Use null propagation.

Rx.NET/Source/src/System.Reactive/Linq/QueryLanguageEx.cs

@@ -458,10 +458,7 @@ namespace System.Reactive.Linq
                         {
                             var curr = new ChainObservable<TSource>(x);
 
-                            if (chain != null)
-                            {
-                                chain.OnNext(curr);
-                            }
+                            chain?.OnNext(curr);
 
                             chain = curr;
 
@@ -471,17 +468,11 @@ namespace System.Reactive.Linq
                         _ => { },
                         exception =>
                         {
-                            if (chain != null)
-                            {
-                                chain.OnError(exception);
-                            }
+                            chain?.OnError(exception);
                         },
                         () =>
                         {
-                            if (chain != null)
-                            {
-                                chain.OnCompleted();
-                            }
+                            chain?.OnCompleted();
                         })
                     .ObserveOn(scheduler)
                     .Select(selector);

Rx.NET/Source/src/System.Reactive/Platforms/UWP/Foundation/AsyncInfoToObservableBridge.cs

@@ -28,10 +28,7 @@ namespace System.Reactive.Windows.Foundation
                     _subject.OnNext(getResult(iai));
                 }
 
-                if (progress != null)
-                {
-                    progress.Report(p);
-                }
+                progress?.Report(p);
             });
 
             Done(info, info.Status, true);