Expose ChannelReader.ReadAllAsync from non-core builds as well

I'm not sure why we didn't fix this before when we shipped Microsoft.Bcl.AsyncInterfaces, but with IAsyncEnumerable available downlevel, there's no need to hide this method away; it can be in all builds.  Doing so makes it easier for others to create their own channel implementations, as they don't _need_ to multitarget in order to override everything they might want to.

I've not changed any C# code, just moved it between files.

src/libraries/System.Threading.Channels/ref/System.Threading.Channels.cs

@@ -49,6 +49,7 @@ protected ChannelReader() { }
         public virtual System.Threading.Tasks.Task Completion { get { throw null; } }
         public virtual int Count { get { throw null; } }
         public virtual System.Threading.Tasks.ValueTask<T> ReadAsync(System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
+        public virtual System.Collections.Generic.IAsyncEnumerable<T> ReadAllAsync(System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public virtual bool TryPeek([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T item) { throw null; }
         public abstract bool TryRead([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T item);
         public abstract System.Threading.Tasks.ValueTask<bool> WaitToReadAsync(System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken));

src/libraries/System.Threading.Channels/ref/System.Threading.Channels.csproj

@@ -14,5 +14,6 @@
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="$(SystemThreadingTasksExtensionsVersion)" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="$(MicrosoftBclAsyncInterfacesVersion)" />
   </ItemGroup>
 </Project>

src/libraries/System.Threading.Channels/ref/System.Threading.Channels.netcoreapp.cs

@@ -14,8 +14,4 @@ public partial class ChannelClosedException : System.InvalidOperationException
 #endif
         protected ChannelClosedException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
     }
-    public abstract partial class ChannelReader<T>
-    {
-        public virtual System.Collections.Generic.IAsyncEnumerable<T> ReadAllAsync(System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
-    }
 }

src/libraries/System.Threading.Channels/src/System.Threading.Channels.csproj

@@ -26,8 +26,6 @@ System.Threading.Channel&lt;T&gt;</PackageDescription>
              Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
     <Compile Include="System\Threading\Channels\ChannelOptions.cs" />
     <Compile Include="System\Threading\Channels\ChannelReader.cs" />
-    <Compile Include="System\Threading\Channels\ChannelReader.netcoreapp.cs"
-             Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
     <Compile Include="System\Threading\Channels\ChannelUtilities.cs" />
     <Compile Include="System\Threading\Channels\ChannelWriter.cs" />
     <Compile Include="System\Threading\Channels\Channel_1.cs" />
@@ -40,7 +38,7 @@ System.Threading.Channel&lt;T&gt;</PackageDescription>
     <Compile Include="$(CommonPath)Internal\Padding.cs"
              Link="Common\Internal\Padding.cs" />
     <Compile Include="$(CommonPath)System\Collections\Concurrent\IProducerConsumerQueue.cs"
-         Link="Common\System\Collections\Concurrent\IProducerConsumerQueue.cs" />
+             Link="Common\System\Collections\Concurrent\IProducerConsumerQueue.cs" />
     <Compile Include="$(CommonPath)System\Collections\Concurrent\MultiProducerMultiConsumerQueue.cs"
              Link="Common\System\Collections\Concurrent\MultiProducerMultiConsumerQueue.cs" />
     <Compile Include="$(CommonPath)System\Collections\Concurrent\SingleProducerSingleConsumerQueue.cs"
@@ -61,6 +59,7 @@ System.Threading.Channel&lt;T&gt;</PackageDescription>
 
   <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))">
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="$(SystemThreadingTasksExtensionsVersion)" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="$(MicrosoftBclAsyncInterfacesVersion)" />
   </ItemGroup>
 
 </Project>

src/libraries/System.Threading.Channels/src/System/Threading/Channels/ChannelReader.cs

@@ -1,7 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 
 namespace System.Threading.Channels
@@ -90,5 +92,23 @@ async ValueTask<T> ReadAsyncCore(CancellationToken ct)
                 }
             }
         }
+
+        /// <summary>Creates an <see cref="IAsyncEnumerable{T}"/> that enables reading all of the data from the channel.</summary>
+        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to use to cancel the enumeration.</param>
+        /// <remarks>
+        /// Each <see cref="IAsyncEnumerator{T}.MoveNextAsync"/> call that returns <c>true</c> will read the next item out of the channel.
+        /// <see cref="IAsyncEnumerator{T}.MoveNextAsync"/> will return false once no more data is or will ever be available to read.
+        /// </remarks>
+        /// <returns>The created async enumerable.</returns>
+        public virtual async IAsyncEnumerable<T> ReadAllAsync([EnumeratorCancellation] CancellationToken cancellationToken = default)
+        {
+            while (await WaitToReadAsync(cancellationToken).ConfigureAwait(false))
+            {
+                while (TryRead(out T? item))
+                {
+                    yield return item;
+                }
+            }
+        }
     }
 }

src/libraries/System.Threading.Channels/tests/ChannelTestBase.cs

@@ -784,6 +784,272 @@ public async Task ReadAsync_ConsecutiveReadsSucceed()
             }
         }
 
