Headless authentication for API calls

I've had the same issue recently and have had to switch to using a device code. I've just dumped a copy of my code below.

There are a few steps required.

1 - Run it initially to get a device code. You will have to go to a verification url and enter a code. This then authenticates the app.

2 - Serialise the token cache from the authentication token.

Next time through, create a token cache from the saved state and you can log back in.

It all feels a bit hacky, but appears to work ok. I'm planning on creating a simple github project with all of this in, but am still debugging some issues at the moment

        public PowerBIClient(string clientId, string key, HttpClient httpClient, ILogger log, byte[] state)
        {
            Log = log;
            ClientId = clientId;
            _key = key;
            _httpClient = httpClient;
            var tokenCache = state == null ? new TokenCache() : new TokenCache(state);
            _authenticationContext = new AuthenticationContext(AuthorityUri, tokenCache);
        }

#pragma warning disable RCS1163 // Unused parameter.
        private async Task<string> GetAccessTokenAsync(CancellationToken ct)
#pragma warning restore RCS1163 // Unused parameter.
        {
            if (_accessToken == null)
            {
                if (_authenticationContext.TokenCache.Count > 0)
                {
                    try
                    {
                        var result = await _authenticationContext.AcquireTokenSilentAsync(ResourceUri, ClientId).ConfigureAwait(false);
                        _accessToken = result.AccessToken;
                        Log.Debug("Got cached access token until {expire} ({duration})",
                            result.ExpiresOn, result.ExpiresOn - DateTimeOffset.UtcNow);
                        return _accessToken;
                    }
                    catch (Exception ex)
                    {
                        Log.Warning("Failed to acquire token from cached {exception}", ex.Message);
                    }
                }
            }

            if (_accessToken == null)
            {
                var codeResult = await _authenticationContext.AcquireDeviceCodeAsync(ResourceUri, ClientId).ConfigureAwait(false);
                Log.Warning("Need authentication from {location} {code}", codeResult.VerificationUrl, codeResult.UserCode);
                var result = await _authenticationContext.AcquireTokenByDeviceCodeAsync(codeResult).ConfigureAwait(false);
                Log.Debug("Got new access token until {expire} ({duration})",
                            result.ExpiresOn, result.ExpiresOn - DateTimeOffset.UtcNow);

                _accessToken = result.AccessToken;
            }
            else
            {
                var result = await _authenticationContext.AcquireTokenSilentAsync(ResourceUri, ClientId).ConfigureAwait(false);
                Log.Debug("Got refreshed access token until {expire} ({duration})",
                            result.ExpiresOn, result.ExpiresOn - DateTimeOffset.UtcNow);
                _accessToken = result.AccessToken;
            }

            return _accessToken;
        }