IMenu.cs

using Spectre.Console;
using Spectre.Console.Rendering;


public interface IMenuLayout {
  IRenderable GetContent();
  IPrompt<string> GetPrompt();
}

public interface IMenuBehavior {
  IMenu? HandleInput(string input);
}
public interface IMenu : IMenuLayout, IMenuBehavior {}

public class MainMenu : IMenu
{
    public IRenderable GetContent()
    {
        return new Table().AddColumn("Key").AddColumn("Value").AddRow(["myKey", "myValue"]);
    }

    public IPrompt<string> GetPrompt()
    {
        return new SelectionPrompt<string>().AddChoices(["SubMenu", "Exit"]);
    }

    public IMenu? HandleInput(string input)
    {
      return input switch {
          "SubMenu" => new SubMenu(),
          "Exit" => null,
          _ => this
        };
    }
}

public class SubMenu : IMenu
{
    public IRenderable GetContent()
    {
        return new Calendar(DateTime.Now);
    }

    public IPrompt<string> GetPrompt()
    {
        return new SelectionPrompt<string>().AddChoices(["Go back", "Do stuff"]);
    }

    public IMenu HandleInput(string input)
    {
        return input switch {
          "Go back" => new MainMenu(),
          _ => this
        };
    }
}

public class MenuManager {
  private IMenu _currentState;
  public MenuManager(IMenu initialState) {
    _currentState = initialState;
  }

  public void Run() {
    while (_currentState != null) {
      // main app-loop

      // clear the screen
      AnsiConsole.Clear();

      // render content
      AnsiConsole.Write(_currentState.GetContent());
      // gather user input
      var userInput = AnsiConsole.Prompt(_currentState.GetPrompt());
      // handle input
      _currentState = _currentState.HandleInput(userInput);
    }
  }
}