Custom Validation Attributes: Comparing two properties in the same model

Is there a way to create a custom attribute in ASP.NET Core to validate if one date property is less than other date property in a model using ValidationAttribute.

Lets say I have this:

public class MyViewModel 
{
    [Required]
    [CompareDates]
    public DateTime StartDate { get; set; }

    [Required]
    public DateTime EndDate { get; set; } = DateTime.Parse("3000-01-01");
}

I am trying to use something like this:

    public class CompareDates : ValidationAttribute
{
    public CompareDates()
        : base("") { }

    public override bool IsValid(object value)
    {
        return base.IsValid(value);
    }

}

I found other SO post that proposes to use another library, But I prefer to stick with ValidationAttribute if that was doable.

You can create a custom validation attribute for comparison two properties. It's a server side validation:

public class MyViewModel
{
    [DateLessThan("End", ErrorMessage = "Not valid")]
    public DateTime Begin { get; set; }

    public DateTime End { get; set; }
}

public class DateLessThanAttribute : ValidationAttribute
{
    private readonly string _comparisonProperty;

    public DateLessThanAttribute(string comparisonProperty)
    {
         _comparisonProperty = comparisonProperty;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        ErrorMessage = ErrorMessageString;
        var currentValue = (DateTime)value;

        var property = validationContext.ObjectType.GetProperty(_comparisonProperty);

        if (property == null)
            throw new ArgumentException("Property with this name not found");

        var comparisonValue = (DateTime)property.GetValue(validationContext.ObjectInstance);

        if (currentValue > comparisonValue)
            return new ValidationResult(ErrorMessage);

        return ValidationResult.Success;
    }
}

Update: If you need a client side validation for this attribute, you need implement an IClientModelValidator interface:

public class DateLessThanAttribute : ValidationAttribute, IClientModelValidator
{
    ...
    public void AddValidation(ClientModelValidationContext context)
    {
        var error = FormatErrorMessage(context.ModelMetadata.GetDisplayName());
        context.Attributes.Add("data-val", "true");
        context.Attributes.Add("data-val-error", error);
    }
}

The AddValidation method will add attributes to your inputs from context.Attributes.

You can read more here IClientModelValidator

As one possible option self-validation:

You just need to Implement an interface IValidatableObject with the method Validate(), where you can put your validation code.

public class MyViewModel : IValidatableObject
{
    [Required]
    public DateTime StartDate { get; set; }

    [Required]
    public DateTime EndDate { get; set; } = DateTime.Parse("3000-01-01");

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        int result = DateTime.Compare(StartDate , EndDate);
        if (result < 0)
        {
            yield return new ValidationResult("start date must be less than the end date!", new [] { "ConfirmEmail" });
        }
    }
}