modified for request

OthelloGame.cs

using System;
using Zenject;

public class OthelloGame : IInitializable {
    private readonly OthelloBoardModel othelloBoard;

    public OthelloGame( OthelloBoardModel othelloBoard ) {
        this.othelloBoard = othelloBoard;
    }

    public void Initialize() {
        StartNewGame();
    }

    public void StartNewGame() {
        othelloBoard.SetSide( Disk.DARK );
        othelloBoard.PlaceDisk( 4, 5 );
        othelloBoard.PlaceDisk( 5, 4 );

        othelloBoard.SetSide( Disk.LIGHT );
        othelloBoard.PlaceDisk( 4, 4 );
        othelloBoard.PlaceDisk( 5, 5 );

        othelloBoard.SetSide( Disk.DARK );

        othelloBoard.OnRequestDisk += ValidatePlacement;
    }

    private void ValidatePlacement( int x, int y, Disk disk ) {
        othelloBoard.PlaceDisk( x, y );

        if ( disk == Disk.DARK ) {
            othelloBoard.SetSide( Disk.LIGHT );
        }
        else if ( disk == Disk.LIGHT ) {
            othelloBoard.SetSide( Disk.DARK );
        }
        else {
            throw new ArgumentException( "Place \"None\" disk which is impossible in the game" );
        }
    }
}