Validate legacy TransformSafeAsync factories before forwarding

src/DynamicData.Tests/Cache/TransformSafeAsyncFixture.ArgumentValidation.cs

@@ -0,0 +1,52 @@
+// Copyright (c) 2011-2026 Roland Pheasant. All rights reserved.
+// 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.Threading.Tasks;
+
+using Randomizer = Bogus.Randomizer;
+
+using DynamicData.Kernel;
+using DynamicData.Tests.Domain;
+
+using FluentAssertions;
+
+using Xunit;
+using Xunit.Abstractions;
+
+namespace DynamicData.Tests.Cache;
+
+public partial class TransformSafeAsyncFixture
+{
+    private const int ArgumentValidationSeed = 0x2409_1165;
+
+    private readonly Randomizer _argumentRandomizer = new(ArgumentValidationSeed);
+
+    public TransformSafeAsyncFixture(ITestOutputHelper output)
+        => output.WriteLine($"{nameof(TransformSafeAsyncFixture)} seed: {ArgumentValidationSeed:X8}");
+
+    /// <summary>Verifies that the options overload rejects a null factory without subscribing to the source.</summary>
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void OptionsOverload_NullFactory_ThrowsBeforeSubscription(bool populateSource)
+    {
+        // Arrange
+        using var source = new SourceCache<Person, string>(static person => person.Key);
+
+        if (populateSource)
+        {
+            source.AddOrUpdate(new Person(_argumentRandomizer.String2(_argumentRandomizer.Int(5, 20)), _argumentRandomizer.Int(1, 100)));
+        }
+
+        Func<Person, Optional<Person>, string, Task<Person>>? transformFactory = null;
+
+        // Act
+        Action action = () => source.Connect().TransformSafeAsync(transformFactory!, static _ => { }, TransformAsyncOptions.Default);
+
+        // Assert
+        action.Should().Throw<ArgumentNullException>(because: "invalid factories must be rejected before processing any items")
+            .WithParameterName(nameof(transformFactory));
+    }
+}

src/DynamicData.Tests/Cache/TransformSafeAsyncFixture.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reactive;
@@ -13,7 +13,7 @@
 
 namespace DynamicData.Tests.Cache;
 
-public class TransformSafeAsyncFixture
+public partial class TransformSafeAsyncFixture
 {
     [Fact]
     public void ReTransformAll()

src/DynamicData/Cache/ObservableCacheEx.TransformSafeAsync.cs

@@ -134,6 +134,10 @@ public static IObservable<IChangeSet<TDestination, TKey>> TransformSafeAsync<TDe
         where TSource : notnull
         where TKey : notnull
     {
+        source.ThrowArgumentNullExceptionIfNull(nameof(source));
+        transformFactory.ThrowArgumentNullExceptionIfNull(nameof(transformFactory));
+        errorHandler.ThrowArgumentNullExceptionIfNull(nameof(errorHandler));
+
         return source.TransformSafeAsync((current, previous, key, cancel) => transformFactory(current, previous, key), errorHandler, options);
     }