.cs

    var boundingBox = BoundingBox;
    int leftTile = (int)Math.Floor(boundingBox.TopLeft.X / 16f);
    int rightTile = (int)Math.Ceiling(boundingBox.BottomRight.X / 16f) - 1;
    int topTile = (int)Math.Floor(boundingBox.TopLeft.Y / 16f);
    int bottomTile = (int)Math.Ceiling(boundingBox.BottomRight.Y / 16f) - 1;
    for (int x = leftTile; x <= rightTile; ++x)
    {
        for (int y = topTile; y <= bottomTile; ++y)
        {
            if (w.Map.GetTileAt(x, y).IsWall)
            {
                Velocity = -Velocity;
                goto endCollisionDetect;
            }
        }
    }
endCollisionDetect:

    Position += Velocity;
    if (dt.ElapsedTime.TotalMilliseconds != 0f)
    {
        Velocity /= Friction / (float)dt.ElapsedTime.TotalMilliseconds;
    }