[C#] Fix and improve project files (#6142)

* [C#] Fix and improve project files

* "net35" target included for Unity 5
* "net46" target included for Unity 2017
* "netstandard2.0" target included for Unity 2018 and general use
* "netstandard2.1" target included for Span<T> support

Included project properties for defining UNSAFE_BYTEBUFFER, BYTEBUFFER_NO_BOUNDS_CHECK, and ENABLE_SPAN_T conditional compilation symbols.

* Add documentation on building for C#

4 files changed, 82 insertions, 74 deletions

diff --git a/net/FlatBuffers/ByteBuffer.cs b/net/FlatBuffers/ByteBuffer.cs
index 4ed00627..6e0fe35b 100644
--- a/net/FlatBuffers/ByteBuffer.cs
+++ b/net/FlatBuffers/ByteBuffer.cs
@@ -14,24 +14,25 @@
  * limitations under the License.
  */
 
-// There are 3 #defines that have an impact on performance / features of this ByteBuffer implementation
+// There are three conditional compilation symbols that have an impact on performance/features of this ByteBuffer implementation.
 //
-//      UNSAFE_BYTEBUFFER 
+//      UNSAFE_BYTEBUFFER
 //          This will use unsafe code to manipulate the underlying byte array. This
 //          can yield a reasonable performance increase.
 //
 //      BYTEBUFFER_NO_BOUNDS_CHECK
 //          This will disable the bounds check asserts to the byte array. This can
-//          yield a small performance gain in normal code..
+//          yield a small performance gain in normal code.
 //
 //      ENABLE_SPAN_T
-//          This will enable reading and writing blocks of memory with a Span<T> instead if just
+//          This will enable reading and writing blocks of memory with a Span<T> instead of just
 //          T[].  You can also enable writing directly to shared memory or other types of memory
 //          by providing a custom implementation of ByteBufferAllocator.
-//          ENABLE_SPAN_T also requires UNSAFE_BYTEBUFFER to be defined
+//          ENABLE_SPAN_T also requires UNSAFE_BYTEBUFFER to be defined, or .NET
+//          Standard 2.1.
 //
 // Using UNSAFE_BYTEBUFFER and BYTEBUFFER_NO_BOUNDS_CHECK together can yield a
-// performance gain of ~15% for some operations, however doing so is potentially 
+// performance gain of ~15% for some operations, however doing so is potentially
 // dangerous. Do so at your own risk!
 //
 
