HttpListener.BeginGetContext(AsyncCallback, Object) Method

Reference

Definition

Namespace:: System.Net

Assembly:: System.Net.HttpListener.dll

Assembly:: System.dll

Assembly:: netstandard.dll

Source:: HttpListener.Managed.cs

Source:: HttpListener.Managed.cs

Source:: HttpListener.Managed.cs

Important

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Begins asynchronously retrieving an incoming request.

public:
 IAsyncResult ^ BeginGetContext(AsyncCallback ^ callback, System::Object ^ state);

public IAsyncResult BeginGetContext (AsyncCallback? callback, object? state);

public IAsyncResult BeginGetContext (AsyncCallback callback, object state);

member this.BeginGetContext : AsyncCallback * obj -> IAsyncResult

Public Function BeginGetContext (callback As AsyncCallback, state As Object) As IAsyncResult

Parameters

callback: AsyncCallback

An AsyncCallback delegate that references the method to invoke when a client request is available.

state: Object

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

Returns

IAsyncResult

An IAsyncResult object that indicates the status of the asynchronous operation.

Exceptions

HttpListenerException

A Win32 function call failed. Check the exception's ErrorCode property to determine the cause of the exception.

InvalidOperationException

This object has not been started or is currently stopped.

ObjectDisposedException

This object is closed.

Examples

The following code example demonstrates using the BeginGetContext method to specify a callback method that will handle incoming client requests.


public static void NonblockingListener(string [] prefixes)
{
    HttpListener listener = new HttpListener();
    foreach (string s in prefixes)
    {
        listener.Prefixes.Add(s);
    }
    listener.Start();
    IAsyncResult result = listener.BeginGetContext(new AsyncCallback(ListenerCallback),listener);
    // Applications can do some work here while waiting for the
    // request. If no work can be done until you have processed a request,
    // use a wait handle to prevent this thread from terminating
    // while the asynchronous operation completes.
    Console.WriteLine("Waiting for request to be processed asyncronously.");
    result.AsyncWaitHandle.WaitOne();
    Console.WriteLine("Request processed asyncronously.");
    listener.Close();
}

Public Shared Sub NonblockingListener(ByVal prefixes As String())
    Dim listener As HttpListener = New HttpListener()

    For Each s As String In prefixes
        listener.Prefixes.Add(s)
    Next

    listener.Start()
    Dim result As IAsyncResult = listener.BeginGetContext(New AsyncCallback(AddressOf ListenerCallback), listener)
    ' Applications can do some work here while waiting for the 
    ' request. If no work can be done until you have processed a request,
    ' use a wait handle to prevent this thread from terminating
    ' while the asynchronous operation completes.
    Console.WriteLine("Waiting for request to be processed asyncronously.")
    result.AsyncWaitHandle.WaitOne()
    Console.WriteLine("Request processed asyncronously.")
    listener.Close()
End Sub

The following code example implements a callback method.

public static void ListenerCallback(IAsyncResult result)
{
    HttpListener listener = (HttpListener) result.AsyncState;
    // Call EndGetContext to complete the asynchronous operation.
    HttpListenerContext context = listener.EndGetContext(result);
    HttpListenerRequest request = context.Request;
    // Obtain a response object.
    HttpListenerResponse response = context.Response;
    // Construct a response.
    string responseString = "<HTML><BODY> Hello world!</BODY></HTML>";
    byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);
    // Get a response stream and write the response to it.
    response.ContentLength64 = buffer.Length;
    System.IO.Stream output = response.OutputStream;
    output.Write(buffer,0,buffer.Length);
    // You must close the output stream.
    output.Close();
}

Public Shared Sub ListenerCallback(ByVal result As IAsyncResult)
    Dim listener As HttpListener = CType(result.AsyncState, HttpListener)
    ' Call EndGetContext to complete the asynchronous operation.
    Dim context As HttpListenerContext = listener.EndGetContext(result)
    Dim request As HttpListenerRequest = context.Request
    ' Obtain a response object.
    Dim response As HttpListenerResponse = context.Response
    ' Construct a response.
    Dim responseString As String = "<HTML><BODY> Hello world!</BODY></HTML>"
    Dim buffer As Byte() = System.Text.Encoding.UTF8.GetBytes(responseString)
    ' Get a response stream and write the response to it.
    response.ContentLength64 = buffer.Length
    Dim output As System.IO.Stream = response.OutputStream
    output.Write(buffer, 0, buffer.Length)
    ' You must close the output stream.
    output.Close()
End Sub

Remarks

The BeginGetContext method begins an asynchronous (non-blocking) call to receive incoming client requests. Before calling this method, you must call the Start method and add at least one Uniform Resource Identifier (URI) prefix to listen for by adding the URI strings to the HttpListenerPrefixCollection returned by the Prefixes property.

The asynchronous operation must be completed by calling the EndGetContext method. Typically, the method is invoked by the callback delegate.

This method does not block while the operation completes. To get an incoming request and block until the operation completes, call the GetContext method.

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

Notes to Callers

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

Applies to

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