How can i avoid breaking LSP in this example? C#

Question

I have a base class called Message like this:

public abstract class Message 
{
    protected int m_id;
    protected bool m_localized;
    protected string m_metaData;

    public int GetID() { return m_id; }
    public bool GetLocalized() { return m_localized; }
    public string GetMetadata() { return m_metaData; }
}

Then, i have two more classes that inherit from Message for example:

public class ClassicMessage : Message
{
     private string m_title;
     private string m_content;

     public void SetTitle(string title) { m_title = title; }
     public void SetContent(string content) { m_content = content; }
     public string GetTitle() { return m_title; }
     public string GetContent() { return m_content; }
}

public class MessageWithCustomContent : Message
{
     private List<CustomContent> m_content;

     public MessageWithCustomContent() 
     {
          m_content = new List<CustomContent>();
     }

     public List<CustomContent> GetContent()
     {
          return m_content;
     }

     public CustomContent GetContentEntry(int id) 
     {
          return m_content.find(x => x.ID.Equals(id));
     }
}

public class CustomContent
{
     private int m_id;
     public int ID { get; set { m_id = value; } }
     private string m_body;
     public string Body { get { return m_body; } set { m_body = value; }
     private Image m_image; 
     public Image Image { get { return m_image; } set { m_image = value; } }
}

In a case like this, how can i unify the app interface if the derived classes has similar methods but these methods have different return types? (even when the methods try to do the same)

I know that with the example i'm breaking the Liskov Substitution Principle and the Open/Closed principle, what's the best approach to get around with that?

Thanks for your help!

Edit:

For more clarity, what i'm trying to achieve is to create a common interface to manage all the possible messages as the base "Message", because i want to avoid using typeof in the consumer class.

for example:

if(message is MessageWithCustomContent) 
{
         // do something with the contents.
}
else if(message is MessageWithCustomContent) 
{
       // do another thing with the contents.
}
etc...

Answer 1

You could change Message to be generic, and the T would specify the Content return type. See example below.

Edit You could use a "IMessage" and a "Message: IMessage" as base. You would then be able to create a IMessage list like so

var messages = new List<IMessage>
{
    new ClassicMessage(),
    new MessageWithCustomContent()
};
foreach (var message in messages)
{
    message.GetContent();
}

Below is how the implementation of IMessagecould be done.

public interface IMessage
{
    int GetID();
    bool GetLocalized();
    string GetMetadata();
    object GetContent();
}

public abstract class Message<T> : IMessage
{
    protected int m_id;
    protected bool m_localized;
    protected string m_metaData;

    public int GetID() { return m_id; }
    public bool GetLocalized() { return m_localized; }
    public string GetMetadata() { return m_metaData; }
    object IMessage.GetContent()
    {
        return GetContent();
    }
    public abstract T GetContent();
}

public class ClassicMessage : Message<string>
{
    private string m_title;
    private string m_content;

    public void SetTitle(string title) { m_title = title; }
    public void SetContent(string content) { m_content = content; }
    public string GetTitle() { return m_title; }
    public override string GetContent()
    {
        return m_content;
    }
}

public class MessageWithCustomContent : Message<List<CustomContent>>
{
    private List<CustomContent> m_content;

    public MessageWithCustomContent()
    {
        m_content = new List<CustomContent>();
    }

    public CustomContent GetCustomContent(int id)
    {
        return null;
    }

    public override List<CustomContent> GetContent()
    {
        return m_content;
    }
}

public class CustomContent
{
    private int m_id;
    public int ID { get; set; }
    private string m_body;

    public string Body
    {
        get { return m_body; }
        set { m_body = value; }
    }
}