public static PropertiesDescriptor <T> Int64 <T>(this PropertiesDescriptor <T> propertiesDescriptor,
                                                         Func <NumberPropertyDescriptor <T>, INumberProperty> selector)
            where T : class
        {
            // Validate parameters.
            if (propertiesDescriptor == null)
            {
                throw new ArgumentNullException(nameof(propertiesDescriptor));
            }
            if (selector == null)
            {
                throw new ArgumentNullException(nameof(selector));
            }

            // Modify and return.
            return(propertiesDescriptor.NumberProperty(selector, NumberType.Long));
        }