System.Collections.

System.Collections.ObjectModel.ReadOnlyCollection

System.Collections.Generic.List <string> weekdays = new System.Collections.Generic.List <string>(); 

weekdays.Add("Monday"); 
weekdays.Add("Tuesday"); 
weekdays.Add("Wednesday"); 
weekdays.Add("Thursday"); 

ReadOnlyCollection<string> readOnlyWeekdays = new System.Collections.ObjectModel.ReadOnlyCollection<string>(weekdays); 

System.Collections.Specialized

Added in ?
These are non generic, strongly typed collections that have a very specific purpose.

System.Collections.Specialized.BitVector32 
System.Collections.Specialized.CollectionsUtil 
System.Collections.Specialized.HybridDictionary 
System.Collections.Specialized.ListDictionary 
System.Collections.Specialized.NameValueCollection 
System.Collections.Specialized.StringCollection 
System.Collections.Specialized.StringDictionary 
System.Collections.Specialized.StringEnumerator 
System.Collections.Specialized.OrderedDictionary 

System.Collections.Concurrent

Added in ?
The System.Collections.Concurrent namespace has the following collections
These collections are highly optimised for high concurrency scenarios and are thread safe.

System.Collections.Concurrent.ConcurrentStack<T> 
Stack<T> 

System.Collections.Concurrent.ConcurrentQueue<T> 
Queue<T> 

System.Collections.Concurrent.ConcurrentBag<T> 

no-non concurrent equivaleent

System.Collections.Concurrent.ConcurrentDictionary<TKey, TValue> 
Dictionary<TKey, TValue> 

System.Collections.Immutable

Added in .NET 5.0