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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)

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public static IAsyncResult BeginGetHostEntry(
	string hostNameOrAddress,
	AsyncCallback requestCallback,
	Object stateObject


Type: System.String

The host name or IP address to resolve.

Type: System.AsyncCallback

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

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.


hostNameOrAddress is null.


The length of hostNameOrAddress is greater than 126 characters.


An error is encountered when resolving hostNameOrAddress.


hostNameOrAddress is an invalid IP address.


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 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.

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. 
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);

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

        new AsyncCallback(GetHostEntryCallback), ioContext);

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

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

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

// Determine the Internet Protocol(IP) addresses for this host.
static void DoResolveToAddresses(String* hostName)
    IPAddress * host[] = Dns::ResolveToAddresses(hostName);

    Console::Write("IPs for {0}: ", hostName);
    IEnumerator* ips = host->GetEnumerator();
    while (ips->MoveNext())
        Console::Write("{0} ", ips->Current);

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0