Validate a field only if it is populated

Question

I am having a problem with validating phone numbers. In our system we have two phone numbers which you can store. The problem I am having is that these are optional fields. So I want it to validate the phone number IF and only IF the user has tried to enter a phone number in there. If not it can be left as blank.

I am using the Phone attribute and have set a MaxLength. I have tried to set a MinLength to 0 but that doesn't work.

    [Phone]
    [MaxLength(24)]
    [MinLength(0)]
    public string PhoneNum1
    {
        get { return phoneNum1; }
        set
        {
            if (phoneNum1 != value)
            {
                phoneNum1 = value;
                RaisePropertyChanged("PhoneNum1");
            }
        }
    }

Additionally, we have a checkbox which if ticked the user would have to add at least one of the phone numbers. I haven't attempted this yet so I am technically not asking for that solution but it would be great if any solutions would bare this in mind.

Here is the WPF which I am using. I am using ValidatesOnDataErrors and NotifyOnValidationError

                    <TextBox Margin="0,10,0,0" Grid.Row="10"  Grid.Column="2" 
                        Text="{Binding PhoneNum1, Mode=TwoWay, ValidatesOnDataErrors=True, 
                    NotifyOnValidationError=True, UpdateSourceTrigger=PropertyChanged}"/>

Answer 1

You can implement your OptionalPhoneAttribute based on the original PhoneAttribute:

 public sealed class OptionalPhoneAttribute : ValidationAttribute
 {        
    public override bool IsValid(object value)
    {
        var phone = new PhoneAttribute();

        //return true when the value is null or empty
        //return original IsValid value only when value is not null or empty 
        return (value == null || string.IsNullOrEmpty(Convert.ToString(value)) || phone.IsValid(value));
    }
}

Then you can just use this new attribute instead:

[OptionalPhone]
[MaxLength(24)]
public string PhoneNum1
{
    get { return phoneNum1; }
    set
    {
        if (phoneNum1 != value)
        {
            phoneNum1 = value;
            RaisePropertyChanged("PhoneNum1");
        }
    }
}