void Main()
{
	var validEmailList = new List<EmailAddress>()
	{
		new EmailAddress("a@b.com"),
		new EmailAddress("b@c.com", true)
	};

	IsValidPrimaryCollection(validEmailList)
		.Dump("One email marked as primary");

	var noPrimaryEmailList = new List<EmailAddress>()
	{
		new EmailAddress("a@b.com"),
		new EmailAddress("b@c.com")
	};

	IsValidPrimaryCollection(noPrimaryEmailList)
		.Dump("No email marked as primary");

	var multiplePrimaryEmailList = new List<EmailAddress>()
	{
		new EmailAddress("a@b.com", true),
		new EmailAddress("b@c.com", true)
	};

	IsValidPrimaryCollection(multiplePrimaryEmailList)
		.Dump("More than one email marked as primary");

	var validPhoneList = new List<PhoneNumber>()
	{
		new PhoneNumber("a@b.com"),
		new PhoneNumber("b@c.com", true)
	};

	IsValidPrimaryCollection(validPhoneList)
		.Dump("One phone marked as primary");

	var noPrimaryPhoneList = new List<PhoneNumber>()
	{
		new PhoneNumber("a@b.com"),
		new PhoneNumber("b@c.com")
	};

	IsValidPrimaryCollection(noPrimaryPhoneList)
		.Dump("No phone marked as primary");

	var multiplePrimaryPhoneList = new List<PhoneNumber>()
	{
		new PhoneNumber("a@b.com", true),
		new PhoneNumber("b@c.com", true)
	};

	IsValidPrimaryCollection(multiplePrimaryPhoneList)
		.Dump("More than one phone marked as primary");
}

public bool IsValidPrimaryCollection<T>(IEnumerable<T> items)
	where T : IHavePrimaryFLag => items.Count(i => i.IsPrimary) == 1;

public interface IHavePrimaryFLag
{
	bool IsPrimary {get;}
}

public class EmailAddress : IHavePrimaryFLag
{
	public bool IsPrimary { get; }
	public int Id { get; set; }
	public string Value { get; }

	public EmailAddress(string value, bool isPrimary = false)
	{
		Value = value;
		IsPrimary = isPrimary;
	}
}

public class PhoneNumber : IHavePrimaryFLag
{
	public bool IsPrimary { get; }
	public int Id { get; set; }
	public string Value { get; }

	public PhoneNumber(string value, bool isPrimary = false)
	{
		Value = value;
		IsPrimary = isPrimary;
	}
}