If there is only one area specified by this line

I am drawing a line to detect objects in a specific area

However, as I draw the line, I know that if you do it wrong once, you can create two areas.

 Vector3 startPosition; 
     Vector2 preivousPos=new Vector2(100,100);  
 
     GameObject currentLineObject;
 
     LineRenderer currentLineRenderer;
 
     public Material lineMaterial;
 
     public float lineThickness;
 
     int vertexnum = 0;
     CameraManager cameraManager;
 
     private void Start()
     {
         cameraManager = Camera.main.GetComponent<CameraManager>();
     }
     void LateUpdate()
     {
         //Draw line
         if (cameraManager.mouseClickMode==3&& Input.GetMouseButtonDown(0))    //한번 누르면 계속눌러도 한번만 
         {
             StartDrawingLine();
         }
         else if (cameraManager.mouseClickMode == 3 && Input.GetMouseButton(0))  // 누르는 동안 계속
         {
             Vector2 vec2 = new Vector2(GetMousePosition().x, GetMousePosition().y);
             if (Vector2.Distance(vec2, preivousPos)>0.05)
             {
                 currentLineRenderer.positionCount++;
                 currentLineRenderer.SetPosition(vertexnum++, GetMousePosition());
             }
 
         }
         else if (cameraManager.mouseClickMode == 3 && Input.GetMouseButtonUp(0)) //누르고 난후에 
         {
             EndDrawingLine();
         }
         //Draw line
         if (cameraManager.mouseClickMode == 4 && Input.GetMouseButtonDown(0))    //한번 누르면 계속눌러도 한번만 
         {
             StartDrawingLine();
         }
         else if (cameraManager.mouseClickMode == 4 && Input.GetMouseButton(0))  // 누르는 동안 계속
         {
             Vector2 vec2 = new Vector2(GetMousePosition().x, GetMousePosition().y);
             if (Vector2.Distance(vec2, preivousPos) > 0.1)
             {
                 currentLineRenderer.positionCount++;
                 currentLineRenderer.SetPosition(vertexnum++, GetMousePosition());
             }
 
         }
         else if (cameraManager.mouseClickMode == 4 && Input.GetMouseButtonUp(0)) //누르고 난후에 
         {
             if(Vector2.Distance(startPosition,
                 new Vector2(GetMousePosition().x, GetMousePosition().y))<0.2)
             {
                 currentLineRenderer.positionCount++;
                 currentLineRenderer.SetPosition(vertexnum++, startPosition);
                 startPosition = Vector3.zero;
                 currentLineObject = null;
                 currentLineRenderer = null;
                 vertexnum = 0;
                 preivousPos = Vector2.zero;
             }
             else
             {
                 Destroy(currentLineObject);
                 startPosition = Vector3.zero;
                 currentLineObject = null;
                 currentLineRenderer = null;
                 vertexnum = 0;
                 preivousPos = Vector2.zero;
             }
         }
     }
 
   
     Vector3 GetMousePosition()
     {
         Vector3 mousePos = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x,
             Input.mousePosition.y, -Camera.main.transform.position.z ));
         return mousePos;
     }
 
     void StartDrawingLine()
     {
         startPosition = GetMousePosition();
         currentLineObject = new GameObject();
         currentLineObject.transform.position = startPosition;
         currentLineObject.transform.SetParent(transform);
         currentLineRenderer = currentLineObject.AddComponent<LineRenderer>();
         currentLineRenderer.positionCount = 0;
 
         currentLineRenderer.material = lineMaterial;
         currentLineRenderer.startWidth = lineThickness;
         currentLineRenderer.endWidth = lineThickness;
 
     }
 
     void EndDrawingLine()
     {
         startPosition = Vector3.zero;
         currentLineObject = null;
         currentLineRenderer = null;
         vertexnum = 0;
         preivousPos = Vector2.zero;
      
     }
 
     public void destoryLine()
     {
         for(int i=0; i<transform.childCount;i++)
         {
             GameObject.Destroy(transform.GetChild(i).gameObject);
         }
     }

1. How can I detect objects in a painted area?

2. Prevent two areas from appearing like photoshop lassotool