How to make sure that a list cannot be modified

Suppose we have a List of integers that we want to pass to a method Reader:

class Example { 

  public void Run() {
     // Add more data to the list
     lst.Add(2); 
     lst.Add(3); 
     /*
        how to pass lst to Reader() here
        as read-only ?
     */
  } 
  //.... more code
  void Reader( ICollection<int> coll ) {
     // some code 
  }
  List<int> lst = new List<int>{ 1 };

}

While passing lst to Reader from Run, how can we enforce that the list is not modifiable inside method Reader?