How to write write a custom message handler?

protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

public class BasicAuthHandler
    {
        private const string BasicAuthResponseHeader = "WWW-Authenticate";
        private const string BasicAuthResponseHeaderValue = "Basic";

        private readonly iUser Repository = new User();

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            AuthenticationHeaderValue authValue = request.Headers.Authorization;


            if (authValue == null || authValue.Scheme != BasicAuthResponseHeaderValue)
            {
                return Unauthorized(request);
            }
            string[] credentials = Encoding.ASCII.GetString(Convert.FromBase64String(authValue.Parameter)).Split(new[] { ':' });
            if (credentials.Length != 2 || string.IsNullOrEmpty(credentials[0]) || string.IsNullOrEmpty(credentials[1]))
            {
                return Unauthorized(request);
            }
            api_login user = Repository.Validate2(credentials[0], credentials[1]);
            if (user == null)
            {
                return Unauthorized(request);
            }
            IPrincipal principal = new GenericPrincipal(new GenericIdentity(user.username, BasicAuthResponseHeaderValue), null);
            Thread.CurrentPrincipal = principal;
            HttpContext.Current.User = principal;

            return base.SendAsync(request, cancellationToken);
        }