C# Unable to validate property using a custom validate attribute

Question

I have a validation class:

public sealed class ValidationSet : ValidationAttribute
{
    private List<string> _set = new List<string>();
    public ValidationSet(params string[] setOfValues)
    {
        _set = setOfValues.ToList();
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (!(_set.Select(s => s.ToLower()).Contains(value.ToString().ToLower())))
        {
            throw new Exception("not in the set");
        }
        return ValidationResult.Success;
    }
}

and here is how I use it:

public class Car
{
    [ValidationSet("honda", "gm")]
    public string CarMake{ get; set; }

}

When I instantiate the Car class by:

...
Car c = new Car();
c.CarMake = "ford";
...

Nothing will happen and if I print c.CarMake, it shows ford - the validation didn't happened.

I am just wondering what do I miss here.

Thanks!

Answer 1

Just instantiating the class and assigning the field is not going to call IsValid, you need to use the class in a framework that examines Car, sees that it has ValidationAttribute on CarMake and will call IsValid.

In this example asp:DynamicValidator is doing the work:

How to: Customize Data Field Validation