R – Convert Web Form to Web Content Form after the fact. (.net)

I would suggest exposing a property or method on the child page which hands the master page the tags to display. For example

partial class myPage : IMyTaggablePage
{
    // the following is an interface member
    public List<string> GetTags()
    {
        return this.Taglist; // assuming taglist was populated somewhere on this page.
    }
}

Then on your master page you can write something like:

if (this.Page is IMyTaggablePage)
    var tags = (Page as IMyTaggablePage).GetTags();

The content type is a header of the content, not of the request, which is why this is failing. AddWithoutValidation as suggested by Robert Levy may work, but you can also set the content type when creating the request content itself (note that the code snippet adds application/json in two places-for Accept and Content-Type headers):

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://example.com/");
client.DefaultRequestHeaders
      .Accept
      .Add(new MediaTypeWithQualityHeaderValue("application/json"));//ACCEPT header

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "relativeAddress");
request.Content = new StringContent("{\"name\":\"John Doe\",\"age\":33}",
                                    Encoding.UTF8, 
                                    "application/json");//CONTENT-TYPE header

client.SendAsync(request)
      .ContinueWith(responseTask =>
      {
          Console.WriteLine("Response: {0}", responseTask.Result);
      });