DragDropSender.cs

public interface IDropSender {
    void OnBeginDrag();
    bool IsCorrespondingObject(GameObject obj);
    void OnOverlappingCorrespondingReceiverChange(GameObject receiverObject);
    void OnDrop(GameObject receiverObject);
}

public interface IDropReceiver {
    void OnOverlappingCorrespondingSenderChange(GameObject senderObject);
    void OnDrop(GameObject senderObject);
}

public class DragDropSender : MonoBehaviour
{   
    private Vector3 prevPos;
    private GameObject currentOverlappingReceiver = null;
    
    private IDropSender sender;
    void Awake() {
        sender = GetComponent<IDropSender>();
    }

    public void OnMouseDown()
    {
        // ドラッグ前の座標を覚えておく
        prevPos = transform.position;

        // ドラッグ開始時の処理
        sender.OnBeginDrag();
    }

    public void OnMouseDrag()
    {
        // ドラッグ中はオブジェクトをマウスに追従
        var currentMousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        currentMousePos.z = prevPos.z;
        transform.position = currentMousePos;

        // ドラッグ中に下に受信オブジェクトがあるかどうかを判別
        var objTarget = GetRecieveObject();
        if (objTarget != prevOverObject)
        {
            // オーバーオブジェクトが変更になった場合、通知する
            sender.OnOverlappingCorrespondingReceiverChange(objTarget);
            prevOverObject?.GetComponent<IDropReceiver>().OnOverlappingCorrespondingSenderChange(null);
            objTarget?.GetComponent<IDropReceiver>().OnOverlappingCorrespondingSenderChange(gameObject);
        }
        currentOverlappingReceiver = gameObject;
    }

    public void OnMouseUp()
    {
        if (currentOverlappingReceiver == null) {
            // オブジェクトをドラッグ前の位置に戻す
            transform.position = prevPos;
        }

        sender.OnDrop(currentOverlappingReceiver);
        currentOverlappingReceiver?.OnDrop(gameObject);
    }

    private GameObject GetRecieveObject()
    {
        var hitAll = Physics2D.RaycastAll(Camera.main.ScreenToWorldPoint(Input.mousePosition), Vector2.zero);
        foreach (var hit in hitAll)
        {
            if (hit.collider != null)
            {
                var receptor = hit.collider.gameObject;
                if (receptor != null && sender.IsCorrespondingObject(receptor))
                {
                    return receptor;
                }
            }
        }

        return null;
    }
}