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Dns.BeginGetHostEntry Method (String, AsyncCallback, Object)

Asynchronously resolves a host name or IP address to an IPHostEntry instance.

Namespace:  System.Net
Assembly:  System (in System.dll)

'Declaration
<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)> _
Public Shared Function BeginGetHostEntry ( _
	hostNameOrAddress As String, _
	requestCallback As AsyncCallback, _
	stateObject As Object _
) As IAsyncResult

Parameters

hostNameOrAddress
Type: System.String

The host name or 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

hostNameOrAddress is Nothing.

ArgumentOutOfRangeException

The length of hostNameOrAddress is greater than 255 characters.

SocketException

An error is encountered when resolving hostNameOrAddress.

ArgumentException

hostNameOrAddress is an invalid IP address.

The BeginGetHostEntry method queries a DNS server for the IP address that is associated with a host name or 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. 
  Dim Shared GetHostEntryFinished As ManualResetEvent = New ManualResetEvent(False)

  ' Define the state object for the callback.  
  ' Use hostName to correlate calls with the proper result. 
  Class ResolveState

      Dim hostName As String 
      Dim resolvedIPs As IPHostEntry

      Public Sub New(host As String)
          hostName = host
      End Sub 

      Public Property IPs AS IPHostEntry
          Get 
              Return resolvedIPs 
          End Get 
          Set
              resolvedIPs = value
          End Set 
      End Property 

      Public Property host As [String]
          Get 
              Return hostName
          End Get      
          Set
              hostName = value
          End Set     
      End Property 
  End Class 


  ' Record the IPs in the state object for later use. 
  Shared Sub GetHostEntryCallback(ar As IAsyncResult)

      Dim ioContext As ResolveState = ar.AsyncState

      ioContext.IPs = Dns.EndGetHostEntry(ar)
      GetHostEntryFinished.Set()
  End Sub        

  ' Determine the Internet Protocol (IP) addresses for  
  ' this host asynchronously. 
  Shared Sub DoGetHostEntryAsync(hostname As String)

      GetHostEntryFinished.Reset()
      Dim ioContext As ResolveState = New ResolveState(hostname)

      Dns.BeginGetHostEntry(ioContext.host,AddressOf GetHostEntryCallback, ioContext)

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

      Console.WriteLine("EndGetHostEntry(" + ioContext.host + ") returns:")

      Dim ip As IPAddress() = ioContext.IPs.AddressList
      Dim index As Integer 
      For index = 0 To ip.Length - 1
          Console.WriteLine(ip(index))
      Next index

  End Sub

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

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