public class Outer {
    protected class ProtectedClass {
        protected required int ProtectedProperty { get; set; }
    }
}

 class Derived : Outer {
     public void Test() {
         var p = new ProtectedClass() // error "Required member 'Outer.ProtectedClass.ProtectedProperty' must be set
         {
           // ProtectedProperty = 1 // cannot set the property because it is inaccessible
         };
     }
 }