The reason you're receiving this error is because the call acquiretokensilentasync is EXPECTED to throw that error when the cache is empty. This call is meant to be caught in a try catch. If it does throw this error it should call the acquiretokenasync call.
In addition to that it looks like you're trying to utilize the client credential flow with the acquiretokensilentasync call, which is not the right method to use per the ADAL wiki docs.
See here on how to do this properly: https://github.com/AzureAD/azure-activedirectory-library-for-dotnet/wiki/Client-credential-flows
It looks like you're using an app id and secret, the method in particular on how to do this per the doc linked above is :
AuthenticationContext authenticationContext = new AuthenticationContext("https://login.microsoftonline.com/<tenantId>");
AuthenticationResult result = await authenticationContext.AcquireTokenAsync("https://resourceUrl", clientCredential);
More documentation specifically for the acquiretokensilentasync call can be found here : https://github.com/AzureAD/azure-activedirectory-library-for-dotnet/wiki/AcquireTokenSilentAsync-using-a-cached-token
From the doc above :
Recommended pattern to acquire a token
Now that you have seen both AcquireTokenAsync, AcquireTokenSilentAsync, it's the right moment to present the recommended usage pattern for calling these methods. The idea is that you want to minimize the number of signings for the user, and therefore you'd want to:
first try to acquire a token silently, and if this call fails you try to get one interactively. Note that, AcquireTokenSilent does not need to be called in the Client credentials flow (when the application acquires token without a user, but in its own name)
Note that AcquireTokenSilent can fail for several reasons, such as the cache does not contain a token for the user, or the token has expired and cannot be refreshed. For these reasons, a call to AcquireTokenAsync will usually get a token. But there are also issues such as network problems, STS unavailability, etc., which won't be directly solvable. You will see them in more details in the article about best practices for Handling errors.
In addition to that, it looks like you're using the ADAL Library, I suggest to move over to the MSAL library since Microsoft is slowly moving towards utilizing the MSAL libraries and will at some point in the future (maybe far future) move off of ADAL/V1.0 endpoint. There are no current hard dates for this however. The doc on moving over from ADAL to MSAL can be found here :
https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/wiki/Adal-to-Msal
I changed the controller the way you wanted and committed it.You can see the change in the GitHub link. (TodoListWebApp/Controllers/TodoListController.cs). That's a method I've tried before. When I use it like this the response turns false when I send a request to the service (response.IsSuccessStatusCode = false). So I still haven't gotten the answer from the get method of Web API.
I think ClaimsPrincipal section in TodoListService/Controllers/TodoListController.cs is the reason why the false result is returned by the service.
the result is false because the ClaimsPrincipal.Current.FindFirst("schemas.microsoft.com/identity/claims/scope") is null in the service. (TodoListService/Controllers/TodoListController.cs)