1. 在Google API控制台中创建一个OAuth客户端ID，并设置重定向URI。保存客户端ID和客户端机密。

2. 安装'Google.Apis.Auth.AspNetCore”和'Google.Apis.Oauth2.v2”。

3. 在'Startup.cs”文件中配置Google授权服务。在'ConfigureServices”方法中添加以下代码：

services
.AddAuthentication(options =>
{
  options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
  options.DefaultChallengeScheme = GoogleOpenIdConnectDefaults.AuthenticationScheme;
})
.AddCookie()
.AddGoogleOpenIdConnect(options =>
{
  options.ClientId = Configuration["Google:ClientId"];
  options.ClientSecret = Configuration["Google:ClientSecret"];
  options.SaveTokens = true;
  options.Events.OnCreatingTicket = async context =>
  {
    // 获取刷新令牌并保存到身份验证令牌中。
    var refresh_token = context.Properties.GetTokenValue("refresh_token").ToString();
    context.Properties.StoreTokens(new[]
    {
      new AuthenticationToken { Name = "refresh_token", Value = refresh_token }
    });
  };
});

注意：应'ClientId”和'ClientSecret”替换为Google API控制台中创建的OAuth客户端ID和客户端机密。

4. 在'Startup.cs”文件的'Configure”方法中使用Google授权服务：

app.UseAuthentication();
app.UseAuthorization();

5. 在需要访问Google API的地方，通过以下代码获取刷新令牌：

string refresh_token = await HttpContext.GetTokenAsync("refresh_token");

注意：应该先通过上述步骤进行身份验证才能获取刷新令牌。

完整的'Startup.cs”文件代码示例：

using Google.Apis.Auth.AspNetCore;
using Google.Apis.Oauth2.v2;
using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace MyAppNamespace
{
  public class Startup
  {
    public Startup(IConfiguration configuration)
    {
      Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
      services
      .AddAuthentication(options =>
      {
        options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        options.DefaultChallengeScheme = GoogleOpenIdConnectDefaults.AuthenticationScheme;
      })
      .AddCookie()
      .AddGoogleOpenIdConnect(options =>
      {
        options.ClientId = Configuration["Google:ClientId"];
        options.ClientSecret = Configuration["Google:ClientSecret"];
        options.SaveTokens = true;