builder.Services.AddDistributedMemoryCache();

        builder.Services.AddSession(options =>
        {
            options.IdleTimeout = TimeSpan.FromMinutes(30);
            options.Cookie.HttpOnly = true;
            options.Cookie.IsEssential = true;
        });

        builder.Services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
  //....
  
    app.UseSession();

@using Microsoft.AspNetCore.Http
@inject Microsoft.AspNetCore.Http.IHttpContextAccessor _httpContextAccessor;

<div class="rtm r3">
 @if(_httpContextAccessor.HttpContext.Session.GetInt32("loginid") == null){
    <a href="javascript:;" class="rg1" onclick="login(2)">注册</a>
        <a href="javascript:;" class="rg2" onclick="login(1)">登录</a>
}
else
{
<span>欢迎您，
    @_httpContextAccessor.HttpContext.Session.GetString("loginname")
    ，</span>
<a href="/home/loginout" class="rg2">退出登录</a>
}
</div>