diff --git a/net/FlatBuffers/FlatBuffers.Core.csproj b/net/FlatBuffers/FlatBuffers.Core.csproj
deleted file mode 100644
index 040e7716..00000000
--- a/net/FlatBuffers/FlatBuffers.Core.csproj
+++ /dev/null
@@ -1,19 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-
-    <PropertyGroup>
-        <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
-    </PropertyGroup>
-
-    <ItemGroup>
-      <Compile Remove="Properties\**" />
-    </ItemGroup>
-
-    <ItemGroup>
-      <EmbeddedResource Remove="Properties\**" />
-    </ItemGroup>
-
-    <ItemGroup>
-      <None Remove="Properties\**" />
-    </ItemGroup>
-
-</Project>
diff --git a/net/FlatBuffers/FlatBuffers.csproj b/net/FlatBuffers/FlatBuffers.csproj
index a0290947..54cba5aa 100644
--- a/net/FlatBuffers/FlatBuffers.csproj
+++ b/net/FlatBuffers/FlatBuffers.csproj
@@ -1,54 +1,23 @@
-<?xml version="1.0" encoding="utf-8"?>
-<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+<Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
-    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
-    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
-    <ProjectGuid>{28C00774-1E73-4A75-AD8F-844CD21A064D}</ProjectGuid>
-    <OutputType>Library</OutputType>
-    <AppDesignerFolder>Properties</AppDesignerFolder>
-    <RootNamespace>FlatBuffers</RootNamespace>
-    <AssemblyName>FlatBuffers</AssemblyName>
-    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
-    <FileAlignment>512</FileAlignment>
+    <TargetFrameworks>netstandard2.1;netstandard2.0;net46</TargetFrameworks>
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
-  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
-    <DebugSymbols>true</DebugSymbols>
-    <DebugType>full</DebugType>
-    <Optimize>false</Optimize>
-    <OutputPath>bin\Debug\</OutputPath>
-    <DefineConstants>DEBUG;TRACE</DefineConstants>
-    <ErrorReport>prompt</ErrorReport>
-    <WarningLevel>4</WarningLevel>
+
+  <PropertyGroup Condition="'$(UNSAFE_BYTEBUFFER)' == 'true'">
+    <DefineConstants>$(DefineConstants);UNSAFE_BYTEBUFFER</DefineConstants>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
-  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
-    <DebugType>pdbonly</DebugType>
-    <Optimize>true</Optimize>
-    <OutputPath>bin\Release\</OutputPath>
-    <DefineConstants>TRACE</DefineConstants>
-    <ErrorReport>prompt</ErrorReport>
-    <WarningLevel>4</WarningLevel>
+  <PropertyGroup Condition="'$(BYTEBUFFER_NO_BOUNDS_CHECK)' == 'true'">
+    <DefineConstants>$(DefineConstants);BYTEBUFFER_NO_BOUNDS_CHECK</DefineConstants>
   </PropertyGroup>
-  <ItemGroup>
-    <Reference Include="System" />
-    <Reference Include="System.Core" />
-  </ItemGroup>
-  <ItemGroup>
-    <Compile Include="ByteBuffer.cs" />
-    <Compile Include="FlatBufferBuilder.cs" />
-    <Compile Include="FlatBufferConstants.cs" />
-    <Compile Include="IFlatbufferObject.cs" />
-    <Compile Include="Offset.cs" />
-    <Compile Include="Properties\AssemblyInfo.cs" />
-    <Compile Include="Struct.cs" />
-    <Compile Include="Table.cs" />
+  <PropertyGroup Condition="'$(ENABLE_SPAN_T)' == 'true'">
+    <DefineConstants>$(DefineConstants);ENABLE_SPAN_T</DefineConstants>
+  </PropertyGroup>
+
+  <ItemGroup Condition="('$(ENABLE_SPAN_T)' == 'true') And (('$(TargetFramework)' == 'netstandard2.0') Or ('$(TargetFramework)' == 'net46'))">
+    <PackageReference Include="System.Memory" Version="4.5.4" />
   </ItemGroup>
-  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
-  <!-- To modify your build process, add your task inside one of the targets below and uncomment it.
-       Other similar extension points exist, see Microsoft.Common.targets.
-  <Target Name="BeforeBuild">
-  </Target>
-  <Target Name="AfterBuild">
-  </Target>
-  -->
-</Project>
\ No newline at end of file
+
+</Project>
diff --git a/net/FlatBuffers/FlatBuffers.net35.csproj b/net/FlatBuffers/FlatBuffers.net35.csproj
new file mode 100644
index 00000000..574580e3
--- /dev/null
+++ b/net/FlatBuffers/FlatBuffers.net35.csproj
@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{28C00774-1E73-4A75-AD8F-844CD21A064D}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>FlatBuffers</RootNamespace>
+    <AssemblyName>FlatBuffers</AssemblyName>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\net35</OutputPath>
+    <IntermediateOutputPath>obj\Debug\net35</IntermediateOutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\net35</OutputPath>
+    <IntermediateOutputPath>obj\Release\net35</IntermediateOutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="ByteBuffer.cs" />
+    <Compile Include="ByteBufferUtil.cs" />
+    <Compile Include="FlatBufferBuilder.cs" />
+    <Compile Include="FlatBufferConstants.cs" />
+    <Compile Include="IFlatbufferObject.cs" />
+    <Compile Include="Offset.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Struct.cs" />
+    <Compile Include="Table.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it.
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>
\ No newline at end of file