+        [Fact]
+        public void ReadAllAsync_NotIdempotent()
+        {
+            Channel<int> c = CreateChannel();
+            IAsyncEnumerable<int> e = c.Reader.ReadAllAsync();
+            Assert.NotNull(e);
+            Assert.NotSame(e, c.Reader.ReadAllAsync());
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task ReadAllAsync_UseMoveNextAsyncAfterCompleted_ReturnsFalse(bool completeWhilePending)
+        {
+            Channel<int> c = CreateChannel();
+            IAsyncEnumerator<int> e = c.Reader.ReadAllAsync().GetAsyncEnumerator();
+
+            ValueTask<bool> vt;
+            if (completeWhilePending)
+            {
+                c.Writer.Complete();
+                vt = e.MoveNextAsync();
+                Assert.True(vt.IsCompletedSuccessfully);
+                Assert.False(vt.Result);
+            }
+            else
+            {
+                vt = e.MoveNextAsync();
+                Assert.False(vt.IsCompleted);
+                c.Writer.Complete();
+                Assert.False(await vt);
+            }
+
+            vt = e.MoveNextAsync();
+            Assert.True(vt.IsCompletedSuccessfully);
+            Assert.False(vt.Result);
+        }
+
+        [Fact]
+        public void ReadAllAsync_AvailableDataCompletesSynchronously()
+        {
+            Channel<int> c = CreateChannel();
+
+            IAsyncEnumerator<int> e = c.Reader.ReadAllAsync().GetAsyncEnumerator();
+            try
+            {
+                for (int i = 100; i < 110; i++)
+                {
+                    Assert.True(c.Writer.TryWrite(i));
+                    ValueTask<bool> vt = e.MoveNextAsync();
+                    Assert.True(vt.IsCompletedSuccessfully);
+                    Assert.True(vt.Result);
+                    Assert.Equal(i, e.Current);
+                }
+            }
+            finally
+            {
+                ValueTask vt = e.DisposeAsync();
+                Assert.True(vt.IsCompletedSuccessfully);
+                vt.GetAwaiter().GetResult();
+            }
+        }
+
+        [Fact]
+        public async Task ReadAllAsync_UnavailableDataCompletesAsynchronously()
+        {
+            Channel<int> c = CreateChannel();
+
+            IAsyncEnumerator<int> e = c.Reader.ReadAllAsync().GetAsyncEnumerator();
+            try
+            {
+                for (int i = 100; i < 110; i++)
+                {
+                    ValueTask<bool> vt = e.MoveNextAsync();
+                    Assert.False(vt.IsCompleted);
+                    Task producer = Task.Run(() => c.Writer.TryWrite(i));
+                    Assert.True(await vt);
+                    await producer;
+                    Assert.Equal(i, e.Current);
+                }
+            }
+            finally
+            {
+                ValueTask vt = e.DisposeAsync();
+                Assert.True(vt.IsCompletedSuccessfully);
+                vt.GetAwaiter().GetResult();
+            }
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(128)]
+        public async Task ReadAllAsync_ProducerConsumer_ConsumesAllData(int items)
+        {
+            Channel<int> c = CreateChannel();
+
+            int producedTotal = 0, consumedTotal = 0;
+            await Task.WhenAll(
+                Task.Run(async () =>
+                {
+                    for (int i = 0; i < items; i++)
+                    {
+                        await c.Writer.WriteAsync(i);
+                        producedTotal += i;
+                    }
+                    c.Writer.Complete();
+                }),
+                Task.Run(async () =>
+                {
+                    IAsyncEnumerator<int> e = c.Reader.ReadAllAsync().GetAsyncEnumerator();
+                    try
+                    {
+                        while (await e.MoveNextAsync())
+                        {
+                            consumedTotal += e.Current;
+                        }
+                    }
+                    finally
+                    {
+                        await e.DisposeAsync();
+                    }
+                }));
+
+            Assert.Equal(producedTotal, consumedTotal);
+        }
+
+        [Fact]
+        public async Task ReadAllAsync_MultipleEnumerationsToEnd()
+        {
+            Channel<int> c = CreateChannel();
+
+            Assert.True(c.Writer.TryWrite(42));
+            c.Writer.Complete();
+
+            IAsyncEnumerable<int> enumerable = c.Reader.ReadAllAsync();
+            IAsyncEnumerator<int> e = enumerable.GetAsyncEnumerator();
+
+            Assert.True(await e.MoveNextAsync());
