fix

shadowsocks-csharp/shadowsocks-csharp.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFrameworks>netcoreapp3.1</TargetFrameworks>
     <RootNamespace>Shadowsocks</RootNamespace>
     <UseWindowsForms>true</UseWindowsForms>
     <Authors>clowwindy &amp; community 2020</Authors>
@@ -13,6 +13,7 @@
     <ApplicationIcon>shadowsocks.ico</ApplicationIcon>
     <StartupObject>Shadowsocks.Program</StartupObject>
     <Nullable>disable</Nullable>
+    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
@@ -55,7 +56,8 @@
     <Reference Include="PresentationFramework" />
     <Reference Include="System.Web" />
     <Reference Include="WindowsBase" />
-    
+    <Reference Include="System.Security" />
+    <Reference Include="System.Runtime" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.1' ">
     <PackageReference Include="System.Windows.Forms.DataVisualization" Version="1.0.0-prerelease.20110.1" />