Custom Validation Attributes: Comparing two properties in the same model

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