在ASP.net MVC .netframework 4.8应用程序中使用Cookie来维护用户身份，而在ASP.net MVC Core 6应用程序中使用Json Web Tokens（JWT）来实现身份验证和授权。因此，它们之间共享用户身份变得困难。但是，可以通过将JWT作为 .netframework 4.8应用程序中的Cookie存储或使用ASP.net Core Identity来实现共享用户身份。

以下是使用ASP.net Core Identity实现身份验证和授权的示例代码：

// Startup.cs
public void ConfigureServices(IServiceCollection services)
{
  services.AddIdentity()
    .AddEntityFrameworkStores()
    .AddDefaultTokenProviders();

  // configure JWT token authentication
  var key = Encoding.ASCII.GetBytes(Configuration.GetValue("JwtKey"));

  services.AddAuthentication(options =>
  {
    options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
  })
  .AddJwtBearer(options =>
  {
    options.RequireHttpsMetadata = false;
    options.SaveToken = true;
    options.TokenValidationParameters = new TokenValidationParameters
    {
      ValidateIssuerSigningKey = true,
      IssuerSigningKey = new SymmetricSecurityKey(key),
      ValidateIssuer = false,
      ValidateAudience = false
    };
  });
}

// controller
[Authorize(AuthenticationSchemes = JwtBearerDefaults.AuthenticationScheme)]
public class MyController : ControllerBase
{
  // controller actions
}

在ASP.net Core 6应用程序中生成JWT并在 .netframework 4.8 应用程序中读取和解析：

// ASP.net Core 6
var tokenHandler = new JwtSecurityTokenHandler();
var key = Encoding.ASCII.GetBytes(Configuration.GetValue("JwtKey"));

var tokenDescriptor = new SecurityTokenDescriptor
{
  Subject = new ClaimsIdentity(new Claim[]
  {
    new Claim(ClaimTypes.Name, username),
    new Claim(ClaimTypes.Role, role)
  }),
  Expires = DateTime.UtcNow.AddHours(1),
  SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};

var token = tokenHandler.CreateToken(tokenDescriptor);
var encodedToken = tokenHandler.WriteToken(token);

// .netframework 4.8
var handler = new JwtSecurityTokenHandler();
var jwt = handler.ReadJwtToken(encodedToken);
var identity = new ClaimsIdentity(jwt.Claims);

使用此方法可以在ASP.net MVC .netframework 4.8和ASP.net MVC Core