Added multiple TargetFrameworks

Author: synercoder

Directory.Build.props

@@ -44,4 +44,14 @@
     <LangVersion>8.0</LangVersion>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1'">
+    <DefineConstants>$(DefineConstants);SUPPORTS_HASHCODE</DefineConstants>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp2.1'">
+    <DefineConstants>$(DefineConstants);SUPPORTS_HASHCODE</DefineConstants>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard2.1'">
+    <DefineConstants>$(DefineConstants);SUPPORTS_HASHCODE</DefineConstants>
+  </PropertyGroup>
+
 </Project>

src/Directory.Build.props

@@ -2,6 +2,7 @@
 <Project>
 
   <PropertyGroup>
+    <TargetFrameworks>netcoreapp3.1;netcoreapp2.1;net472;netstandard2.1;netstandard2.0;netstandard1.6</TargetFrameworks>
     <MSBuildAllProjects>$(MSBuildAllProjects);$(MSBuildThisFileDirectory)..\Directory.Build.props</MSBuildAllProjects>
     <SynercodingProjectCategory>src</SynercodingProjectCategory>
   </PropertyGroup>

src/Synercoding.Primitives/Synercoding.Primitives.csproj

@@ -1,11 +1,18 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.1</TargetFramework>
     <Nullable>enable</Nullable>
     <LangVersion>8.0</LangVersion>
   </PropertyGroup>
 
   <Import Project="$(MSBuildThisFileDirectory)\PackageDetails.props" />
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'net472' Or '$(TargetFramework)' == 'netstandard2.0' Or '$(TargetFramework)' == 'netstandard1.6'">
+    <PackageReference Include="System.Memory " Version="4.5.4" />
+  </ItemGroup>
+  
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.6'">
+    <PackageReference Include="System.ValueTuple" Version="4.5.0" />
+  </ItemGroup>
+
 </Project>