C#

public abstract class Base
{
    public int GetInt()
    {
        return 1;
    }
}

public abstract class AbstractChild : Base
{
    public abstract int GetInt();
}

public class Child : AbstractChild
{
    public override int GetInt()
    {
        return 2;
    }
}

Base b = new Child();
Console.Out.WriteLine(b.GetInt());// prints 1

[Serializable]
    public class NewOrder
    {
        //constructor
        public NewOrder()
        {
        }

        //Destructor
        ~NewOrder()
        {
        }
        private string accountGroupfield;
        private string accountNumberfield;

        public string AccountGroup
        {
            get
            {
                return accountGroupfield;
            }
            set
            {
                accountGroupfield = value;
            }
        }

        
        public string AccountNumber
        {
            get
            {
                return accountNumberfield;
            }
            set
            {
                accountNumberfield = value;
            }
        }

[Serializable]
public class CardDetail
        {
                   

            private string cardFirstNamefield;
            private string cardLastNamefield;
            private string cardNumberfield;

            public string CardFirstName
            {
                get
                {
                    return cardFirstNamefield;
                }
                set
                {
                    cardFirstNamefield = value;
                }
            }

            
           
            public string CardLastName
            {
                get
                {
                    return cardLastNamefield;
                }
                set
                {
                    cardLastNamefield = value;
                }
            }

            public string CardNumber
            {
                get
                {
                    return cardNumberfield;
                }
                set
                {
                    cardNumberfield = value;
                }
            }

 }

<?xml version="1.0"?>
<NewOrder xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <AccountGroup>100</AccountGroup>
  <AccountNumber>66302</AccountNumber>
</NewOrder>

<?xml version="1.0"?>
<NewOrder  xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <AccountGroup>100</AccountGroup>
  <AccountNumber>66302</AccountNumber>
  <CardDetail>
        <CardFirstName>George</CardFirstName>
        <CardSurname>Balden</CardSurname>
        <CardNumber>6123 2222 4444 6666</CardNumber>
  </CardDetail>
</NewOrder>