不能使用AuthorizationHandlerContext

Question

使用AuthorizationHandlerContext问题。 ASP.NET Core 3预览版7

NullReferenceException：

System.NullReferenceException: Object reference not set to an instance of an object.
   at GWAdmin.Auth.PermissionHandler.HandleRequirementAsync(AuthorizationHandlerContext context, PermissionRequirement requirement) in H:\GWAdmin\GWAdmin\Auth\PermissionHandler.cs:line 51
   at Microsoft.AspNetCore.Authorization.AuthorizationHandler`1.HandleAsync(AuthorizationHandlerContext context)
   at Microsoft.AspNetCore.Authorization.DefaultAuthorizationService.AuthorizeAsync(ClaimsPrincipal user, Object resource, IEnumerable`1 requirements)
   at Microsoft.AspNetCore.Authorization.Policy.PolicyEvaluator.AuthorizeAsync(AuthorizationPolicy policy, AuthenticateResult authenticationResult, HttpContext context, Object resource)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

        protected override async Task HandleRequirementAsync(AuthorizationHandlerContext context, PermissionRequirement requirement)
        {

            Requirement = requirement;

            var httpContext = (context.Resource as Microsoft.AspNetCore.Mvc.Filters.AuthorizationFilterContext).HttpContext;
...


        }