use firstOrDefault for a default value, for person1 and person2

See more:

C#5.0

I use VS2012, WindowsApp,c#. For my Person list, I would like to use FirstOrDefault for PrimaryPersonID and SecondaryPersonID properties.

My controls work fine in the UI, but I need logic to display properly.
If both PrimaryPersonID and SecondPersonID exists, set Primary as first.
If only a PrimaryPersonID exists and no SecondaryPersonID, Secondary can be null.
If neither exists, set null as default. So null is acceptable for both.

//CaseRecord is my class
protected void ValidatePersonType(ValidationResults results)
{
    ValidationResult vr = new ValidationResult();     
    if (this.CaseRecord.PrimaryPersonID != null &&             this.CaseRecord.SecondaryPersonID != null)
{
     //this does not work
     PrimaryPersonID = CaseRecord.GetPersonByID(PersonID.Value).FirstOrDefault();
}
else
{
     if (PersonList.Count == 0)
     {
        //Not sure how to do here also	
        PrimaryPersonID == null;
        SecondaryPersonID == null;
     }
}

//and do the same for SecondaryPersonID

Any help is appreciated. If using FirstOrDefault is wrong, I just need help with correct logic. Thanks.