Fixed premature evaluation of the collection within the Cache TrueFor operators, causing premature and potentially incorrect emissions to occur, when items in the collection publish values immediately upon subscription. (#923)

Author: JakenVeina

.editorconfig

@@ -345,7 +345,7 @@ dotnet_diagnostic.SA1110.severity = error
 dotnet_diagnostic.SA1111.severity = error
 dotnet_diagnostic.SA1112.severity = error
 dotnet_diagnostic.SA1113.severity = error
-dotnet_diagnostic.SA1114.severity = error
+dotnet_diagnostic.SA1114.severity = none
 dotnet_diagnostic.SA1115.severity = error
 dotnet_diagnostic.SA1116.severity = none
 dotnet_diagnostic.SA1117.severity = error

src/DynamicData.Tests/Cache/TrueForAnyFixture.cs

@@ -2,6 +2,8 @@
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
 
+using DynamicData.Tests.Utilities;
+
 using FluentAssertions;
 
 using Xunit;
@@ -77,6 +79,26 @@ public void MultipleValuesReturnTrue()
         subscribed.Dispose();
     }
 
+    // https://github.com/reactivemarbles/DynamicData/issues/922
+    [Fact]
+    public void ValuesPublishedOnSubscriptionDoNotTriggerPrematureOutput()
+    {
+        var item1 = new ObjectWithObservable(1);
+        var item2 = new ObjectWithObservable(2);
+
+        item2.InvokeObservable(true);
+
+        _source.AddOrUpdate(item1);
+        _source.AddOrUpdate(item2);
+
+        using var subscription = _observable
+            .ValidateSynchronization()
+            .RecordValues(out var results);
+
+        results.RecordedValues.Count.Should().Be(1, because: "No items were added to the source, and no value changes were made to the items");
+        results.RecordedValues[0].Should().Be(true, because: "One of the two items in the source has a true value");
+    }
+
     private class ObjectWithObservable(int id)
     {
         private readonly ISubject<bool> _changed = new Subject<bool>();

src/DynamicData/Cache/Internal/TrueFor.cs

@@ -18,16 +18,26 @@ internal sealed class TrueFor<TObject, TKey, TValue>(IObservable<IChangeSet<TObj
 
     private readonly IObservable<IChangeSet<TObject, TKey>> _source = source ?? throw new ArgumentNullException(nameof(source));
 
-    public IObservable<bool> Run() => Observable.Create<bool>(
-            observer =>
-            {
-                var transformed = _source.Transform(t => new ObservableWithValue<TObject, TValue>(t, _observableSelector(t))).Publish();
-                var inlineChanges = transformed.MergeMany(t => t.Observable);
-                var queried = transformed.ToCollection();
-
-                // nb: we do not care about the inline change because we are only monitoring it to cause a re-evaluation of all items
-                var publisher = queried.CombineLatest(inlineChanges, (items, _) => _collectionMatcher(items)).DistinctUntilChanged().SubscribeSafe(observer);
-
-                return new CompositeDisposable(publisher, transformed.Connect());
-            });
+    public IObservable<bool> Run()
+        => Observable.Create<bool>(observer =>
+        {
+            var itemsWithValues = _source
+                .Transform(item => new ObservableWithValue<TObject, TValue>(
+                    item: item,
+                    source: _observableSelector.Invoke(item)))
+                .Publish();
+
+            var subscription = Observable.CombineLatest(
+                    // Make sure we subscribe to ALL of the items before we make the first evaluation of the collection, so any values published on-subscription don't trigger a re-evaluation of the matcher method.
+                    first: itemsWithValues.MergeMany(item => item.Observable),
+                    second: itemsWithValues.ToCollection(),
+                    // We don't need to actually look at the changed values, we just need them as a trigger to re-evaluate the matcher method.
+                    resultSelector: (_, itemsWithValues) => _collectionMatcher.Invoke(itemsWithValues))
+                .DistinctUntilChanged()
+                .SubscribeSafe(observer);
+
+            return new CompositeDisposable(
+                subscription,
+                itemsWithValues.Connect());
+        });
 }