spectre.console/test/Spectre.Console.Tests/Data/Validators/EvenNumberValidatorAttribute.cs

using System;
using Spectre.Console.Cli;

namespace Spectre.Console.Tests.Data
{
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = true)]
    public sealed class EvenNumberValidatorAttribute : ParameterValidationAttribute
    {
        public EvenNumberValidatorAttribute(string errorMessage)
            : base(errorMessage)
        {
        }

        public override ValidationResult Validate(CommandParameterContext context)
        {
            if (context.Value is int integer)
            {
                if (integer % 2 == 0)
                {
                    return ValidationResult.Success();
                }

                return ValidationResult.Error($"Number is not even ({context.Parameter.PropertyName}).");
            }

            throw new InvalidOperationException($"Parameter is not a number ({context.Parameter.PropertyName}).");
        }
    }
}