Marking a field in ToList as readonly.

Rx.NET/Source/src/System.Reactive/Linq/Observable/ToList.cs

@@ -24,7 +24,7 @@ namespace System.Reactive.Linq.ObservableImpl
 
         private sealed class _ : Sink<IList<TSource>>, IObserver<TSource>
         {
-            private List<TSource> _list;
+            private readonly List<TSource> _list;
 
             public _(IObserver<IList<TSource>> observer, IDisposable cancel)
                 : base(observer, cancel)