根据kisp解决方案，这是我的编辑版本工作异步:

类WebConnection.cs

internal class WebConnection : WebClient
{
    internal int Timeout { get; set; }

    protected override WebRequest GetWebRequest(Uri Address)
    {
        WebRequest WebReq = base.GetWebRequest(Address);
        WebReq.Timeout = Timeout * 1000 // Seconds
        return WebReq;
    }
}

异步任务

private async Task GetDataAsyncWithTimeout()
{
    await Task.Run(() =>
    {
        using (WebConnection webClient = new WebConnection())
        {
            webClient.Timeout = 5; // Five seconds (the multiplication is in the override)
            webClient.DownloadData("https://www.yourwebsite.com");
        }
    });
} // await GetDataAsyncWithTimeout()

否则，如果你不想使用async:

private void GetDataSyncWithTimeout()
{
    using (WebConnection webClient = new WebConnection())
    {
        webClient.Timeout = 5; // Five seconds (the multiplication is in the override)
        webClient.DownloadData("https://www.yourwebsite.com");
    }
} // GetDataSyncWithTimeout()

对于任何需要WebClient超时工作的异步/任务方法的人，建议的解决方案将不起作用。以下是有效的方法:

public class WebClientWithTimeout : WebClient
{
    //10 secs default
    public int Timeout { get; set; } = 10000;

    //for sync requests
    protected override WebRequest GetWebRequest(Uri uri)
    {
        var w = base.GetWebRequest(uri);
        w.Timeout = Timeout; //10 seconds timeout
        return w;
    }

    //the above will not work for async requests :(
    //let's create a workaround by hiding the method
    //and creating our own version of DownloadStringTaskAsync
    public new async Task<string> DownloadStringTaskAsync(Uri address)
    {
        var t = base.DownloadStringTaskAsync(address);
        if(await Task.WhenAny(t, Task.Delay(Timeout)) != t) //time out!
        {
            CancelAsync();
        }
        return await t;
    }
}

我在博客上写了完整的解决方法

在某些情况下，有必要在头文件中添加用户代理:

WebClient myWebClient = new WebClient();
myWebClient.DownloadFile(myStringWebResource, fileName);
myWebClient.Headers["User-Agent"] = "Mozilla/4.0 (Compatible; Windows NT 5.1; MSIE 6.0) (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";

这就是我案子的解决办法。

信贷:

http://genjurosdojo.blogspot.com/2012/10/the-remote-server-returned-error-504.html

我昨天不得不与这个问题作斗争，我也最终写了我的自定义扩展类。

通过查看下面的代码并将其与接受的答案进行比较，您可以看到，我试图稍微调整建议，以便拥有一个更通用的类:通过这种方式，您可以在实例化对象时或在使用使用内部WebRequest处理程序的方法之前设置精确的超时。

using System;
using System.Net;

namespace Ryadel.Components.Web
{
    /// <summary>
    /// An extension of the standard System.Net.WebClient
    /// featuring a customizable constructor and [Timeout] property.
    /// </summary>
    public class RyadelWebClient : WebClient
    {
        /// <summary>
        /// Default constructor (30000 ms timeout)
        /// NOTE: timeout can be changed later on using the [Timeout] property.
        /// </summary>
        public RyadelWebClient() : this(30000) { }

        /// <summary>
        /// Constructor with customizable timeout
        /// </summary>
        /// <param name="timeout">
        /// Web request timeout (in milliseconds)
        /// </param>
        public RyadelWebClient(int timeout)
        {
            Timeout = timeout;
        }

        #region Methods
        protected override WebRequest GetWebRequest(Uri uri)
        {
            WebRequest w = base.GetWebRequest(uri);
            w.Timeout = Timeout;
            ((HttpWebRequest)w).ReadWriteTimeout = Timeout;
            return w;
        }
        #endregion

        /// <summary>
        /// Web request timeout (in milliseconds)
        /// </summary>
        public int Timeout { get; set; }
    }
}

当我在那里时，我也抓住机会将默认的Timeout值降低到30秒，因为100秒对我来说似乎太多了。

如果你需要关于这个类或如何使用它的额外信息，请查看我在博客上写的这篇文章。

为了完整起见，下面是kisp移植到VB的解决方案(不能向注释中添加代码)

Namespace Utils

''' <summary>
''' Subclass of WebClient to provide access to the timeout property
''' </summary>
Public Class WebClient
    Inherits System.Net.WebClient

    Private _TimeoutMS As Integer = 0

    Public Sub New()
        MyBase.New()
    End Sub
    Public Sub New(ByVal TimeoutMS As Integer)
        MyBase.New()
        _TimeoutMS = TimeoutMS
    End Sub
    ''' <summary>
    ''' Set the web call timeout in Milliseconds
    ''' </summary>
    ''' <value></value>
    Public WriteOnly Property setTimeout() As Integer
        Set(ByVal value As Integer)
            _TimeoutMS = value
        End Set
    End Property


    Protected Overrides Function GetWebRequest(ByVal address As System.Uri) As System.Net.WebRequest
        Dim w As System.Net.WebRequest = MyBase.GetWebRequest(address)
        If _TimeoutMS <> 0 Then
            w.Timeout = _TimeoutMS
        End If
        Return w
    End Function

End Class

End Namespace

正如Sohnee所说，使用System.Net.HttpWebRequest并设置Timeout属性，而不是使用System.Net.WebClient。

然而，你不能设置一个无限的超时值(它不受支持，试图这样做会抛出一个argumentoutofranceexception)。

我建议首先执行HEAD HTTP请求，检查返回的Content-Length头值，以确定您正在下载的文件中的字节数，然后为后续的GET请求相应地设置超时值，或者简单地指定一个非常长的超时值，您不会期望超过这个超时值。