gistfile1.cs

 public class RequestPreprocessor : DelegatingHandler {
        public RequestPreprocessor(HttpMessageHandler handler) {
            this.InnerHandler = handler;
        }

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
        {

            request.Content.LoadIntoBufferAsync().RunSynchronously();
            // Do your stuff!

            return base.SendAsync(request, cancellationToken);
        }
    }


var client = new HttpClient(new RequestPreprocessor(new HttpClientHandler()));
client.PostAsync(new Uri("http://example.org"), new StringContent("Hello World"));