Allocation-free IMemoryGroup<T> enumeration

src/ImageSharp/Memory/DiscontiguousBuffers/IMemoryGroup{T}.cs

@@ -33,5 +33,15 @@ public interface IMemoryGroup<T> : IReadOnlyList<Memory<T>>
         /// the image buffers internally.
         /// </remarks>
         bool IsValid { get; }
+
+        /// <summary>
+        /// Returns a value-type implementing an allocation-free enumerator of the memory groups in the current
+        /// instance. The return type shouldn't be used directly: just use a <see langword="foreach"/> block on
+        /// the <see cref="IMemoryGroup{T}"/> instance in use and the C# compiler will automatically invoke this
+        /// method behind the scenes. This method takes precedence over the <see cref="IEnumerable{T}.GetEnumerator"/>
+        /// implementation, which is still available when casting to one of the underlying interfaces.
+        /// </summary>
+        /// <returns>A new <see cref="MemoryGroupEnumerator{T}"/> instance mapping the current <see cref="Memory{T}"/> values in use.</returns>
+        new MemoryGroupEnumerator<T> GetEnumerator();
     }
 }

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroupEnumerator{T}.cs

@@ -0,0 +1,69 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Memory
+{
+    /// <summary>
+    /// A value-type enumerator for <see cref="MemoryGroup{T}"/> instances.
+    /// </summary>
+    /// <typeparam name="T">The element type.</typeparam>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public ref struct MemoryGroupEnumerator<T>
+        where T : struct
+    {
+        private readonly IMemoryGroup<T> memoryGroup;
+        private readonly int count;
+        private int index;
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        internal MemoryGroupEnumerator(MemoryGroup<T>.Owned memoryGroup)
+        {
+            this.memoryGroup = memoryGroup;
+            this.count = memoryGroup.Count;
+            this.index = -1;
+        }
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        internal MemoryGroupEnumerator(MemoryGroup<T>.Consumed memoryGroup)
+        {
+            this.memoryGroup = memoryGroup;
+            this.count = memoryGroup.Count;
+            this.index = -1;
+        }
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        internal MemoryGroupEnumerator(MemoryGroupView<T> memoryGroup)
+        {
+            this.memoryGroup = memoryGroup;
+            this.count = memoryGroup.Count;
+            this.index = -1;
+        }
+
+        /// <inheritdoc cref="System.Collections.Generic.IEnumerator{T}.Current"/>
+        public Memory<T> Current
+        {
+            [MethodImpl(InliningOptions.ShortMethod)]
+            get => this.memoryGroup[this.index];
+        }
+
+        /// <inheritdoc cref="System.Collections.IEnumerator.MoveNext"/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public bool MoveNext()
+        {
+            int index = this.index + 1;
+
+            if (index < this.count)
+            {
+                this.index = index;
+
+                return true;
+            }
+
+            return false;
+        }
+    }
+}

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroupView{T}.cs

@@ -5,6 +5,7 @@
 using System.Buffers;
 using System.Collections;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
@@ -37,6 +38,7 @@ public MemoryGroupView(MemoryGroup<T> owner)
 
         public int Count
         {
+            [MethodImpl(InliningOptions.ShortMethod)]
             get
             {
                 this.EnsureIsValid();
@@ -73,7 +75,15 @@ public Memory<T> this[int index]
             }
         }
 
-        public IEnumerator<Memory<T>> GetEnumerator()
+        /// <inheritdoc/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public MemoryGroupEnumerator<T> GetEnumerator()
+        {
+            return new MemoryGroupEnumerator<T>(this);
+        }
+
+        /// <inheritdoc/>
+        IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
         {
             this.EnsureIsValid();
             for (int i = 0; i < this.Count; i++)
@@ -82,7 +92,8 @@ public IEnumerator<Memory<T>> GetEnumerator()
             }
         }
 
-        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+        /// <inheritdoc/>
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<Memory<T>>)this).GetEnumerator();
 
         internal void Invalidate()
         {

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.Consumed.cs

@@ -2,16 +2,17 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Buffers;
 using System.Collections.Generic;
-using System.Linq;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
     internal abstract partial class MemoryGroup<T>
     {
-        // Analogous to the "consumed" variant of MemorySource
-        private sealed class Consumed : MemoryGroup<T>
+        /// <summary>
+        /// A <see cref="MemoryGroup{T}"/> implementation that consumes the underlying memory buffers.
+        /// </summary>
+        public sealed class Consumed : MemoryGroup<T>, IEnumerable<Memory<T>>
         {
             private readonly Memory<T>[] source;
 
@@ -22,16 +23,31 @@ public Consumed(Memory<T>[] source, int bufferLength, long totalLength)
                 this.View = new MemoryGroupView<T>(this);
             }
 
-            public override int Count => this.source.Length;
+            public override int Count
+            {
+                [MethodImpl(InliningOptions.ShortMethod)]
+                get => this.source.Length;
+            }
 
             public override Memory<T> this[int index] => this.source[index];
 
