AuthenticatingHandler.cs

public class AuthenticatingHandler<T> : DelegatingHandler where T : ISecurityTokenAccessor
    {
        private readonly Policy<HttpResponseMessage> _policy;
        private readonly T _securityTokenAccessor;
        private IAccessToken _accessToken;
        private AuthenticationHeaderValue _authenticationHeader;

        public AuthenticatingHandler(T securityTokenAccessor)
        {
            _securityTokenAccessor = securityTokenAccessor;

            // Create a policy that tries to renew the access token if a 403 Unauthorized is received.
            _policy = Policy.HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized).RetryAsync(1, async (response, attemp) =>
            {
                await AuthenticateAsync();
            });
        }

        protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            // Request an access token if we don't have one yet or if it has expired.
            if (!_securityTokenAccessor.ValidateAccessToken(_accessToken))
            {
                await AuthenticateAsync();
            }

            // Try to perform the request, re-authenticating gracefully if the call fails due to an expired or revoked access token.
            var result = await _policy.ExecuteAndCaptureAsync(() =>
            {
                request.Headers.Authorization = _authenticationHeader;
                return base.SendAsync(request, cancellationToken);
            });

            return result.Result;
        }

        private async Task AuthenticateAsync()
        {
            _accessToken = await _securityTokenAccessor.RenewAccessTokenAsync();
            _authenticationHeader = new AuthenticationHeaderValue(_accessToken.TokenType, _accessToken.Token);
        }
    }