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Dns.BeginGetHostEntry Method (IPAddress, AsyncCallback, Object)

Asynchronously resolves an IP address to an IPHostEntry instance.

Namespace:  System.Net
Assembly:  System (in System.dll)
[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public static IAsyncResult BeginGetHostEntry(
	IPAddress address,
	AsyncCallback requestCallback,
	Object stateObject
)

Parameters

address
Type: System.Net.IPAddress

The IP address to resolve.

requestCallback
Type: System.AsyncCallback

An AsyncCallback delegate that references the method to invoke when the operation is complete.

stateObject
Type: System.Object

A user-defined object that contains information about the operation. This object is passed to the requestCallback delegate when the operation is complete.

Return Value

Type: System.IAsyncResult
An IAsyncResult instance that references the asynchronous request.
ExceptionCondition
ArgumentNullException

address is null.

SocketException

An error is encountered when resolving address.

ArgumentException

address is an invalid IP address.

The BeginGetHostEntry method asynchronously queries a DNS server for the IP addresses and aliases associated with an IP address.

Note   This member emits trace information when you enable network tracing in your application. For more information, see Network Tracing in the .NET Framework.

The asynchronous BeginGetHostEntry operation must be completed by calling the EndGetHostEntry method. Typically, the method is invoked by the requestCallback delegate.

This method does not block until the operation is complete. To block until the operation is complete, use the GetHostEntry method.

For detailed information about using the asynchronous programming model, see Calling Synchronous Methods Asynchronously

NoteNote

The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The following code example uses the BeginGetHostEntry method to resolve an IP address to an IPHostEntry instance.

      // Signals when the resolve has finished. 
      public static ManualResetEvent GetHostEntryFinished = 
          new ManualResetEvent(false);

      // Define the state object for the callback.  
      // Use hostName to correlate calls with the proper result. 
      public class ResolveState
      {
          string hostName;
          IPHostEntry resolvedIPs;

          public ResolveState(string host)
          {
              hostName = host;
          }

          public IPHostEntry IPs
          {
              get { return resolvedIPs; } 
              set {resolvedIPs = value;}}
          public string host {get {return hostName;} 
              set {hostName = value;}}
      }

      // Record the IPs in the state object for later use. 
      public static void GetHostEntryCallback(IAsyncResult ar)
      {
          ResolveState ioContext = (ResolveState)ar.AsyncState;

          ioContext.IPs = Dns.EndGetHostEntry(ar);
          GetHostEntryFinished.Set();
      }       

      // Determine the Internet Protocol (IP) addresses for  
      // this host asynchronously. 
      public static void DoGetHostEntryAsync(string hostname)
      {
          GetHostEntryFinished.Reset();
          ResolveState ioContext= new ResolveState(hostname);

          Dns.BeginGetHostEntry(ioContext.host, 
              new AsyncCallback(GetHostEntryCallback), ioContext);

          // Wait here until the resolve completes (the callback  
          // calls .Set())
          GetHostEntryFinished.WaitOne();

          Console.WriteLine("EndGetHostEntry({0}) returns:", ioContext.host);

          foreach (IPAddress ip in ioContext.IPs.AddressList)
          {
              Console.WriteLine("    {0}", ip);
          }

      }

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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