fix: invalid cast exception with nullable appstate

src/ReactiveUI.Tests/Suspension/DummyAppState.cs

@@ -0,0 +1,6 @@
+namespace ReactiveUI.Tests.Suspension
+{
+    public class DummyAppState
+    {
+    }
+}

src/ReactiveUI.Tests/Suspension/SuspensionHostExtensionsTests.cs

@@ -0,0 +1,58 @@
+using System;
+using Shouldly;
+using Xunit;
+
+namespace ReactiveUI.Tests.Suspension
+{
+    public class SuspensionHostExtensionsTests
+    {
+        [Fact]
+        public void GetAppStateReturns()
+        {
+            var fixture = new SuspensionHost();
+            fixture.AppState = new DummyAppState();
+
+            var result = fixture.GetAppState<DummyAppState>();
+
+            result.ShouldBe(fixture.AppState);
+        }
+
+        [Fact]
+        public void NullSuspensionHostThrowsException()
+        {
+            var result = Record.Exception(() => ((SuspensionHost)null!).SetupDefaultSuspendResume());
+
+            result.ShouldBeOfType<ArgumentNullException>();
+        }
+
+        [Fact]
+        public void NullAppStateDoesNotThrowException()
+        {
+            var fixture = new SuspensionHost();
+
+            var result = Record.Exception(() => fixture.SetupDefaultSuspendResume());
+
+            result.ShouldBeNull();
+        }
+
+        [Fact]
+        public void ObserveAppStateDoesNotThrowException()
+        {
+            var fixture = new SuspensionHost();
+
+            var result = Record.Exception(() => fixture.ObserveAppState<DummyAppState>().Subscribe());
+
+            result.ShouldBeNull();
+        }
+
+        [Fact]
+        public void ObserveAppStateDoesNotThrowInvalidCastException()
+        {
+            var fixture = new SuspensionHost();
+
+            var result = Record.Exception(() => fixture.ObserveAppState<DummyAppState>().Subscribe());
+
+            result.ShouldNotBeOfType<InvalidCastException>();
+        }
+    }
+}

src/ReactiveUI/Suspension/SuspensionHostExtensions.cs

@@ -24,8 +24,9 @@ public static class SuspensionHostExtensions
         public static IObservable<T> ObserveAppState<T>(this ISuspensionHost item)
             where T : class
         {
-            return item.WhenAny(x => x.AppState, x => (T)x)
-                        .Where(x => x != null);
+            return item.WhenAny(suspensionHost => suspensionHost.AppState, observedChange => observedChange.Value)
+                        .Where(x => x != null)
+                        .Cast<T>();
         }
 
         /// <summary>
@@ -41,12 +42,7 @@ public static T GetAppState<T>(this ISuspensionHost item)
                 throw new ArgumentNullException(nameof(item));
             }
 
-            if (item.AppState == null)
-            {
-                throw new NullReferenceException(nameof(item.AppState));
-            }
-
-            return (T)item.AppState;
+            return (T)item.AppState!;
         }
 
         /// <summary>
@@ -63,11 +59,6 @@ public static IDisposable SetupDefaultSuspendResume(this ISuspensionHost item, I
                 throw new ArgumentNullException(nameof(item));
             }
 
-            if (item.AppState == null)
-            {
-                throw new NullReferenceException(nameof(item.AppState));
-            }
-
             var ret = new CompositeDisposable();
             driver ??= Locator.Current.GetService<ISuspensionDriver>();
 
@@ -77,7 +68,7 @@ public static IDisposable SetupDefaultSuspendResume(this ISuspensionHost item, I
                          .Subscribe(_ => item.Log().Info("Invalidated app state")));
 
             ret.Add(item.ShouldPersistState
-                         .SelectMany(x => driver.SaveState(item.AppState).Finally(x.Dispose))
+                         .SelectMany(x => driver.SaveState(item.AppState!).Finally(x.Dispose))
                          .LoggedCatch(item, Observables.Unit, "Tried to persist app state")
                          .Subscribe(_ => item.Log().Info("Persisted application state")));