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)

<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.

Exception Condition
ArgumentNullException

hostNameOrAddress is null.

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.

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

DnsPermission

for accessing DNS information. Associated enumeration: PermissionState.Unrestricted

.NET Framework
Available since 2.0
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