AcquireTokenSilentAsync fails to authenticate user - asp.net-mvc

Question

I am trying to get the silent token request to initialize the ConfidentialClientApp object as in the 'Microsoft Graph SDK ASPNET Connect' project and outlined in Add sign-in with Microsoft to an ASP.NET web app
With my code mirroring the samples above, I expect that my call will return a successful result with an access to.
var result = await cca.AcquireTokenSilentAsync(graphScopes, cca.Users.First());
return result.AccessToken;
However, I get an error where it says the user needs to be authenticated. I am not sure what I am missing from the examples that make this work in them.

Answer 1

You can only acquire the token silently if there is already a cached token for that user that includes the scopes you're requesting (it can have more, but it needs to have at least what you've asked for).
This is why AcquireTokenSilentAsync should always be wrapped in a Try/Catch block. If it fails to find a matching token, you need to launch an interactive flow. Here is an example from the MSAL Wiki:
AuthenticationResult result = null;
try
{
result = await app.AcquireTokenSilentAsync(scopes, app.Users.FirstOrDefault());
}
catch (MsalUiRequiredException ex)
{
// A MsalUiRequiredException happened on AcquireTokenSilentAsync.
// This indicates you need to call AcquireTokenAsync to acquire a token
System.Diagnostics.Debug.WriteLine($"MsalUiRequiredException: {ex.Message}");
try
{
result = await app.AcquireTokenAsync(scopes);
}
catch (MsalException msalex)
{
ResultText.Text = $"Error Acquiring Token:{System.Environment.NewLine}{msalex}";
}
}
catch (Exception ex)
{
ResultText.Text = $"Error Acquiring Token Silently:{System.Environment.NewLine}{ex}";
return;
}
if (result != null)
{
string accessToken = result.AccessToken;
// Use the token
}