Creating Custom Validators with NValidate

Introduction

The initial release of NValidate supports custom validators through the use of the IValidatable interface. In this article, we'll look at how to write your own tests and have NValidate execute them.

A custom validator must implement the IValidatable interface. To simplify your work, NValidate provides the CustomValidatorBase class, which provides the base implementation of the interface for you. All you should have to do is implement the constructor and Validate method.

The sample below shows a basic validator that ensures that a string is not null, not empty, and matches a ZIP+4 pattern:

       
namespace Validation
{
   public class ZipPlus4Validator
      : CustomValidatorBase
   {
      public ZipPlus4Validator (string value)
         : base()
      {
         m_value = value;
      }

      public bool> Validate()
      {
         // Clear the error message; set it if a failure occurs.
         // If the error message is anything but an empty string
         // by the time we reach the bottom, the test has failed.
         m_errorMessage = string.Empty;
         if(null == m_value)
            m_errorMessage = "Null values not permitted.";
         else if(string.Empty == m_value)
            m_errorMessage = "Empty strings not permitted.";
         else if(!(new Regex(expression).IsMatch("\\d[5]-\\d[4]")))
            m_errorMessage = "Not a valid ZIP+4 value.";
         return (string.Empty = m_errorMessage);
      }
      private string m_value = string.Empty;
      private string m_errorMessage = string.Empty;
   }
}
    

Note that this is a class prepared for demonstration purposes only; you'll want to do something far more robust in your own code.)

Using Your Custom Test

Once you've created your custom test class, all you have to do is pass it to NValidate's Demand.That method. For this example, we'll assume you're validating a ZIP code from Visual Basic code:

Public Sub InsertZipCode(ByVal zipCode As String)
   Demand.That(New ZipPlus4Validator(zipCode), "zipCode").IsValid()
   ' Continue processing
End Sub
    

That's it. When your InsertZipCode method is called, Demand.That will create a new custom validator instance and pass the zipCode parameter to it. InsertZipCode will then invoke the IsValid method on it, which will then invoke the custom validator's Validate method. If any test within Validate fails, Validate sets the ErrorMessage propety and returns False; otherwise, it simply returns True.

When the Validate method on your custom validator has finished processing, IsValid examines the return value. If the return value is False, an ArgumentException is thrown, using the ErrorMessage property and the ParameterName (in this case, "zipCode").

An Important Note

Note that if a custom validator fails, NValidate can only throw a generic ArgumentException, but the error message is determined by your custom validator class. For this reason, it's imperitive that your class correctly set the ErrorMessage property at runtime and return the appropriate boolean value.