在startup.cs中
services.AddIdentity<ApplicationUser, IdentityRole>()
更改以下代码将处理此问题。

services.AddIdentity<ApplicationUser, IdentityRole>(options =>
            {
                options.Cookies.ApplicationCookie.Events = new CookieAuthenticationEvents
                {
                    OnRedirectToAccessDenied = context => {
                        context.Response.StatusCode = 403;
                        return Task.FromResult(0);
                    },
                    OnRedirectToLogin = context =>
                    {
                        context.Response.StatusCode = 401;
                        return Task.FromResult(0);
                    }
                };
                options.Cookies.ApplicationCookie.AutomaticAuthenticate = true;
            })

Maxisam的回答很有帮助，但事情又发生了变化。IdentityOptions上不再有Cookies属性。然而，这起作用：

services.ConfigureApplicationCookie(options =>
{
    options.Events = new CookieAuthenticationEvents
    {
        OnRedirectToAccessDenied = context =>
        {
            context.Response.StatusCode = 403;
            return Task.FromResult(0);
        },
        OnRedirectToLogin = context =>
        {
            context.Response.StatusCode = 401;
            return Task.FromResult(0);
        }
    };
});

您被重定向，因为默认情况下ApplicationCookie将AutomaticChallenge设置为true。
要防止重定向，请用途：

options.Cookies.ApplicationCookie.AutomaticChallenge = false;