It is very easy to implement new validations. We will demonstrate it through the looking at the implementation of the RegexValidation and MatchesAttribute classes.

Implement the pure validation

Create a class and derive from ValidationBase. Now implement your validation logic:

public class RegexValidation : ValidationBase
{
  private readonly Regex _regex;

  public RegexValidation (string pattern, RegexOptions options = RegexOptions.None)
  {
    _regex = new Regex (pattern, options);
  }

  // actual implementation of IValidation.Validate
  public override void Validate (object value, string paramName,
                                    IValidationContext context)
  {
    if (value is string)
      Validate ((string) value, paramName);
    else
      // throw an exception if the type is not as expected
      ThrowExceptionForIncompatibleType (paramName, typeof (string));
  }

  private void Validate (string value, string paramName)
  {
    if (!_regex.IsMatch (value))
    {
      // throw an exception for invalid values
      var message = string.Format ("Value must match pattern '{0}'", _regex);
      throw new ValidationException (message, paramName);
    }
  }
}

You can not only access the appropriate value, but also the context of validation:

// MethodInfo, ConstructorInfo, or PropertyInfo
MethodBase Method { get; }

// object whose method call is being validated
object Instance { get; }

// arguments passed to the method
object[] Arguments { get; }

// return value of the method
object ReturnValue { get; }

Implement the validation attribute

Create a class and derive from ValidationAttributeBase. Now pass the validation object to the base constructor:

public class MatchesAttribute : ValidationAttributeBase
{
  public MatchesAttribute (string pattern, RegexOptions options = RegexOptions.None)
      : base(new RegexValidation(pattern, options))
  {
  }
}