Add LoadFromAsync and SaveToAsync helper methods to DataContent (#7159)

Co-authored-by: stephentoub 
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: Copilot

eng/MSBuild/LegacySupport.props

@@ -75,7 +75,16 @@
     <Compile Include="$(MSBuildThisFileDirectory)\..\..\src\LegacySupport\CollectionBuilder\*.cs" LinkBase="LegacySupport\CollectionBuilder" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(InjectMediaTypeMapOnLegacy)' == 'true' AND ('$(TargetFramework)' == 'net462' or '$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netcoreapp3.1' or '$(TargetFramework)' == 'net8.0' or '$(TargetFramework)' == 'net9.0' or '$(TargetFramework)' == 'net10.0')">
+    <Compile Include="$(MSBuildThisFileDirectory)\..\..\src\LegacySupport\MediaTypeMap\*.cs" LinkBase="LegacySupport\MediaTypeMap" />
+  </ItemGroup>
+
   <ItemGroup Condition="'$(InjectNullabilityInfoContextOnLegacy)' == 'true' AND ('$(TargetFramework)' == 'net462' or '$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netcoreapp3.1')">
     <Compile Include="$(MSBuildThisFileDirectory)\..\..\src\LegacySupport\NullabilityInfoContext\*.cs" LinkBase="LegacySupport\NullabilityInfoContext" />
   </ItemGroup>
+
+  <!-- FilePolyfills: Adds C# 14 extension members for File methods not available on legacy frameworks -->
+  <ItemGroup Condition="'$(InjectFilePolyfillsOnLegacy)' == 'true'">
+    <Compile Include="$(MSBuildThisFileDirectory)\..\..\src\LegacySupport\FilePolyfills\*.cs" LinkBase="LegacySupport\FilePolyfills" />
+  </ItemGroup>
 </Project>

src/LegacySupport/FilePolyfills/FilePolyfills.cs

@@ -0,0 +1,82 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NET9_0_OR_GREATER
+
+#pragma warning disable CA1835 // Prefer the 'Memory'-based overloads for 'ReadAsync' and 'WriteAsync'
+
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.InteropServices;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace System.IO;
+
+/// <summary>
+/// Provides polyfill extension members for <see cref="File"/> for older frameworks.
+/// </summary>
+[ExcludeFromCodeCoverage]
+internal static class FilePolyfills
+{
+    extension(File)
+    {
+#if !NET
+        /// <summary>
+        /// Asynchronously reads all bytes from a file.
+        /// </summary>
+        /// <param name="path">The file to read from.</param>
+        /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+        /// <returns>A task that represents the asynchronous read operation, which wraps the byte array containing the contents of the file.</returns>
+        public static async Task<byte[]> ReadAllBytesAsync(string path, CancellationToken cancellationToken = default)
+        {
+            using var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 4096, useAsync: true);
+            byte[] data = new byte[stream.Length];
+            int totalRead = 0;
+            while (totalRead < data.Length)
+            {
+                int read = await stream.ReadAsync(data, totalRead, data.Length - totalRead, cancellationToken).ConfigureAwait(false);
+                if (read == 0)
+                {
+                    break;
+                }
+
+                totalRead += read;
+            }
+
+            return data;
+        }
+#endif
+
+        /// <summary>
+        /// Asynchronously writes all bytes to a file.
+        /// </summary>
+        /// <param name="path">The file to write to.</param>
+        /// <param name="bytes">The bytes to write to the file.</param>
+        /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+        /// <returns>A task that represents the asynchronous write operation.</returns>
+        public static async Task WriteAllBytesAsync(string path, ReadOnlyMemory<byte> bytes, CancellationToken cancellationToken = default)
+        {
+            // Try to avoid ToArray() if the data is backed by a byte[] with offset 0 and matching length
+            byte[] byteArray;
+            if (MemoryMarshal.TryGetArray(bytes, out ArraySegment<byte> segment) &&
+                segment.Offset == 0 &&
+                segment.Count == segment.Array!.Length)
+            {
+                byteArray = segment.Array;
+            }
+            else
+            {
+                byteArray = bytes.ToArray();
+            }
+
+#if NET
+            await File.WriteAllBytesAsync(path, byteArray, cancellationToken).ConfigureAwait(false);
+#else
+            using var stream = new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.None, bufferSize: 4096, useAsync: true);
+            await stream.WriteAsync(byteArray, 0, byteArray.Length, cancellationToken).ConfigureAwait(false);
+#endif
+        }
+    }
+}
+
+#endif

src/LegacySupport/FilePolyfills/README.md

@@ -0,0 +1,9 @@
+# About FilePolyfills
+
+This folder contains C# 14 extension member polyfills for `System.IO.File` methods
+that are not available on older frameworks.
+
+- `File.ReadAllBytesAsync` - Added in .NET Core 2.0, not available in .NET Framework 4.6.2 or .NET Standard 2.0
+
+The polyfill uses C# 14 extension members so the call site can use `File.ReadAllBytesAsync` naturally
+and it will use the real one on supported platforms and the polyfill elsewhere.