Fix shared state issue in List.Transform. Fixes #618

src/DynamicData.Tests/List/TransformFixture.cs

@@ -144,4 +144,22 @@ public void Update()
         _results.Messages[0].Adds.Should().Be(1, "Should be 1 adds");
         _results.Messages[0].Replaced.Should().Be(0, "Should be 1 update");
     }
+
+
+    [Fact]
+    public void MultipleSubscribersShouldNotShareState()
+    {
+        _source.AddRange(new Person[]
+        {
+            new ("Adult1", 50),
+            new ("Adult2", 51)
+        });
+
+        var transformed = _source.Connect()
+            .Transform(o => o);
+
+        // will throw if state of ChangeAwareList is shared
+        transformed.Transform(o => o).Subscribe();
+        transformed.Transform(o => o).Subscribe();
+    }
 }

src/DynamicData/List/Internal/TransformAsync.cs

@@ -2,12 +2,8 @@
 // Roland Pheasant licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Reactive.Linq;
 using System.Reactive.Threading.Tasks;
-using System.Threading.Tasks;
 
 namespace DynamicData.List.Internal;
 

src/DynamicData/List/Internal/TransformMany.cs

@@ -2,11 +2,8 @@
 // Roland Pheasant licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using System;
 using System.Collections;
-using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System.Linq;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 

src/DynamicData/List/Internal/Transformer.cs

@@ -31,9 +31,9 @@ public Transformer(IObservable<IChangeSet<TSource>> source, Func<TSource, Option
         _containerFactory = (item, prev, index) => new TransformedItemContainer(item, factory(item, prev, index));
     }
 
-    public IObservable<IChangeSet<TDestination>> Run()
-    {
-        return _source.Scan(new ChangeAwareList<TransformedItemContainer>(), (state, changes) =>
+    public IObservable<IChangeSet<TDestination>> Run() => Observable.Defer(RunImpl);
+
+    private IObservable<IChangeSet<TDestination>> RunImpl() => _source.Scan(new ChangeAwareList<TransformedItemContainer>(), (state, changes) =>
             {
                 Transform(state, changes);
                 return state;
@@ -43,7 +43,6 @@ public IObservable<IChangeSet<TDestination>> Run()
                 var changed = transformed.CaptureChanges();
                 return changed.Transform(container => container.Destination);
             });
-    }
 
     private void Transform(ChangeAwareList<TransformedItemContainer> transformed, IChangeSet<TSource> changes)
     {
@@ -125,7 +124,7 @@ private void Transform(ChangeAwareList<TransformedItemContainer> transformed, IC
                         }
                         else
                         {
-                            TransformedItemContainer? toRemove = transformed.FirstOrDefault(t => ReferenceEquals(t.Source, change.Current));
+                            var toRemove = transformed.FirstOrDefault(t => ReferenceEquals(t.Source, change.Current));
 
                             if (toRemove is not null)
                             {
@@ -188,15 +187,9 @@ public TransformedItemContainer(TSource source, TDestination destination)
 
         public TSource Source { get; }
 
-        public static bool operator ==(TransformedItemContainer left, TransformedItemContainer right)
-        {
-            return Equals(left, right);
-        }
+        public static bool operator ==(TransformedItemContainer left, TransformedItemContainer right) => Equals(left, right);
 
-        public static bool operator !=(TransformedItemContainer left, TransformedItemContainer right)
-        {
-            return !Equals(left, right);
-        }
+        public static bool operator !=(TransformedItemContainer left, TransformedItemContainer right) => !Equals(left, right);
 
         public bool Equals(TransformedItemContainer? other)
         {
@@ -233,9 +226,6 @@ public override bool Equals(object? obj)
             return Equals((TransformedItemContainer)obj);
         }
 
-        public override int GetHashCode()
-        {
-            return Source is null ? 0 : EqualityComparer<TSource>.Default.GetHashCode(Source);
-        }
+        public override int GetHashCode() => Source is null ? 0 : EqualityComparer<TSource>.Default.GetHashCode(Source);
     }
 }