From 1cee50a19de220ed16a03d9715adbb829f05e61a Mon Sep 17 00:00:00 2001
From: Matt Ellis <matell@microsoft.com>
Date: Thu, 18 Mar 2021 07:28:31 -0700
Subject: [PATCH] [System.Memory.Data] Add `Empty` to `BinaryData` (#49777)

This is useful for APIs which return `BinaryData` and want to use a
singleton instead of allocating a new `BinaryData` each
time. Previously, developers would need to create their own static
copy of an empty binary data, now they can just use `Empty`

Fixes #49670
---
 .../System.Memory.Data/ref/System.Memory.Data.cs    |  1 +
 .../System.Memory.Data/src/System/BinaryData.cs     |  5 +++++
 .../System.Memory.Data/tests/BinaryDataTests.cs     | 13 +++++++++++++
 3 files changed, 19 insertions(+)

diff --git a/src/libraries/System.Memory.Data/ref/System.Memory.Data.cs b/src/libraries/System.Memory.Data/ref/System.Memory.Data.cs
index 4d0266a607cfd..d2bb7a7622d1d 100644
--- a/src/libraries/System.Memory.Data/ref/System.Memory.Data.cs
+++ b/src/libraries/System.Memory.Data/ref/System.Memory.Data.cs
@@ -12,6 +12,7 @@ public BinaryData(byte[] data) { }
         public BinaryData(object? jsonSerializable, System.Text.Json.JsonSerializerOptions? options = null, System.Type? type = null) { }
         public BinaryData(System.ReadOnlyMemory<byte> data) { }
         public BinaryData(string data) { }
+        public static BinaryData Empty { get; }
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
         public override bool Equals([System.Diagnostics.CodeAnalysis.NotNullWhen(true)] object? obj) { throw null; }
         public static System.BinaryData FromBytes(byte[] data) { throw null; }
diff --git a/src/libraries/System.Memory.Data/src/System/BinaryData.cs b/src/libraries/System.Memory.Data/src/System/BinaryData.cs
index 8e9744a74c881..35c1fcf9ae1cd 100644
--- a/src/libraries/System.Memory.Data/src/System/BinaryData.cs
+++ b/src/libraries/System.Memory.Data/src/System/BinaryData.cs
@@ -22,6 +22,11 @@ public class BinaryData
         /// </summary>
         private readonly ReadOnlyMemory<byte> _bytes;
 
+        /// <summary>
+        /// Returns an empty BinaryData.
+        /// </summary>
+        public static BinaryData Empty { get; } = new BinaryData(ReadOnlyMemory<byte>.Empty);
+
         /// <summary>
         /// Creates a <see cref="BinaryData"/> instance by wrapping the
         /// provided byte array.
diff --git a/src/libraries/System.Memory.Data/tests/BinaryDataTests.cs b/src/libraries/System.Memory.Data/tests/BinaryDataTests.cs
index b3ffa187f454b..9486ac8270f46 100644
--- a/src/libraries/System.Memory.Data/tests/BinaryDataTests.cs
+++ b/src/libraries/System.Memory.Data/tests/BinaryDataTests.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Runtime.InteropServices;
@@ -572,6 +573,18 @@ public void CloseStreamValidation()
 
         }
 
+        [Fact]
+        public void EmptyIsEmpty()
+        {
+            Assert.Equal(Array.Empty<byte>(), BinaryData.Empty.ToArray());
+        }
+
+        [Fact]
+        public void EmptyIsSingleton()
+        {
+            Assert.Same(BinaryData.Empty, BinaryData.Empty);
+        }
+
         private class TestModel
         {
             public string A { get; set; }