I wrote this helper class to assist with processing an Http Request via Silverlight 2. Since we can't provide a custom class by deriving from BrowserHttpWebRequest because it is internal I wrote this helper that will create and send the request and raise an event when the request completes. I am using it in my current game to send commands to the server and process the responses.

Edit: I revised the class slightly based on finding from Mike Briseno:

Edit: Some folks have experienced issues with this class after installing Silverlight 2 Beta 2. Be sure to check your domain access policy xml file for the new changes in beta 2 but if you experience issues feel free to contact me and we can try and figure them out.

Edit: Added HttpUtility.UrlEncode to the post values before writing them to the stream. Thanks for the suggestion Stefan!

using System;
using System.Collections.Generic;
using System.IO;
using System.Windows;
using System.Windows.Browser;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Net;

namespace Lionsguard
{
    public class HttpHelper
    {
        private HttpWebRequest Request { get; set; }
        public Dictionary<string, string> PostValues { get; private set; }

        public event HttpResponseCompleteEventHandler ResponseComplete;
        private void OnResponseComplete(HttpResponseCompleteEventArgs e)
        {
            if (this.ResponseComplete != null)
            {
                this.ResponseComplete(e);
            }
        }

        public HttpHelper(Uri requestUri, string method, params KeyValuePair<string, string>[] postValues)
        {
            this.Request = (HttpWebRequest)WebRequest.Create(requestUri);
            this.Request.ContentType = "application/x-www-form-urlencoded";
            this.Request.Method = method;
            this.PostValues = new Dictionary<string, string>();
            if (postValues != null && postValues.Length > 0)
            {
                foreach (var item in postValues)
                {
                    this.PostValues.Add(item.Key, item.Value);
                }
            }
        }

        public void Execute()
        {
            this.Request.BeginGetRequestStream(new AsyncCallback(HttpHelper.BeginRequest), this);
        }

        private static void BeginRequest(IAsyncResult ar)
        {
            HttpHelper helper = ar.AsyncState as HttpHelper;
            if (helper != null)
            {
                if (helper.PostValues.Count > 0)
                {
                    using (StreamWriter writer = new StreamWriter(helper.Request.EndGetRequestStream(ar)))
                    {
                        foreach (var item in helper.PostValues)
                        {
                            writer.Write("{0}={1}&", item.Key, HttpUtility.UrlEncode(item.Value));
                        }
                    }
                }
                helper.Request.BeginGetResponse(new AsyncCallback(HttpHelper.BeginResponse), helper);
            }
        }

        private static void BeginResponse(IAsyncResult ar)
        {
            HttpHelper helper = ar.AsyncState as HttpHelper;
            if (helper != null)
            {
                HttpWebResponse response = (HttpWebResponse)helper.Request.EndGetResponse(ar);
                if (response != null)
                {
                    Stream stream = response.GetResponseStream();
                    if (stream != null)
                    {
                        using (StreamReader reader = new StreamReader(stream))
                        {
                            helper.OnResponseComplete(new HttpResponseCompleteEventArgs(reader.ReadToEnd()));
                        }
                    }
                }
            }
        }
    }

    public delegate void HttpResponseCompleteEventHandler(HttpResponseCompleteEventArgs e);
    public class HttpResponseCompleteEventArgs : EventArgs
    {
        public string Response { get; set; }

        public HttpResponseCompleteEventArgs(string response)
        {
            this.Response = response;
        }
    }
}

And this is how I am using it in my app:

        private void ProcessCommand(short cmd, string msg)
        {
            App app = App.Current as App;
            HttpHelper helper = new HttpHelper(app.ServerUri, "POST", 
                new KeyValuePair<string, string>("authKey", app.AuthKey),
                new KeyValuePair<string, string>("cmd", cmd.ToString()),
                new KeyValuePair<string, string>("msg", msg));
            helper.ResponseComplete += new HttpResponseCompleteEventHandler(this.CommandComplete);
            helper.Execute();

        }

        private void CommandComplete(HttpResponseCompleteEventArgs e)
        {
            txtAlert.Text = e.Response;
        }

For the VB Developers out there David Thiessen has converted my code to VB:

' Usage....

