How to check either both properties or none of them are present in configuration element?
===

Is there way to write some `ConfigurationValidatorAttribute` or in some other way that will allow either both `Prop1` and `Prop2` are present or none of them?

For example for following config `domain3` element will cause error because it must have both `Prop1` and `Prop2` or none of them, but not just one of them!

    <MySection>
            <Domains>
              <Domain Name="domain1" Prop1="1" Prop2="4" /> 
              <Domain Name="domain2" /> 
              <Domain Name="domain3" Prop1="1" /> 
            </Domains>         
        </MySection>

    public class ConfigElement : ConfigurationElement
    {     
        [ConfigurationProperty("Name", IsRequired = true)]
        public string Name
        {
            get { return (string)this["Name"]; }
            set { this["Name"] = value; }
        }        
           
        [ConfigurationProperty("Prop1")]
        public int Prop1
        {
            get { return (int)this["Prop1"]; }
            set { this["Prop1"] = value; }
        }

        [ConfigurationProperty("Prop2")]
        public int Prop2
        {
            get { return (int)this["Prop2"]; }
            set { this["Prop2"] = value; }
        }
    }

   
