Add target for .NET Standard 2.1

Author: brantburnett

Microsoft.IO.RecyclableMemoryStream.sln

@@ -1,14 +1,15 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25420.1
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.28803.156
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.IO.RecyclableMemoryStream", "src\Microsoft.IO.RecyclableMemoryStream.csproj", "{4B52F0CE-5F4C-43E7-A054-50CA0A502CF3}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.IO.RecyclableMemoryStream", "src\Microsoft.IO.RecyclableMemoryStream.csproj", "{4B52F0CE-5F4C-43E7-A054-50CA0A502CF3}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "UnitTests", "UnitTests\UnitTests.csproj", "{419C9F3D-71CC-4F95-8324-E15CD0D1E754}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "UnitTests", "UnitTests\UnitTests.csproj", "{419C9F3D-71CC-4F95-8324-E15CD0D1E754}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{DF4A161A-7C52-46A2-A0ED-427C1C6F29AA}"
 	ProjectSection(SolutionItems) = preProject
+		global.json = global.json
 		LICENSE = LICENSE
 		README.md = README.md
 	EndProjectSection
@@ -31,4 +32,7 @@ Global
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {D5E147AF-721B-469D-AD77-A00A290340AC}
+	EndGlobalSection
 EndGlobal

global.json

@@ -0,0 +1,5 @@
+{
+  "sdk": {
+    "version": "3.0.100-preview3-010431"
+  }
+}

src/Microsoft.IO.RecyclableMemoryStream.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.4;net40;net45;netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>netstandard1.4;netstandard2.1;net40;net45;netcoreapp2.1</TargetFrameworks>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
   <!-- for assembly signing we use a magic variable coupled with a special build definition which skips UTs -->

src/RecyclableMemoryStream.cs

@@ -530,7 +530,7 @@ public int SafeRead(byte[] buffer, int offset, int count, ref int streamPosition
             return amountRead;
         }
 
-#if NETCOREAPP2_1
+#if NETCOREAPP2_1 || NETSTANDARD2_1
         /// <summary>
         /// Reads from the current position into the provided buffer
         /// </summary>
@@ -640,7 +640,7 @@ public override void Write(byte[] buffer, int offset, int count)
             this.length = Math.Max(this.position, this.length);
         }
 
-#if NETCOREAPP2_1
+#if NETCOREAPP2_1 || NETSTANDARD2_1
         /// <summary>
         /// Writes the buffer to the stream
         /// </summary>
@@ -898,7 +898,7 @@ private int InternalRead(byte[] buffer, int offset, int count, int fromPosition)
             return amountToCopy;
         }
 
-#if NETCOREAPP2_1
+#if NETCOREAPP2_1 || NETSTANDARD2_1
         private int InternalRead(Span<byte> buffer, int fromPosition)
         {
             if (this.length - fromPosition <= 0)