'Private Sub ProcessCommand(ByVal cmd As Short, ByVal msg As String)
'    Dim app As App = TryCast(App.Current, App)
'    Dim helper As New HttpHelper(app.ServerUri, "POST", New KeyValuePair(Of String, String)("authKey", app.AuthKey), New KeyValuePair(Of String, String)("cmd", cmd.ToString()), New KeyValuePair(Of String, String)("msg", msg))
'    AddHandler helper.ResponseComplete, AddressOf CommandComplete
'    helper.Execute()

'End Sub

'Private Sub CommandComplete(ByVal e As HttpResponseCompleteEventArgs)
'    txtAlert.Text = e.Response
'End Sub

''' <summary>
''' Converted C# code from http://www.cameronalbert.com/post/2008/03/HttpWebRequest-Helper-for-Silverlight-2.aspx
''' </summary>
''' <remarks></remarks>
Public Class HttpHelper

    Private Property Request() As HttpWebRequest
        Get
            Return _Request
        End Get
        Set(ByVal value As HttpWebRequest)
            _Request = value
        End Set
    End Property
    Private _Request As HttpWebRequest

    Public Property PostValues() As Dictionary(Of String, String)
        Get
            Return _PostValues
        End Get
        Private Set(ByVal value As Dictionary(Of String, String))
            _PostValues = value
        End Set
    End Property
    Private _PostValues As Dictionary(Of String, String)

    Public Event ResponseComplete As HttpResponseCompleteEventHandler
    Private Sub OnResponseComplete(ByVal e As HttpResponseCompleteEventArgs)
        RaiseEvent ResponseComplete(e)
    End Sub

    Public Sub New(ByVal requestUri As Uri, ByVal method As String, ByVal ParamArray postValues As KeyValuePair(Of String, String)())
        Me.Request = DirectCast(WebRequest.Create(requestUri), HttpWebRequest)
        Me.Request.ContentType = "application/x-www-form-urlencoded"
        Me.Request.Method = method
        Me.PostValues = New Dictionary(Of String, String)()
        For Each item In postValues
            Me.PostValues.Add(item.Key, item.Value)
        Next
    End Sub

    Public Sub Execute()
        Me.Request.BeginGetRequestStream(New AsyncCallback(AddressOf HttpHelper.BeginRequest), Me)
    End Sub

    Private Shared Sub BeginRequest(ByVal ar As IAsyncResult)
        Dim helper As HttpHelper = TryCast(ar.AsyncState, HttpHelper)
        If helper IsNot Nothing Then
            Using writer As New StreamWriter(helper.Request.EndGetRequestStream(ar))
                For Each item In helper.PostValues
                    writer.Write("{0}={1}&", item.Key, HttpUtility.UrlEncode(item.Value))
                Next
            End Using
            helper.Request.BeginGetResponse(New AsyncCallback(AddressOf HttpHelper.BeginResponse), helper)
        End If
    End Sub

    Private Shared Sub BeginResponse(ByVal ar As IAsyncResult)
        Dim helper As HttpHelper = TryCast(ar.AsyncState, HttpHelper)
        If helper IsNot Nothing Then
            Dim response As HttpWebResponse = DirectCast(helper.Request.EndGetResponse(ar), HttpWebResponse)
            If response IsNot Nothing Then
                Dim stream As Stream = response.GetResponseStream()
                If stream IsNot Nothing Then
                    Using reader As New StreamReader(stream)
                        helper.OnResponseComplete(New HttpResponseCompleteEventArgs(reader.ReadToEnd()))
                    End Using
                End If
            End If
        End If
    End Sub
End Class

Public Delegate Sub HttpResponseCompleteEventHandler(ByVal e As HttpResponseCompleteEventArgs)

Public Class HttpResponseCompleteEventArgs
    Inherits EventArgs

    Public Property Response() As String
        Get
            Return _Response
        End Get
        Set(ByVal value As String)
            _Response = value
        End Set
    End Property
    Private _Response As String

    Public Sub New(ByVal response As String)
        Me.Response = response
    End Sub

End Class