OAuth 2 Google API 刷新令牌为空

我正在开发一个 Asp.NET MVC5 应用程序按照这个谷歌示例代码 https://developers.google.com/api-client-library/dotnet/guide/aaa_oauth.

我希望应用程序经过身份验证并由用户创建访问令牌（配置阶段），稍后我需要能够使用刷新令牌调用 Google API'（在我的情况下是目录 API）（无需用户干预，例如“离线”）。

控制器：

public async Task<ActionResult> IndexAsync(CancellationToken cancellationToken)
{
    var result = await new AuthorizationCodeMvcApp(this, new AppFlowMetadata()).
                        AuthorizeAsync(cancellationToken);

        if (result.Credential != null)
        {
          var service = new Google.Apis.Admin.Directory.directory_v1.DirectoryService(new BaseClientService.Initializer
                        {
                            HttpClientInitializer = result.Credential,
                            ApplicationName = "My Application"
                        });
                      return View();
        }
        else
        {
             return new RedirectResult(result.RedirectUri);
        }
}         

流元数据实现。 （我正在使用稍微修改过的 FiledataStore (GoogleFileDataStore)）

public class AppFlowMetadata : FlowMetadata
    {
        //Move to settings
        static readonly string clientId = "xxxccnvofsdfsfoj.apps.googleusercontent.com";
        static readonly string clientsecret = "xxxxxxxxxxLvtC6Qbqpp4x_";
        static readonly string datastore = "AdminSDK.Api.Auth.Store";


         private static readonly IAuthorizationCodeFlow flow =
            new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer
                {
                    ClientSecrets = new ClientSecrets
                    {
                        ClientId = clientId,
                        ClientSecret = clientsecret
                    },
                    Scopes = new[] { DirectoryService.Scope.AdminDirectoryUser, DirectoryService.Scope.AdminDirectoryUserAliasReadonly },
                    DataStore = new GoogleFileDataStore(datastore)
                });


        public override string GetUserId(Controller controller)
        {           
            return ConfigHelper.UserID.ToString();
        }

        public override IAuthorizationCodeFlow Flow
        {
            get { return flow; }
        }
    }

当我调用 IndexAsync 控制器时，由于用户之前没有访问令牌，因此它会在用户登录 Google 帐户后创建一个新的访问令牌。
这是一个示例访问令牌，

{"access_token":"ya29.5gBOszGO-oYJJt4YZfF6FeaZth1f69_.....","token_type":"Bearer","expires_in":3600,"Issued":"2014-12-24T16:02:32.014+05:30"}

问题1：为什么这里没有存储Refreshtoken？如何检索刷新令牌？ 问题2：如果获得了refreshtoken，我应该如何修改代码来创建新的access token并调用API（当accesstoken过期时）？ [我提到这个问题 https://stackoverflow.com/questions/25144667/using-the-youtube-v3-data-api-for-net-how-is-it-possible-to-get-a-refresh-toke，但对于缺少刷新令牌没有正确的答案。

找到了Google API离线访问获取refresh token并使用refresh token创建新的accesstoken的解决方案，

问题1：为什么没有存储刷新令牌？如何检索刷新令牌？

我必须在请求中将 access_type 设置为离线（默认情况下为在线）。正如这里提到的 https://developers.google.com/accounts/docs/OAuth2WebServer#overview

我必须为 GoogleAuthorizationCodeFlow 类编写自己的实现。谢谢这个帖子。 https://stackoverflow.com/questions/22094599/google-analytics-oauth-with-accesstype-offline-in-c-sharp

 public class ForceOfflineGoogleAuthorizationCodeFlow : GoogleAuthorizationCodeFlow
    {
        public ForceOfflineGoogleAuthorizationCodeFlow(GoogleAuthorizationCodeFlow.Initializer initializer) : base(initializer) { }

        public override AuthorizationCodeRequestUrl CreateAuthorizationCodeRequest(string redirectUri)
        {
            return new GoogleAuthorizationCodeRequestUrl(new Uri(AuthorizationServerUrl))
            {
                ClientId = ClientSecrets.ClientId,
                Scope = string.Join(" ", Scopes),
                RedirectUri = redirectUri,
                AccessType = "offline",
                ApprovalPrompt = "force"
            };
        }
    };

问题2：..我应该如何修改代码来创建新的访问令牌并调用API？

    //try to get results
    var result = await new AuthorizationCodeMvcApp(this, new AppFlowMetadata()).
                    AuthorizeAsync(cancellationToken);


    //// This bit checks if the token is out of date, 
    //// and refreshes the access token using the refresh token.
    if (result.Credential.Token.IsExpired(SystemClock.Default))
    {
           Google.Apis.Auth.OAuth2.Responses.TokenResponse token = new Google.Apis.Auth.OAuth2.Responses.TokenResponse();
           //If the token is expired recreate the token
           token = await result.Credential.Flow.RefreshTokenAsync(ConfigHelper.UserID.ToString(), result.Credential.Token.RefreshToken, CancellationToken.None);

            //Get the authorization details back
            result = await new AuthorizationCodeMvcApp(this, new AppFlowMetadata()).AuthorizeAsync(cancellationToken);
     }

这对我有用！希望这会帮助其他人......