You might develop a helper method to unbox and join the boxed collection if you don’t know the type.

public string UnboxAndJoin(string separator, object obj)
{
      if (obj is IEnumerable enumerable)
   { 
      return string.Join(separator, enumerable.Cast<object>());
   }
   else
   {
     throw new ArgumentException("Object is not an IEnumerable");
   }
}

Usage:

object myList = new List<string> { “str1”, “str2” };
Console.WriteLine(UnboxAndJoin(“,”, myList)); // str1,str2