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Using GetRequestStreamAsync and GetResponseAsync in .Net 4.0 Portable Class Library

I've been building a small Portable Class Library and it makes use of the WebRequest class. I'm targeting .Net 4.0 (and above), Silverlight 5 and Windows App Store. I would target the phone as well but it doesn't support the TPL at the moment - WP8 is just around the corner:
I wanted to use async method GetRequestStreamAsync  & GetResponseAsync but these aren't supported in .Net 4.0 and when creating a Portable Class Library you only get the commonly supported methods across the configured platforms - so no support then or may be not...

Why not just create a couple of extension methods?

   1:  public static class HttpWebRequestExtensions
   2:  {
   3:      public static Task<Stream> GetRequestStreamAsync(this HttpWebRequest request)
   4:      {
   5:          var tcs = new TaskCompletionSource<Stream>();
   6:   
   7:          try
   8:          {
   9:              request.BeginGetRequestStream(iar =>
  10:              {
  11:                  try
  12:                  {
  13:                      var response = request.EndGetRequestStream(iar);
  14:                      tcs.SetResult(response);
  15:                  }
  16:                  catch (Exception exc)
  17:                  {
  18:                      tcs.SetException(exc);
  19:                  }
  20:              }, null);
  21:          }
  22:          catch (Exception exc)
  23:          {
  24:              tcs.SetException(exc);
  25:          }
  26:   
  27:          return tcs.Task;
  28:      }
  29:   
  30:      public static Task<HttpWebResponse> GetResponseAsync(this HttpWebRequest request)
  31:      {
  32:          var tcs = new TaskCompletionSource<HttpWebResponse>();
  33:   
  34:          try
  35:          {
  36:              request.BeginGetResponse(iar =>
  37:              {
  38:                  try
  39:                  {
  40:                      var response = (HttpWebResponse)request.EndGetResponse(iar);
  41:                      tcs.SetResult(response);
  42:                  }
  43:                  catch (Exception exc)
  44:                  {
  45:                      tcs.SetException(exc);
  46:                  }
  47:              }, null);
  48:          }
  49:          catch (Exception exc)
  50:          {
  51:              tcs.SetException(exc);
  52:          }
  53:   
  54:          return tcs.Task;
  55:      }
  56:  }

When used with the AsyncBridge NuGet package to provide async support in .Net 4.0 \ Silverlight 5 you get exactly the same syntax as if this was a standard .Net 4.5 library:

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