Can I manually validate a property using a custom validation attribute?

Question

I have a custom ValidationAttribute , however I only want to validate this property if a CheckBox is checked.

I've made my class inherit from IValidationObject and am using the Validate method to perform any custom validation, however can I use a custom ValidationAttribute here instead of duplicating the code? And if so, how?

public class MyClass : IValidatableObject
{
    public bool IsReminderChecked { get; set; }
    public bool EmailAddress { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (IsReminderChecked)
        {
            // How can I validate the EmailAddress field using
            // the Custom Validation Attribute found below?
        }
    }
}


// Custom Validation Attribute - used in more than one place
public class EmailValidationAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var email = value as string;

        if (string.IsNullOrEmpty(email))
            return false;

        try
        {
            var testEmail = new MailAddress(email).Address;
        }
        catch (FormatException)
        {
            return false;
        }

        return true;
    }
}

Answer 1

It's possible to validate a property based on the value of another property, but there are a few hoops to jump through to make sure the validation engine works the way you expect. Simon Ince's RequiredIfAttribute has a good approach and it should be easy to modify it into a ValidateEmailIfAttribute just by adding your e-mail validation logic to the IsValid method.

For example, you could have your base validation attribute, just like you do now:

public class ValidateEmailAttribute : ValidationAttribute
{
  ...
}

and then define the conditional version, using Ince's approach:

public class ValidateEmailIfAttribute : ValidationAttribute, IClientValidatable
{
  private ValidateEmailAttribute _innerAttribute = new ValidateEmailAttribute();

  public string DependentProperty { get; set; }
  public object TargetValue { get; set; }

  public ValidateEmailIfAttribute(string dependentProperty, object targetValue)
  {
    this.DependentProperty = dependentProperty;
    this.TargetValue = targetValue;
  }

  protected override ValidationResult IsValid(object value, ValidationContext validationContext)
  {
    // get a reference to the property this validation depends upon
    var containerType = validationContext.ObjectInstance.GetType();
    var field = containerType.GetProperty(this.DependentProperty);

    if (field != null)
    {
      // get the value of the dependent property
      var dependentvalue = field.GetValue(validationContext.ObjectInstance, null);

      // compare the value against the target value
      if ((dependentvalue == null && this.TargetValue == null) ||
          (dependentvalue != null && dependentvalue.Equals(this.TargetValue)))
      {
        // match => means we should try validating this field
        if (!_innerAttribute.IsValid(value))
          // validation failed - return an error
          return new ValidationResult(this.ErrorMessage, new[] { validationContext.MemberName });
      }
    }

    return ValidationResult.Success;
  }

  // Client-side validation code omitted for brevity
}

Then you could just have something like:

[ValidateEmailIf("IsReminderChecked", true)]
public bool EmailAddress { get; set; }