using System;

using System.ComponentModel.DataAnnotations;

using System.Collections.Generic;

​

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]

public class TestAttribute : ValidationAttribute

{

    public TestAttribute() : base("Error message"){}

    public override bool IsValid(object o)

    {

        int value = (int) o;

        ErrorMessage = "{0} is not even, it is " + value;

        return (value%2)==0;

    }

}

​

public class TestModel 

{

    [TestAttribute]

    public int TestProperty { get; set; }

}

​

public class Program {

    public static void Main(string[] args){

        TestModel invalidModel = new TestModel() { TestProperty = 1 };

        Validate(invalidModel);

        TestModel validModel = new TestModel() { TestProperty = 2 };

        Validate(validModel);

    }

    private static void Validate(TestModel model)

    {

        ValidationContext vc = new ValidationContext(model);

        List<ValidationResult> results = new List<ValidationResult>();

        bool isValid = Validator.TryValidateObject(model, vc, results, true);

        Console.Out.WriteLine(isValid);

        results.ForEach(c => Console.Out.WriteLine(c.ErrorMessage));

    }

}