+            Assert.Equal(42, e.Current);
+
+            Assert.False(await e.MoveNextAsync());
+            Assert.False(await e.MoveNextAsync());
+
+            await e.DisposeAsync();
+
+            e = enumerable.GetAsyncEnumerator();
+            Assert.Same(enumerable, e);
+
+            Assert.False(await e.MoveNextAsync());
+            Assert.False(await e.MoveNextAsync());
+        }
+
+        [Theory]
+        [InlineData(false, false)]
+        [InlineData(false, true)]
+        [InlineData(true, false)]
+        [InlineData(true, true)]
+        public void ReadAllAsync_MultipleSingleElementEnumerations_AllItemsEnumerated(bool sameEnumerable, bool dispose)
+        {
+            Channel<int> c = CreateChannel();
+            IAsyncEnumerable<int> enumerable = c.Reader.ReadAllAsync();
+
+            for (int i = 0; i < 10; i++)
+            {
+                Assert.True(c.Writer.TryWrite(i));
+                IAsyncEnumerator<int> e = (sameEnumerable ? enumerable : c.Reader.ReadAllAsync()).GetAsyncEnumerator();
+                ValueTask<bool> vt = e.MoveNextAsync();
+                Assert.True(vt.IsCompletedSuccessfully);
+                Assert.True(vt.Result);
+                Assert.Equal(i, e.Current);
+                if (dispose)
+                {
+                    ValueTask dvt = e.DisposeAsync();
+                    Assert.True(dvt.IsCompletedSuccessfully);
+                    dvt.GetAwaiter().GetResult();
+                }
+            }
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task ReadAllAsync_DualConcurrentEnumeration_AllItemsEnumerated(bool sameEnumerable)
+        {
+            if (RequiresSingleReader)
+            {
+                return;
+            }
+
+            Channel<int> c = CreateChannel();
+
+            IAsyncEnumerable<int> enumerable = c.Reader.ReadAllAsync();
+
+            IAsyncEnumerator<int> e1 = enumerable.GetAsyncEnumerator();
+            IAsyncEnumerator<int> e2 = (sameEnumerable ? enumerable : c.Reader.ReadAllAsync()).GetAsyncEnumerator();
+            Assert.NotSame(e1, e2);
+
+            ValueTask<bool> vt1, vt2;
+            int producerTotal = 0, consumerTotal = 0;
+            for (int i = 0; i < 10; i++)
+            {
+                vt1 = e1.MoveNextAsync();
+                vt2 = e2.MoveNextAsync();
+
+                await c.Writer.WriteAsync(i);
+                producerTotal += i;
+                await c.Writer.WriteAsync(i * 2);
+                producerTotal += i * 2;
+
+                Assert.True(await vt1);
+                Assert.True(await vt2);
+                consumerTotal += e1.Current;
+                consumerTotal += e2.Current;
+            }
+
+            vt1 = e1.MoveNextAsync();
+            vt2 = e2.MoveNextAsync();
+            c.Writer.Complete();
+            Assert.False(await vt1);
+            Assert.False(await vt2);
+
+            Assert.Equal(producerTotal, consumerTotal);
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task ReadAllAsync_CanceledBeforeMoveNextAsync_Throws(bool dataAvailable)
+        {
+            Channel<int> c = CreateChannel();
+            if (dataAvailable)
+            {
+                Assert.True(c.Writer.TryWrite(42));
+            }
+
+            var cts = new CancellationTokenSource();
+            cts.Cancel();
+
+            IAsyncEnumerator<int> e = c.Reader.ReadAllAsync(cts.Token).GetAsyncEnumerator();
+            ValueTask<bool> vt = e.MoveNextAsync();
+            Assert.True(vt.IsCompleted);
+            Assert.False(vt.IsCompletedSuccessfully);
+            OperationCanceledException oce = await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await vt);
+            Assert.Equal(cts.Token, oce.CancellationToken);
+        }
+
+        [Fact]
+        public async Task ReadAllAsync_CanceledAfterMoveNextAsync_Throws()
+        {
+            Channel<int> c = CreateChannel();
+            var cts = new CancellationTokenSource();
+
+            IAsyncEnumerator<int> e = c.Reader.ReadAllAsync(cts.Token).GetAsyncEnumerator();
+            ValueTask<bool> vt = e.MoveNextAsync();
+            Assert.False(vt.IsCompleted);
+
+            cts.Cancel();
+            OperationCanceledException oce = await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await vt);
+            Assert.Equal(cts.Token, oce.CancellationToken);
+
+            vt = e.MoveNextAsync();
+            Assert.True(vt.IsCompletedSuccessfully);
+            Assert.False(vt.Result);
+        }
+
         [Fact]
         public async Task WaitToReadAsync_ConsecutiveReadsSucceed()
         {