-            public override IEnumerator<Memory<T>> GetEnumerator()
+            /// <inheritdoc/>
+            [MethodImpl(InliningOptions.ShortMethod)]
+            public override MemoryGroupEnumerator<T> GetEnumerator()
+            {
+                return new MemoryGroupEnumerator<T>(this);
+            }
+
+            /// <inheritdoc/>
+            IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
             {
-                for (int i = 0; i < this.source.Length; i++)
-                {
-                    yield return this.source[i];
-                }
+                /* The runtime sees the Array class as if it implemented the
+                 * type-generic collection interfaces explicitly, so here we
+                 * can just cast the source array to IList<Memory<T>> (or to
+                 * an equivalent type), and invoke the generic GetEnumerator
+                 * method directly from that interface reference. This saves
+                 * having to create our own iterator block here. */
+                return ((IList<Memory<T>>)this.source).GetEnumerator();
             }
 
             public override void Dispose()

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.Owned.cs

@@ -5,13 +5,16 @@
 using System.Buffers;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
-    // Analogous to the "owned" variant of MemorySource
     internal abstract partial class MemoryGroup<T>
     {
-        private sealed class Owned : MemoryGroup<T>
+        /// <summary>
+        /// A <see cref="MemoryGroup{T}"/> implementation that owns the underlying memory buffers.
+        /// </summary>
+        public sealed class Owned : MemoryGroup<T>, IEnumerable<Memory<T>>
         {
             private IMemoryOwner<T>[] memoryOwners;
 
@@ -29,6 +32,7 @@ public Owned(IMemoryOwner<T>[] memoryOwners, int bufferLength, long totalLength,
 
             public override int Count
             {
+                [MethodImpl(InliningOptions.ShortMethod)]
                 get
                 {
                     this.EnsureNotDisposed();
@@ -45,7 +49,15 @@ public override Memory<T> this[int index]
                 }
             }
 
-            public override IEnumerator<Memory<T>> GetEnumerator()
+            /// <inheritdoc/>
+            [MethodImpl(InliningOptions.ShortMethod)]
+            public override MemoryGroupEnumerator<T> GetEnumerator()
+            {
+                return new MemoryGroupEnumerator<T>(this);
+            }
+
+            /// <inheritdoc/>
+            IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
             {
                 this.EnsureNotDisposed();
                 return this.memoryOwners.Select(mo => mo.Memory).GetEnumerator();
@@ -69,14 +81,21 @@ public override void Dispose()
                 this.IsValid = false;
             }
 
+            [MethodImpl(InliningOptions.ShortMethod)]
             private void EnsureNotDisposed()
             {
-                if (this.memoryOwners == null)
+                if (this.memoryOwners is null)
                 {
-                    throw new ObjectDisposedException(nameof(MemoryGroup<T>));
+                    ThrowObjectDisposedException();
                 }
             }
 
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            private static void ThrowObjectDisposedException()
+            {
+                throw new ObjectDisposedException(nameof(MemoryGroup<T>));
+            }
+
             internal static void SwapContents(Owned a, Owned b)
             {
                 a.EnsureNotDisposed();

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.cs

@@ -6,7 +6,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
-using SixLabors.ImageSharp.Memory.Internals;
 
 namespace SixLabors.ImageSharp.Memory
 {
@@ -48,10 +47,21 @@ private MemoryGroup(int bufferLength, long totalLength)
         public abstract void Dispose();
 
         /// <inheritdoc />
-        public abstract IEnumerator<Memory<T>> GetEnumerator();
+        public abstract MemoryGroupEnumerator<T> GetEnumerator();
 
         /// <inheritdoc />
-        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+        IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
+        {
+            /* This method is implemented in each derived class.
+             * Implementing the method here as non-abstract and throwing,
+             * then reimplementing it explicitly in each derived class, is
+             * a workaround for the lack of support for abstract explicit
+             * interface method implementations in C#. */
+            throw new NotImplementedException($"The type {this.GetType()} needs to override IEnumerable<Memory<T>>.GetEnumerator()");
+        }
+
+        /// <inheritdoc />
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<Memory<T>>)this).GetEnumerator();
 
         /// <summary>
         /// Creates a new memory group, allocating it's buffers with the provided allocator.

tests/ImageSharp.Tests/Memory/DiscontiguousBuffers/MemoryGroupTests.cs

@@ -1,8 +1,10 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
 using System.Buffers;
+using System.Collections;
+using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Memory;
@@ -165,7 +167,7 @@ public void GetBoundedSlice_WhenOverlapsBuffers_Throws(long totalLength, int buf
         }
 
         [Fact]
-        public void Fill()
+        public void FillWithFastEnumerator()
         {
             using MemoryGroup<int> group = this.CreateTestGroup(100, 10, true);
             group.Fill(42);
@@ -177,6 +179,34 @@ public void Fill()
             }
         }
 
+        [Fact]
+        public void FillWithSlowGenericEnumerator()
+        {
+            using MemoryGroup<int> group = this.CreateTestGroup(100, 10, true);
+            group.Fill(42);
+
+            int[] expectedRow = Enumerable.Repeat(42, 10).ToArray();
+            IReadOnlyList<Memory<int>> groupAsList = group;
+            foreach (Memory<int> memory in groupAsList)
+            {
+                Assert.True(memory.Span.SequenceEqual(expectedRow));
+            }
+        }
+
+        [Fact]
+        public void FillWithSlowEnumerator()
+        {
+            using MemoryGroup<int> group = this.CreateTestGroup(100, 10, true);
+            group.Fill(42);
+
+            int[] expectedRow = Enumerable.Repeat(42, 10).ToArray();
+            IEnumerable groupAsList = group;
+            foreach (Memory<int> memory in groupAsList)
+            {
+                Assert.True(memory.Span.SequenceEqual(expectedRow));
+            }
+        }
+
         [Fact]
         public void Clear()
         {