how to put observablecollection in List

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C#

WPF

Hello
I want to put observablecollection into the List<string> like this :

List<string> test = new List<string>();
          foreach (CheckInData coll in CheckInCollection)
          {
              for (int i = 0; i < _CheckInCollection.Count; i++)
              {
               test.Add(coll.RoomNumber[i].ToString());
              }
              
          }

but its only put the first character of collection for example there is 2424 test content will be only 2.
Please help me anybody

This is my Collection:

ObservableCollection<CheckInData> _CheckInCollection = new ObservableCollection<CheckInData>();

   public ObservableCollection<CheckInData> CheckInCollection
   {
       get { return _CheckInCollection; }
   }
   public class CheckInData
   {
       public string RoomNumber { get; set; }
       public decimal Price { get; set; }
       public string Currecny { get; set; }
       public decimal Discount { get; set; }
       public string CheckOut { get; set; }
       public int TotalDay { get; set; }
       public decimal TotalPrice { get; set; }
       public int CheckOutYear { get; set; }
       public int CheckOutMonth { get; set; }
       public int CheckOutDay { get; set; }
       public Boolean IncToday { get; set; }
   }