MainActivity.cs

Button adalButton = FindViewById<Button>(Resource.Id.adalButton);
adalButton.Click += async (sender, args) =>
{
    var authContext = new AuthenticationContext(commonAuthority);
    if (authContext.TokenCache.ReadItems().Count() > 0)
    {
        authContext = new AuthenticationContext(authContext.TokenCache.ReadItems().First().Authority);
    }
    authResult = await authContext.AcquireTokenAsync(graphResourceUri, clientId, returnUri, new PlatformParameters(this));

    if ((authResult != null) && !string.IsNullOrEmpty(authResult.AccessToken))
    {
        Toast.MakeText(this, string.Format("Welcome {0} {1}.", authResult.UserInfo.GivenName,
                                           authResult.UserInfo.FamilyName), ToastLength.Long).Show();
        StartActivity(typeof(RememberListActivity));
    }
};