InvalidAuthenticationToken when trying to invoke Microsoft Graph client - microsoft-graph-api

Question

I'm following a sample provided at the Microsoft Graph repository on github. In it, it describes using the DelegateAuthenticationProvider which needs an access token.
Marc LaFleur's primer on auth flow specified a way to get the access token which I combined with guidance on making service-service authentication work.
Even though I receive what appears to be a valid access token, when I issue the command via the graph client, I get a ServiceException from Microsoft Graph stating,
"{
Code: InvalidAuthenticationToken
Message: Access token validation failure.
Inner error
}"
I haven't seen any other inner error.
What am I doing incorrectly?
This is the code I've come up:
var clientId = "[app-guid]";
var clientSecret = "[secret-from-app-dashboard]";
var resource = "[app-guid]";
var baseUrl = "https://login.microsoftonline.com";
var loginUrl = "/[tenant guid]/oauth2/token";
using (var client = new HttpClient())
{
client.BaseAddress = new Uri(baseUrl);
var content = new FormUrlEncodedContent(new[]
{
new KeyValuePair<string, string>("grant_type", "client_credentials"),
new KeyValuePair<string, string>("client_id", clientId),
new KeyValuePair<string, string>("client_secret", clientSecret),
new KeyValuePair<string, string>("resource", resource)
});
var result = client.PostAsync(loginUrl, content).Result;
string resultContent = result.Content.ReadAsStringAsync().Result;
var json = JObject.Parse(resultContent);
accessToken = (string)json["access_token"];
}
var header = new AuthenticationHeaderValue("Bearer", accessToken);
var graphserviceClient = new GraphServiceClient(
new DelegateAuthenticationProvider(
(requestMessage) =>
{
requestMessage.Headers.Authorization = header;
return Task.FromResult(0);
}));
try
{
var users = await graphserviceClient.Users.Request().GetAsync();
var user = users.First();
}
catch (Microsoft.Graph.ServiceException servex)
{
Console.WriteLine(servex);
}

Answer 1

You set your resource to point to your website, but it should point to the external web resource you wish to access. Try this:
var resource = "https://graph.microsoft.com/";