExceptionStatus.ProtocolError只能检测到发生错误，而不能给出错误的确切代码。

例如：

catch (WebException ex)
{
    if (ex.Status != WebExceptionStatus.ProtocolError)
    {
        throw ex;
    }
}

只是不够有用...协议异常可能是401、503、403，实际上是任何东西。

HttpStatusCode Enumeration专门使用HttpStatusCode.NotFound

就像是：

HttpWebResponse errorResponse = we.Response as HttpWebResponse;
if (errorResponse.StatusCode == HttpStatusCode.NotFound) {
  //
}

哪里
we是WebException。

try
{
    var request = WebRequest.Create(uri);
    using (var response = request.GetResponse())
    {
        using (var responseStream = response.GetResponseStream())
        {
            // Process the stream
        }
    }
}
catch (WebException ex)
{
    if (ex.Status == WebExceptionStatus.ProtocolError &&
        ex.Response != null)
    {
        var resp = (HttpWebResponse) ex.Response;
        if (resp.StatusCode == HttpStatusCode.NotFound)
        {
            // Do something
        }
        else
        {
            // Do something else
        }
    }
    else
    {
        // Do something else
    }
}

在C＃6中，可以使用异常过滤器。

try
{
    var request = WebRequest.Create(uri);
    using (var response = request.GetResponse())
    using (var responseStream = response.GetResponseStream())
    {
        // Process the stream
    }
}
catch(WebException ex) when ((ex.Response as HttpWebResponse)?.StatusCode == HttpStatusCode.NotFound)
{
    // handle 404 exceptions
}
catch (WebException ex)
{
    // handle other web exceptions
}

我没有测试过，但是应该可以

try
{
    // TODO: Make request.
}
catch (WebException ex)
{
    if (ex.Status == WebExceptionStatus.ProtocolError) {
        HttpWebResponse resp = ex.Response as HttpWebResponse;
        if (resp != null && resp.StatusCode == HttpStatusCode.NotFound)
        {
            // TODO: Handle 404 error.
        }
        else
            throw;
    }
    else
        throw;
}

我认为，如果您捕获了WebException，则可以在其中使用一些信息来确定它是否是404。这是我目前所知道的唯一方式...我会对了解其他信息感兴趣。

catch(WebException e) {
    if(e.Status == WebExceptionStatus.ProtocolError) {
        var statusCode = (HttpWebResponse)e.Response).StatusCode);
        var description = (HttpWebResponse)e.Response).StatusDescription);
    }
}

看看这个snipit。GetResponse将引发WebRequestException。赶上它，您可以从响应中获取状态代码。

try {
   // Create a web request for an invalid site. Substitute the "invalid site" strong in the Create call with a invalid name.
     HttpWebRequest myHttpWebRequest = (HttpWebRequest) WebRequest.Create("invalid site");

    // Get the associated response for the above request.
     HttpWebResponse myHttpWebResponse = (HttpWebResponse) myHttpWebRequest.GetResponse();
    myHttpWebResponse.Close();
}
catch(WebException e) {
    Console.WriteLine("This program is expected to throw WebException on successful run."+
                        "\n\nException Message :" + e.Message);
    if(e.Status == WebExceptionStatus.ProtocolError) {
        Console.WriteLine("Status Code : {0}", ((HttpWebResponse)e.Response).StatusCode);
        Console.WriteLine("Status Description : {0}", ((HttpWebResponse)e.Response).StatusDescription);
    }
}
catch(Exception e) {
    Console.WriteLine(e.Message);
}

这来自http://msdn.microsoft.com/en-us/library/system.net.webexception.status.aspx

捕获适当的异常类型WebException：

try
{
    var request = (HttpWebRequest) WebRequest.Create(String.Format("http://www.gravatar.com/avatar/{0}?d=404", hashe));

    using(var response = (HttpWebResponse)request.GetResponse())
        Response.Write("has avatar");
}
catch(WebException e) 
{
  if(e.Response.StatusCode == 404) 
    Response.Write("No avatar");
}

有关响应状态，请参见MSDN：

...
catch(WebException e) {
  Console.WriteLine("The following error occured : {0}",e.Status);  
}
...

对于浏览此内容的VB.NET人员，我相信只有在它确实是404的情况下，我们才能捕获该异常。

Try
    httpWebrequest.GetResponse()
Catch we As WebException When we.Response IsNot Nothing _
                              AndAlso TypeOf we.Response Is HttpWebResponse _
                              AndAlso (DirectCast(we.Response, HttpWebResponse).StatusCode = HttpStatusCode.NotFound)

    ' ...

End Try

当使用WebRequest类将POST或GET数据发送到服务器时，异常类型将为WebException。以下是找不到文件异常的代码

        //Create a web request with the specified URL
            string path = @"http://localhost/test.xml1";
            WebRequest myWebRequest = WebRequest.Create(path);

       //Senda a web request and wait for response.
                try
                {
                    WebResponse objwebResponse = myWebRequest.GetResponse();
                    Stream stream= objwebResponse.GetResponseStream();

                }
                catch (WebException ex) {
                    if (((HttpWebResponse)(ex.Response)).StatusCode == HttpStatusCode.NotFound) {
                        throw new FileNotFoundException(ex.Message);
                    }

                }