How to remove duplicate matching?

Question

There are my shapes (for example):

public abstract class Shape
{
  protected int _id;
  protected string _description;

  public abstract string ToXml();
}

public sealed class Triangle : Shape
{
  public int TriangleId { get { return _id; } set { _id = value; } }
  public string TriangleDescription { get { return _description; } set { _description = value; } }
  public ConsoleColor color { get; set; }

  public override string ToXml()
  {
    // First implementation
    throw new NotImplementedException();
  }
}

public sealed class Circle : Shape
{
  public int CircleId { get { return _id; } set { _id = value; } }
  public string CircleDescription { get { return _description; } set { _description = value; } }
  public int Radius { get; set; }

  public override string ToXml()
  {
    // Second implementation
    throw new NotImplementedException();
  }
}

public sealed class Square : Shape
{
  public int SquareId { get { return _id; } set { _id = value; } }
  public string SquareDescription { get { return _description; } set { _description = value; } }
  public int Edge { get; set; }

  public override string ToXml()
  {
    // Third implementation
    throw new NotImplementedException();
  }
}

Client have to choose one kind of shape to get xml-view:

public enum ShapeType
{
  Triangle,
  Circle,
  Square
}

public class Module
{
  public string GetXml( ShapeType type, int id )
  {
    switch( type )
    {
      case ShapeType.Circle:
        return DB.GetEntity( id ).ToTypedEntity<Circle>().ToXml();
      case ShapeType.Square:
        return DB.GetEntity( id ).ToTypedEntity<Square>().ToXml();
      case ShapeType.Triangle:
        return DB.GetEntity( id ).ToTypedEntity<Triangle>().ToXml();
    }
  }
}

As you can see, there are too much the same code parts that are matching enumerations with shape's types. It is not good.

How can I avoid it?

Answer 1

Just expose the class types to the user (which they already appear to be) rather than forcing them to use a gawd-awful enum.

public class Module
{
  public string GetXml<TShape>( int id ) where TShape : Shape
  {    
    return DB.GetEntity( id ).ToTypedEntity<TShape>().ToXml();    
  }
}

The the user can just make a normal call like

new Module().GetXml<Triangle>(34);

Answer 2

I would consider decoupling the Xml serialization from the entity objects. The Xml serialization namespace gives you very fine grained control over how an object is represented in Xml plus a lot fo built in functionality that you do not need to reinvent.

The advantages of this decoupling is that your Shape object hierarchy implementations become smaller, you get better on single responsibilty for the shape objects.

public class Module
{
  public string GetXml<TShape>( int id ) where TShape : Shape
  {   
        XmlSerializer ser = new XmlSerializer(typeof(TShape));
        StringBuilder builder = new StringBuilder();
        using(StringWriter sww = new StringWriter(builder))
        using (XmlWriter writer = XmlWriter.Create(sww))
            ser.Serialize(writer, DB.GetEntity( id ).ToTypedEntity<TShape>());
        return builder.ToString(); // Your xml   
  }
}