Capture ExceptionDispatchInfo in ToAsyncEnumerable

Author: saul

Ix.NET/Source/System.Linq.Async.Tests/System/Linq/AsyncEnumerableTests.cs

@@ -15,22 +15,16 @@ public class AsyncEnumerableTests
     {
         protected static readonly IAsyncEnumerable<int> Return42 = new[] { 42 }.ToAsyncEnumerable();
 
-        protected async Task AssertThrowsAsync<TException>(Task t)
+        protected async Task<TException> AssertThrowsAsync<TException>(Task t)
             where TException : Exception
         {
-            await Assert.ThrowsAsync<TException>(() => t);
+            return await Assert.ThrowsAsync<TException>(() => t);
         }
 
         protected async Task AssertThrowsAsync(Task t, Exception e)
         {
-            try
-            {
-                await t;
-            }
-            catch (Exception ex)
-            {
-                Assert.Same(e, ex);
-            }
+            var ex = await Assert.ThrowsAnyAsync<Exception>(() => t);
+            Assert.Same(e, ex);
         }
 
         protected Task AssertThrowsAsync<T>(ValueTask<T> t, Exception e)

Ix.NET/Source/System.Linq.Async.Tests/System/Linq/Operators/ToAsyncEnumerable.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -283,6 +284,48 @@ public async Task ToAsyncEnumerable_Observable_Throw()
             await AssertThrowsAsync(e.MoveNextAsync(), ex);
         }
 
+        [Fact]
+        public async Task ToAsyncEnumerable_Observable_Throw_ActiveException()
+        {
+            // No inlining as we're asserting that this function must be mentioned in the exception stacktrace
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            void ThrowsException()
+            {
+                throw new Exception("Bang!");
+            }
+
+            var subscribed = false;
+
+            var xs = new MyObservable<int>(obs =>
+            {
+                subscribed = true;
+
+                try
+                {
+                    ThrowsException();
+                }
+                catch (Exception ex)
+                {
+                    // `ex` is active at this point as it has been thrown.
+                    // Therefore the stack trace should be captured by ToAsyncEnumerable.
+                    // See 'Remarks' at https://docs.microsoft.com/en-us/dotnet/api/system.runtime.exceptionservices.exceptiondispatchinfo.capture
+                    obs.OnError(ex);
+                }
+
+                return new MyDisposable(() => { });
+            }).ToAsyncEnumerable();
+
+            Assert.False(subscribed);
+
+            var e = xs.GetAsyncEnumerator();
+
+            // NB: Breaking change to align with lazy nature of async iterators.
+            // Assert.True(subscribed);
+
+            var actual = await AssertThrowsAsync<Exception>(e.MoveNextAsync().AsTask());
+            Assert.Contains(nameof(ThrowsException), actual.StackTrace);
+        }
+
         [Fact]
         public async Task ToAsyncEnumerable_Observable_Dispose()
         {

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToAsyncEnumerable.Observable.cs

@@ -4,6 +4,7 @@
 
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Runtime.ExceptionServices;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -31,7 +32,7 @@ private sealed class ObservableAsyncEnumerable<TSource> : AsyncIterator<TSource>
             private readonly IObservable<TSource> _source;
 
             private ConcurrentQueue<TSource>? _values = new ConcurrentQueue<TSource>();
-            private Exception? _error;
+            private ExceptionDispatchInfo? _error;
             private bool _completed;
             private TaskCompletionSource<bool>? _signal;
             private IDisposable? _subscription;
@@ -95,7 +96,7 @@ protected override async ValueTask<bool> MoveNextCore()
 
                                 if (error != null)
                                 {
-                                    throw error;
+                                    error.Throw();
                                 }
 
                                 return false;
@@ -120,7 +121,7 @@ public void OnCompleted()
 
             public void OnError(Exception error)
             {
-                _error = error;
+                _error = ExceptionDispatchInfo.Capture(error);
                 Volatile.Write(ref _completed, true);
 
                 DisposeSubscription();