How to identify property is user defined or primitive , using reflection in c#?

I'm using reflection to identify object properties , Now i need to identify object property is user defined or not. i have try as following way

 foreach (System.Reflection.PropertyInfo prop in typeof(Domain.Entities.UrlClass).GetProperties().Where(p => p.CanWrite))
            {
                if (prop.Name.ToLower() == column.ToLower() && prop.ReflectedType.IsPrimitive)
                {
                    prop.SetValue(entityToMap, Convert.ChangeType(cellValue, prop.PropertyType));
                    break;
                }
            }

But this prop.ReflectedType.IsPrimitive is always false for string property is anyone have idea to separate user defined types and primitive types?

thanks.