How to debug message handler code?

I am writing to seek help/advice, a better way to debug the following code below.  The code below, authenticates user details (username & password) but currently its not allowing me login.  I have tried debugging it but the application it not notify me with any error in the local patameter window.  
public class BasicAuthMessageHandler : DelegatingHandler { private const string BasicAuthResponseHeader = "WWW-Authenticate"; private const string BasicAuthResponseHeaderValue = "Basic"; //[Inject] //public iUser Repository { get; set; } // private readonly iUser Repository; private readonly iUser Repository = new User(); protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { AuthenticationHeaderValue authValue = request.Headers.Authorization; ...