update to c# 8.0

src/Net_40/HandyControl_Net_40/HandyControl_Net_40.csproj

@@ -23,7 +23,7 @@
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
     <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>pdbonly</DebugType>
@@ -34,6 +34,7 @@
     <WarningLevel>4</WarningLevel>
     <DocumentationFile>bin\Release\HandyControl.xml</DocumentationFile>
     <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
+    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\..\Shared\HandyControl_Shared\Properties\AssemblyInfo.cs">