Conditional data annotation.

RequiredIfAttribute.cs

namespace De.Lenidh.Misc
{
    using System;
    using System.ComponentModel.DataAnnotations;

    /// <summary>
    /// Specifies that a data field value is conditional required.
    /// </summary>
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = false)]
    public class RequiredIfAttribute : RequiredAttribute
    {
        private string dependencyName;
        private object requiredValue;

        public RequiredIfAttribute(string dependencyName, object requiredValue)
        {
            this.dependencyName = dependencyName;
            this.requiredValue = requiredValue;
        }

        protected override ValidationResult IsValid(object value,
            ValidationContext validationContext)
        {
            // Get the dependency property
            var dependencyType = validationContext.ObjectInstance.GetType();
            var dependency = dependencyType.GetProperty(this.dependencyName);

            if (dependency == null)
            {
                var msg = string.Format("Property '{0}' not found.", this.dependencyName);
                throw new InvalidOperationException(msg);
            }

            var dependencyValue = dependency.GetValue(validationContext.ObjectInstance);

            if (this.requiredValue == null && dependencyValue == null)
                return base.IsValid(value, validationContext);

            if (this.requiredValue != null &&  this.requiredValue.Equals(dependencyValue))
                return base.IsValid(value, validationContext);

            // Always successful, if dependency value and required value not equal.
            return ValidationResult.Success;
        }
    }
}