add System.Memory dependency (#5835)

also add useful Span-based methods for ByteString

csharp/src/Google.Protobuf/ByteString.cs

@@ -67,15 +67,6 @@ namespace Google.Protobuf
             {
                 return new ByteString(bytes);
             }
-
-            /// <summary>
-            /// Provides direct, unrestricted access to the bytes contained in this instance.
-            /// You must not modify or resize the byte array returned by this method.
-            /// </summary>
-            internal static byte[] GetBuffer(ByteString bytes)
-            {
-                return bytes.bytes;
-            }
         }
 
         /// <summary>
@@ -119,6 +110,14 @@ namespace Google.Protobuf
             get { return Length == 0; }
         }
 
+#if NETSTANDARD2_0
+        /// <summary>
+        /// Provides read-only access to the data of this <see cref="ByteString"/>.
+        /// No data is copied so this is the most efficient way of accessing.
+        /// </summary>
+        public ReadOnlySpan<byte> Span => new ReadOnlySpan<byte>(bytes);
+#endif
+
         /// <summary>
         /// Converts this <see cref="ByteString"/> into a byte array.
         /// </summary>
@@ -161,7 +160,7 @@ namespace Google.Protobuf
             int capacity = stream.CanSeek ? checked((int) (stream.Length - stream.Position)) : 0;
             var memoryStream = new MemoryStream(capacity);
             stream.CopyTo(memoryStream);
-#if NETSTANDARD1_0
+#if NETSTANDARD1_0 || NETSTANDARD2_0
             byte[] bytes = memoryStream.ToArray();
 #else
             // Avoid an extra copy if we can.
@@ -187,7 +186,7 @@ namespace Google.Protobuf
             // We have to specify the buffer size here, as there's no overload accepting the cancellation token
             // alone. But it's documented to use 81920 by default if not specified.
             await stream.CopyToAsync(memoryStream, 81920, cancellationToken);
-#if NETSTANDARD1_0
+#if NETSTANDARD1_0 || NETSTANDARD2_0
             byte[] bytes = memoryStream.ToArray();
 #else
             // Avoid an extra copy if we can.
@@ -219,6 +218,18 @@ namespace Google.Protobuf
             return new ByteString(portion);
         }
 
+#if NETSTANDARD2_0
+        /// <summary>
+        /// Constructs a <see cref="ByteString" /> from a read only span. The contents
+        /// are copied, so further modifications to the span will not
+        /// be reflected in the returned <see cref="ByteString" />.
+        /// </summary>
+        public static ByteString CopyFrom(ReadOnlySpan<byte> bytes)
+        {
+            return new ByteString(bytes.ToArray());
+        }
+#endif
+
         /// <summary>
         /// Creates a new <see cref="ByteString" /> by encoding the specified text with
         /// the given encoding.

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -7,7 +7,7 @@
     <VersionPrefix>3.7.0</VersionPrefix>
     <LangVersion>6</LangVersion>
     <Authors>Google Inc.</Authors>
-    <TargetFrameworks>netstandard1.0;net45</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;netstandard2.0;net45</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
@@ -28,9 +28,13 @@
     - Visual Studio.
     -->
   <PropertyGroup Condition="'$(OS)' != 'Windows_NT'">
-    <TargetFrameworks>netstandard1.0</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;netstandard2.0</TargetFrameworks>
   </PropertyGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+    <PackageReference Include="System.Memory" Version="4.5.2" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-18618-05" PrivateAssets="All" />
   </ItemGroup>