install-package Microsoft.AspNetCore.Session -ver 2.0

install-package Microsoft.AspNetCore.Http.Extensions -ver 2.0

services.AddSession();

app.UseSession();

dotnet new razor -n SessionSample

public class Startup
{
  public void ConfigureServices(IServiceCollection services)
  {
    ...
    //添加session服务
    services.AddSession();
  }
  public void Configure(IApplicationBuilder app, IHostingEnvironment env)
  {
    ...
    //启用session
    app.UseSession();
    app.UseMvc();
  }
}

public class MyPage:PageModel
{
  protected bool IsLogin
  {
    get
    {
      string userId = null;
      //从sessin中获取UserId对应的用户信息来判断用户是否登陆
      if (HttpContext.Session.TryGetValue("UserId", out byte[] bytes))
      {
        userId = Encoding.UTF8.GetString(bytes);
      }
      return !string.IsNullOrWhiteSpace(userId);
    }
  }
}

dotnet new page -n Login

public class LoginModel : MyPage
{
  public void OnGet()
  {
    if(IsLogin)
    {
      //如果已经登陆就跳转到/Index页面
      RedirectToPage("/Index");
    }
  }
  public IActionResult OnPost()
  {
    //这里我们将UserId写入session中
    HttpContext.Session.SetString("UserId", Guid.NewGuid().ToString());
    return RedirectToPage("/Index");
  }
}

public class IndexModel : MyPage
{
  public IActionResult OnGet()
  {
    if (!IsLogin)
    {
      return RedirectToPage("/Login");
    }
    return Page();
  }
}