1. 添加Identity服务：在Startup.cs的ConfigureServices方法中添加Identity服务

services.AddDefaultIdentity()
   .AddEntityFrameworkStores();

2. 添加第三方登录服务：在Startup.cs的ConfigureServices方法中添加第三方登录服务

services.AddAuthentication()
   .AddGoogle(options =>
   {
       options.ClientId = Configuration["Authentication:Google:ClientId"];
       options.ClientSecret = Configuration["Authentication:Google:ClientSecret"];
   })
   .AddFacebook(options =>
   {
       options.AppId = Configuration["Authentication:Facebook:AppId"];
       options.AppSecret = Configuration["Authentication:Facebook:AppSecret"];
   });

3. 创建账户控制器AccountController：在Controllers文件夹下创建AccountController，包含方法Login、ExternalLoginCallback、ExternalLogin

public class AccountController : Controller
{
   private readonly SignInManager _signInManager;
   private readonly UserManager _userManager;

   public AccountController(SignInManager signInManager, UserManager userManager)
   {
       _signInManager = signInManager;
       _userManager = userManager;
   }

   [HttpPost]
   [AllowAnonymous]
   [ValidateAntiForgeryToken]
   public async Task Login(LoginViewModel model)
   {
       if (ModelState.IsValid)
       {
           var result = await _signInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, lockoutOnFailure: false);
           if (result.Succeeded)
           {
               return RedirectToAction("Index", "Home");
           }
           ModelState.AddModelError(string.Empty, "Invalid login attempt.");
           return View(model);
       }
       return View(model);
   }

   [HttpGet]
   [AllowAnonymous]
   public async Task ExternalLoginCallback(string returnUrl = null, string remoteError = null)
   {
       if (remoteError != null)
       {
           ModelState.AddModelError(string.Empty, $"Error from external provider: {remoteError}");
           return View("Login", new LoginViewModel());
       }
       var info = await _signInManager.GetExternalLoginInfoAsync();
       if (info == null)
       {
           ModelState.AddModelError(string.Empty, "Error loading external login information.");
           return View("Login", new LoginViewModel());
       }