Custom validation using MVC3 ValidationAttribute

In this Demo, Let's create a custom validation attribute "DateGreaterAttribute" that validates that a Template EndDate is later than the given StartDate. This validation attribute will be applied like any other Data Annotations.

    public partial class Template
    {
        public Template()
        {
        }
    
        public int TemplateId { get; set; }
        public string Name { get; set; }
        public byte Active { get; set; }
        public System.DateTime Effective { get; set; }
       [DateGreater(DateStartProperty="Effective", ErrorMessage="The Expiration Date must be after the Effective Date")]        
        public Nullable Expiring { get; set; }
        public string Description { get; set; }
        public string Notes { get; set; }
        public int ChangedBy { get; set; }
        public System.DateTime ChangedAt { get; set; }
    
     }

        [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
 public class DateGreaterAttribute : ValidationAttribute
 {
      public DateGreaterAttribute() : base("The End Date must be later then Start Date")
      {
      }
      public string DateStartProperty { get; set; }

      protected override ValidationResult IsValid(object value, ValidationContext validationContext)
      {
         string dateEndString = (value != null) ? value.ToString() : string.Empty;
         if (string.IsNullOrWhiteSpace(dateEndString))
        {
          // A blank end date means no comparison. Use Required attribute to force an end date entry.
          return ValidationResult.Success;
        }

        // Get the start date and compare to the end date
        bool valid = false;
        PropertyInfo dateStartPropertyInfo = validationContext.ObjectType.GetProperty (DateStartProperty);

        if (dateStartPropertyInfo != null)
        {
         object dateStartValue = dateStartPropertyInfo.GetValue(validationContext.ObjectInstance, null);
         string dateStartString = (dateStartValue != null) ? dateStartValue.ToString() : string.Empty;
         DateTime dateStart;

            if (DateTime.TryParse(dateStartString, out dateStart))
            {
              DateTime dateEnd;
              if (DateTime.TryParse(dateEndString, out dateEnd))
              {
                  valid = (dateStart < dateEnd);
              }
            }
        }

      return valid ? ValidationResult.Success : new ValidationResult(ErrorMessage);
  }
 }

In the above code, we are implementing a class initializer and IsValid() from the base class. The constructor allows you to provide a default error message in case of validation failure.
The IsValid() method provides 2 parameters,
1. object value - It's the value entered by the user for End Date field.
2. validationContext - Describes the context in which a validation check is performed. The 2 important property that we use here are -
a) ObjectType - Get the type of the object to validate i.e. Template. The GetProperty()method provided by it, returns property information on the Start Date parameter.
b) ObjectInstance - Get the object to validate i.e. Template. This instance is used to get the value of Start Date parameter.