如何在.net google api v3中刷新token？

using Google.Apis.Auth.OAuth2;
using Google.Apis.Auth.OAuth2.Flows;
using Google.Apis.Auth.OAuth2.Responses;
using System.IO;

// 从存储中加载客户端凭据
var clientSecrets = GoogleClientSecrets.Load(stream);
var tokenFile = "tokens.json"; // 存储刷新令牌的文件路径

// 构建授权流程
var flow = new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer
{
    ClientSecrets = clientSecrets.Secrets
});

// 从文件中加载刷新令牌
var token = File.ReadAllText(tokenFile);

// 构建刷新令牌请求
var tokenRequest = new TokenRequest
{
    ClientId = clientSecrets.Secrets.ClientId,
    ClientSecret = clientSecrets.Secrets.ClientSecret,
    RefreshToken = token,
    GrantType = "refresh_token"
};

// 刷新令牌
var refreshToken = flow.RefreshTokenAsync("", tokenRequest, CancellationToken.None).Result;

// 将刷新后的令牌保存到文件中
File.WriteAllText(tokenFile, refreshToken.RefreshToken);