How to read data from a text file into a collection? (JSON preferred)

Well, I do it using FileStream. I first save the length of the array and then do a for loop to save the data.

Here is an example of saving an array on integers in C#

 using System.IO;

 public class SomeClass
 {
     FileStream file;
     
     public void SaveArray(int[] arr)
     {
         file = File.open("myfile.format", FileMode.OpenOrCreate);
         
         byte[] arraySize = System.BitConverter.GetBytes(arr.Length);
         file.Write(arraySize, 0, sizeof(int));
         
         for(int i = 0; i < arr.Length; i++)
         {
             byte[] arrayElement = System.BitConverter.GetBytes(arr[i]);
             file.Write(arrayElement, 0, sizeof(int));
         }
         
         file.Close();
     }
     
     public void LoadArray(int[] arr)
     {
         if (file = File.Open("myfile.format", FileMode.Open))
         {
             byte[] arraySize = new byte[sizeof(int)];
             file.Read(arraySize, 0, sizeof(int));
             
             arr = new int[arraySize];
             
             for (int i = 0; i < arraySize; i++)
             {
                 byte[] arrayElement = new byte[sizeof(int)];
                 file.Read(arrayElement, 0, sizeof(int));
                 
                 arr[i] = System.BitConverter.ToInt32(arrayElement);
             }
             
             file.Close();
         }
     }
 }

You need to remember, when saving, things come out in the exact same way they go in. So since you are doing an array of 2D coordinates. That means in the loop, you would do 2 writes and 2 reads instead of one. One for each float in the coordinate.

 for(int i = 0; i < arr.Length; i++)
 {
     byte[] arrayElement = System.BitConverter.GetBytes(arr[i].x);
     file.Write(arrayElement, 0, sizeof(int));

     arrayElement = System.BitConverter.GetBytes(arr[i].y);
     file.Write(arrayElement, 0, sizeof(int));
 }

The same principles should apply with whatever format you choose to use. You just need to find out how the co-ordinates are stored in the file so you know what datatypes to pull out the data as.