UML class diagram 

using System;

// "AbstractFactory"

abstract class AbstractFactory
{
  // Methods
  abstract public AbstractProductA CreateProductA();
  abstract public AbstractProductB CreateProductB();
}

// "ConcreteFactory1"

class ConcreteFactory1 : AbstractFactory
{
  // Methods
  override public AbstractProductA CreateProductA()
  {
    return new ProductA1();
  }
  override public AbstractProductB CreateProductB()
  {
    return new ProductB1();
  }
}

// "ConcreteFactory2"

class ConcreteFactory2 : AbstractFactory
{
  // Methods
  override public AbstractProductA CreateProductA()
  {
    return new ProductA2();
  }
  override public AbstractProductB CreateProductB()
  {
    return new ProductB2();
  }
}

// "AbstractProductA"

abstract class AbstractProductA
{
}

// "AbstractProductB"

abstract class AbstractProductB
{
  // Methods
  abstract public void Interact( AbstractProductA a );
}

// "ProductA1"

class ProductA1 : AbstractProductA
{
}

// "ProductB1"

class ProductB1 : AbstractProductB
{
  // Methods
  override public void Interact( AbstractProductA a )
  {
    Console.WriteLine( this + " interacts with " + a );
  }
}

// "ProductA2"

class ProductA2 : AbstractProductA
{
}

// "ProductB2"

class ProductB2 : AbstractProductB
{
  // Methods
  override public void Interact( AbstractProductA a )
  {
    Console.WriteLine( this + " interacts with " + a );
  }
}

// "Client" - the interaction environment of the products

class Environment
{
  // Fields
  private AbstractProductA AbstractProductA;
  private AbstractProductB AbstractProductB;

  // Constructors
  public Environment( AbstractFactory factory )
  {
    AbstractProductB = factory.CreateProductB();
    AbstractProductA = factory.CreateProductA();
  }
  
  // Methods
  public void Run()
  {
    AbstractProductB.Interact( AbstractProductA );
  }
}

/// <summary>
///  ClientApp test environment
/// </summary>

class ClientApp
{
  public static void Main(string[] args)
  {
    AbstractFactory factory1 = new ConcreteFactory1();
    Environment e1 = new Environment( factory1 );
    e1.Run();

    AbstractFactory factory2 = new ConcreteFactory2();
    Environment e2 = new Environment( factory2 );
    e2.Run();

    Console.Read();
  }
}