Offline Refresh Token Can't Generate

Hi,

I am using Google AdWords Version v201603 in that taking adwords account data Offline using Dot Net Web Service that time i trying to generate tokens offline using(delegate) and store that in Database and then accessing this tokens for data retrieve that time access token generating each time correctly but refresh token not generating.

 

follow is code i am using: 

         oAuth.OnOAuthTokensObtained += delegate(AdsOAuthProvider pp)

               

           {

                        OAuth2ProviderForApplications oAuth =(provider as OAuth2ProviderForApplications);

                       string refresht = oAuth.RefreshToken;

                       string accesst = oAuth.AccessToken;

                       Date updateo= oAuth.UpdatedOn;

                       int expire = oAuth.ExpiresIn;

            }

Its throwing following error:

             Failed to refresh access token.

              {

                  "error" : "invalid_grant"

              }

Please suggest me any another way or any correction/mistaken in existing code to generate offline Refresh Token. 

Thanks & Regards,

  Pravin Yadav

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