Creating Custom Validation Attribute With Dependency To Other Property

In the last post, I went through the steps to make a custom validation attribute - if you missed it, check it out here: Creating Custom Validation Attribute. Now let's say we want to make our validator to look up on the value "37" from another property in our class (instead of hard-coding it). How do we do that? With the help of reflection, we can do that quite easily.

Here is our modified class:

public class MyClass {
   // ... more code
   [MagicNumber("ThirtySeven", ErrorMessage = "Sum is not correct")]
   public int MyData { get; set; }
   // ... more code
   public int ThirtySeven {
      get { return 37; }
   }
   // ... more code
}

In our new custom validator, we override a different IsValid method (the original one is commented out):

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
sealed public class MagicNumberAttribute : ValidationAttribute {
   // constructor to accept the comparison property name
   public MagicNumberAttribute(string thirtySevenProperty) : base() {
      if (thirtySevenProperty == null) {
         throw new ArgumentNullException("thirtySevenProperty");
      }
      ThirtySevenProperty= thirtySevenProperty;
   }
 
   // property to store the comparison field name
   public string ThirtySevenProperty{ get; private set; }
 
   // public override bool IsValid(object value) {
   //    var num = int.Parse(value.ToString());
   //    return num == 37;
   // }
 
   protected override ValidationResult IsValid(object value, ValidationContext validationContext) {
      // get property info of the "ThirtySevenProperty"
      PropertyInfo thirtySevenPropertyInfo = validationContext.ObjectType.GetProperty(ThirtySevenProperty);
 
      // check if that property exists / valid
      if (thirtySevenPropertyInfo == null) {
         return new ValidationResult(String.Format(CultureInfo.CurrentCulture, "UNKNOWN PROPERTY", ThirtySevenProperty));
      }
 
      // get the value of the property
      object thirtySevenPropertyValue = thirtySevenPropertyInfo.GetValue(validationContext.ObjectInstance, null);
 
      // do comparison and return validation result with error if not equal
      if (!Equals(value, thirtySevenPropertyValue)) {
         return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
      }
 
      // return null if everything is ok
      return null;
   }
}

That's it!
Additional reading:

• Creating Custom Validation Attribute
• Custom Validation Attribute - Client Side Enabled!
• Custom Validation Attribute - Client Side Enabled With Custom Rule
• Creating Validation Adapter for Validation Attribute