Пример валидации значений одних полей в зависимости от других.

Отсюда: https://gist.github.com/elizabeth-young/5875161

public enum Comparison
{
    IsEqualTo,
    IsNotEqualTo
}
/// 

/// Validation attribute for required value if another property is set
/// 
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public sealed class RequiredIfAttribute : ValidationAttribute, IClientValidatable
{
    private const string DefaultErrorMessageFormatString = "The {0} field is required.";
    public string OtherProperty { get; private set; }
    public Comparison Comparison { get; private set; }
    public Object Value { get; private set; }

    public RequiredIfAttribute(string otherProperty, Comparison comparison, object value)
    {
        if (string.IsNullOrEmpty(otherProperty))
        {
            throw new ArgumentNullException("otherProperty");
        }

        OtherProperty = otherProperty;
        Comparison = comparison;
        Value = value;
    }

    public override string FormatErrorMessage(string name)
    {
        return string.Format(ErrorMessageString, name, OtherProperty);
    }

    public bool Validate(object actualPropertyValue)
    {
        switch (Comparison)
        {
            case Comparison.IsNotEqualTo:
                return actualPropertyValue == null ? Value != null : !actualPropertyValue.Equals(Value);
            default:
                return actualPropertyValue == null ? Value == null : actualPropertyValue.Equals(Value);
        }
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null)
        {
            try
            {
                var property = validationContext.ObjectInstance.GetType().GetProperty(OtherProperty);
                var propertyValue = property.GetValue(validationContext.ObjectInstance, null);

                if (Validate(propertyValue))
                {
                    return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
                }
            }
            catch (Exception e)
            {
                if (OtherProperty.Contains('_'))
                {
                    var prop = OtherProperty.Split('_');

                    var property = validationContext.ObjectInstance.GetType().GetProperty(prop.Last());
                    var propertyValue = property.GetValue(validationContext.ObjectInstance, null);

                    if (!Validate(propertyValue))
                    {
                        return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
                    }
                }
            }
        }
        return ValidationResult.Success;
    }

    public IEnumerable GetClientValidationRules(ModelMetadata metadata,
                                                                           ControllerContext context)
    {
        return new[]
            {
                new ModelClientValidationRequiredIfRule(
                    FormatErrorMessage(metadata.GetDisplayName()), OtherProperty, Comparison, Value, metadata,
                    context)
            };
    }
}

public class ModelClientValidationRequiredIfRule : ModelClientValidationRule
{
    public ModelClientValidationRequiredIfRule(string errorMessage, string otherProperty, Comparison comparison,
                                               object value, ModelMetadata metadata, ControllerContext context)
    {
        ErrorMessage = errorMessage;
        ValidationType = "requiredif";
        ValidationParameters.Add("other", BuildDependentPropertyId(metadata, context as ViewContext, otherProperty));
        ValidationParameters.Add("comp", comparison.ToString().ToLower());
        ValidationParameters.Add("value", value.ToString().ToLower());
    }

    private string BuildDependentPropertyId(ModelMetadata metadata, ViewContext viewContext, string otherProperty)
    {
        return QualifyFieldId(metadata, otherProperty, viewContext);
    }

    protected string QualifyFieldId(ModelMetadata metadata, string fieldId, ViewContext viewContext)
    {
        // build the ID of the property
        string depProp = viewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(fieldId);
        // unfortunately this will have the name of the current field appended to the beginning,
        // because the TemplateInfo's context has had this fieldname appended to it. Instead, we
        // want to get the context as though it was one level higher (i.e. outside the current property,
        // which is the containing object (our Person), and hence the same level as the dependent property.
        var thisField = metadata.PropertyName + "_";
        if (depProp.StartsWith(thisField))
            // strip it off again
            depProp = depProp.Substring(thisField.Length);
        else if (depProp.Contains(thisField))
        {
            depProp = depProp.Remove(depProp.IndexOf(thisField), thisField.Length);
        }
        return depProp;
    }
}