我想检查主机是否可以解析,但我不想等待很长时间,所以我想设置一个超时.
我试过了
public static bool ResolveTest(string hostNameOrAddress, int millisecond_time_out)
{
var timeoutSpan = TimeSpan.FromMilliseconds(millisecond_time_out);
var result = Dns.BeginGetHostEntry(hostNameOrAddress, null, null);
var success = result.AsyncWaitHandle.WaitOne(timeoutSpan);
if (!success) {
//throw new Exception("Failed to resolve the domain.");
return false;
}
return true;
}
但是现在这个工作正常,当它是一个错误的主机时,它也可以返回true.那么如何设置DnsResolve的超时?
最佳答案 原始答案
看看更新的答案
无法实际取消Dns.BeginGetHostEntry(),但您可以尝试实现可在设定时间后取消的内容.这有点hackish但可以按照你的要求做.
然而,DNS查找(通常)非常快,并且在测试中通常在10ms内解析.
这是一个例子.基本上基于任务的方法允许您执行一个返回bool的任务,无论结果是否已完成.
public static Task<bool> Resolve(string hostNameOrAddress, int millisecond_time_out)
{
return Task.Run(async () =>
{
bool completed = false;
var asCallBack = new AsyncCallback(ar =>
{
ResolveState context = (ResolveState)ar.AsyncState;
if (context.Result == ResolveType.Pending)
{
try
{
var ipList = Dns.EndGetHostEntry(ar);
if (ipList == null || ipList.AddressList == null || ipList.AddressList.Length == 0)
context.Result = ResolveType.InvalidHost;
else
context.Result = ResolveType.Completed;
}
catch
{
context.Result = ResolveType.InvalidHost;
}
}
completed = true;
});
ResolveState ioContext = new ResolveState(hostNameOrAddress);
var result = Dns.BeginGetHostEntry(ioContext.HostName, asCallBack, ioContext);
int miliCount = 0;
while (!completed)
{
miliCount++;
if (miliCount >= millisecond_time_out)
{
result.AsyncWaitHandle.Close();
result = null;
ioContext.Result = ResolveType.Timeout;
break;
}
await Task.Delay(1);
}
Console.WriteLine($"The result of {ioContext.HostName} is {ioContext.Result}");
return ioContext.Result == ResolveType.Completed;
});
}
public class ResolveState
{
public ResolveState(string hostName)
{
if (string.IsNullOrWhiteSpace(hostName))
throw new ArgumentNullException(nameof(hostName));
_hostName = hostName;
}
readonly string _hostName;
public ResolveType Result { get; set; } = ResolveType.Pending;
public string HostName => _hostName;
}
public enum ResolveType
{
Pending,
Completed,
InvalidHost,
Timeout
}
然后可以将该方法称为
public static async void RunTest()
{
await Resolve("asdfasdfasdfasdfasdfa.ca", 60);
await Resolve("google.com", 60);
}
执行Resolve时,它会创建一个内部AsyncCallBack委托.然后将该委托传递给Dns.BeginGetHostEntry()方法.另外,我们传入ResolveState的状态对象.该状态对象可用于管理上下文之间的状态.因此,当执行AsyncCallBack时,我们可以设置ResolveState的Result.接下来,设置completed标志以指示回调委托已完成.
最后(这是我不喜欢的部分)然后我们有一个循环,每1ms执行一次,如果超时到期,则将我们的状态对象的Result设置为Timeout.
这又是一种轻微的hackish,但确实产生了预期的效果.现在这也是异步,所以你可以使用async&等待功能.
更新的答案
在第一个答案后,我不喜欢结果,并审查了您的初始问题.你在正确的轨道上,除了我们需要像你一样检查完整的旗帜.但是,如果在WaitOne调用之后结果未完成,则成功将返回false.如果完成则成功将成立(即使没有解析IP地址),您需要使用Dns.EndGetHostEntry检查结果.
修订代码:
public static bool ResolveTest(string hostNameOrAddress, int millisecond_time_out)
{
ResolveState ioContext = new ResolveState(hostNameOrAddress);
var result = Dns.BeginGetHostEntry(ioContext.HostName, null, null);
var success = result.AsyncWaitHandle.WaitOne(TimeSpan.FromMilliseconds(millisecond_time_out), true);
if (!success)
{
ioContext.Result = ResolveType.Timeout;
}
else
{
try
{
var ipList = Dns.EndGetHostEntry(result);
if (ipList == null || ipList.AddressList == null || ipList.AddressList.Length == 0)
ioContext.Result = ResolveType.InvalidHost;
else
ioContext.Result = ResolveType.Completed;
}
catch
{
ioContext.Result = ResolveType.InvalidHost;
}
}
Console.WriteLine($"The result of ResolveTest for {ioContext.HostName} is {ioContext.Result}");
return ioContext.Result == ResolveType.Completed;
}
这是完整的要点https://gist.github.com/Nico-VanHaaster/35342c1386de752674d0c37ceaa65e00.