DrawRectangleCommand.cs

public void Draw()
{
    // Save state
    SavePreviousState();

    // Carry out the command execution
    var brush = new SolidBrush(_color);
    var rectangle = new System.Drawing.Rectangle(_point.X, _point.Y, Width, Height);
    _graphics.FillRectangle(brush, rectangle);
}

// Saving all pixels in the area we are going to draw
// We are storing the color of each pixel
// When we perform an undo, all pixels will be restored to initial state
private void SavePreviousState()
{
    for (int i = _point.X; i < _point.X + Width; i++)
    {
        for (int j = _point.Y; j < _point.Y + Height; j++)
        {
            _previousState.Add(_bitmap.GetPixel(i, j));
        }
    }
}