/// <summary>
/// Handles async host lookup completions.
/// </summary>
/// <param name="ar"></param>
private static void OnGetHostByName(IAsyncResult ar)
{
// Queue operations that completed synchronously so that they'll
// be dispatched on a different thread.
ar = QueuedAsyncResult.QueueSynchronous(ar, onGetHostByName);
if (ar == null)
{
return;
}
// Handle the completion
EnhancedDnsResult dnsAR = (EnhancedDnsResult)ar.AsyncState;
try
{
dnsAR.HostEntry = Dns.EndGetHostEntry(ar);
dnsAR.Notify();
}
catch (Exception e)
{
dnsAR.Notify(e);
}
}
//---------------------------------------------------------------------
// Implementation Note:
//
// I'm going to perform the cache lookup in BeginGetHostByName() and
// if I find the entry and it hasn't expired, I'll set the HostEntry
// field in the async result as well as setting Cached=true. EndGetHostByName()
// will be able to distinguish between a cached and non-cached situations
// by looking at the Cached field.
/// <summary>
/// Initiates an asynchronous operation to perform an host name lookup.
/// </summary>
/// <param name="host">The host name.</param>
/// <param name="callback">The operation completion callback (or <c>null</c>).</param>
/// <param name="state">Application state.</param>
/// <returns>The asynchronous result used to track the operation.</returns>
public static IAsyncResult BeginGetHostByName(string host, AsyncCallback callback, object state)
{
var dnsAR = new EnhancedDnsResult("SafeDns", callback, state);
DnsRecord dnsRec = null;
dnsAR.Host = host.ToUpper(); // Doing this once
if (enableCache)
{
lock (syncLock)
{
dnsRec = (DnsRecord)cache[dnsAR.Host];
if (dnsRec != null)
{
if (dnsRec.TTD <= SysTime.Now)
{
// Remove expired entries
cache.Remove(dnsRec.Host);
dnsRec = null;
}
}
}
}
if (dnsRec == null)
{
Dns.BeginGetHostEntry(host, onGetHostByName, dnsAR);
dnsAR.Started();
}
else
{
dnsAR.HostEntry = dnsRec.Entry;
dnsAR.Cached = true;
dnsAR.Started();
dnsAR.Notify(); // Indicate completion
}
return(dnsAR);
}
