Enable multi-targeting to NetFX 4.6.1 on Windows

Enable building the class libraries and head CLI project against
.NET Framework 4.6.1 when compiling on Windows.

This enables the Windows-only authentication helpers (which build
against the Microsoft.Git.CredentialManager.dll library) to avoid
pulling in the .NET Standard type-forwarding libraries, which not
only allows us to deploy the helpers and the main GCM executable
in to the same directory without conflicting type-fwd libraries,
but also greatly reduces the number of assemblies required to
ship on Windows.

Author: mjcheetham

common/src/Git-Credential-Manager/Git-Credential-Manager.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework Condition="'$(OS)'!='Unix'">net461</TargetFramework>
     <RuntimeIdentifiers>win-x64;osx-x64</RuntimeIdentifiers>
     <AssemblyName>git-credential-manager</AssemblyName>
     <RootNamespace>Microsoft.Git.CredentialManager</RootNamespace>
@@ -16,4 +17,8 @@
     <ProjectReference Include="..\Microsoft.Git.CredentialManager\Microsoft.Git.CredentialManager.csproj" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)'=='net461'">
+    <ProjectReference Include="..\..\..\windows\src\Microsoft.Authentication.Helper\Microsoft.Authentication.Helper.csproj" />
+  </ItemGroup>
+
 </Project>

common/src/GitHub/GitHub.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFrameworks>netstandard2.0</TargetFrameworks>
+        <TargetFrameworks Condition="'$(OS)'!='Unix'">netstandard2.0;net461</TargetFrameworks>
         <RootNamespace>GitHub</RootNamespace>
         <AssemblyName>GitHub</AssemblyName>
         <IsTestProject>false</IsTestProject>
@@ -12,4 +13,8 @@
       <ProjectReference Include="..\Microsoft.Git.CredentialManager\Microsoft.Git.CredentialManager.csproj" />
     </ItemGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' == 'net461'">
+        <Reference Include="System.Net.Http" />
+    </ItemGroup>
+
 </Project>

common/src/Microsoft.AzureRepos/Microsoft.AzureRepos.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFrameworks>netstandard2.0</TargetFrameworks>
+        <TargetFrameworks Condition="'$(OS)'!='Unix'">netstandard2.0;net461</TargetFrameworks>
         <IsTestProject>false</IsTestProject>
         <LangVersion>latest</LangVersion>
         <RootNamespace>Microsoft.AzureRepos</RootNamespace>
@@ -12,4 +13,8 @@
       <ProjectReference Include="..\Microsoft.Git.CredentialManager\Microsoft.Git.CredentialManager.csproj" />
     </ItemGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' == 'net461'">
+        <Reference Include="System.Net.Http" />
+    </ItemGroup>
+
 </Project>

common/src/Microsoft.Git.CredentialManager/Microsoft.Git.CredentialManager.csproj

@@ -1,11 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFrameworks>netstandard2.0</TargetFrameworks>
+        <TargetFrameworks Condition="'$(OS)'!='Unix'">netstandard2.0;net461</TargetFrameworks>
         <IsTestProject>false</IsTestProject>
         <LangVersion>latest</LangVersion>
         <RootNamespace>Microsoft.Git.CredentialManager</RootNamespace>
         <AssemblyName>Microsoft.Git.CredentialManager</AssemblyName>
     </PropertyGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' == 'net461'">
+        <Reference Include="System.Net.Http" />
+    </ItemGroup>
+
 </Project>

common/src/Microsoft.Git.CredentialManager/PlatformUtils.cs

@@ -26,7 +26,11 @@ public static PlatformInformation GetPlatformInformation()
         /// <returns>True if running on macOS, false otherwise.</returns>
         public static bool IsMacOS()
         {
+#if NETFRAMEWORK
+            return Environment.OSVersion.Platform == PlatformID.MacOSX;
+#elif NETSTANDARD
             return RuntimeInformation.IsOSPlatform(OSPlatform.OSX);
+#endif
         }
 
         /// <summary>
@@ -35,7 +39,11 @@ public static bool IsMacOS()
         /// <returns>True if running on Windows, false otherwise.</returns>
         public static bool IsWindows()
         {
+#if NETFRAMEWORK
+            return Environment.OSVersion.Platform == PlatformID.Win32NT;
+#elif NETSTANDARD
             return RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+#endif
         }
 
         /// <summary>
@@ -44,7 +52,11 @@ public static bool IsWindows()
         /// <returns>True if running on a Linux distribution, false otherwise.</returns>
         public static bool IsLinux()
         {
+#if NETFRAMEWORK
+            return Environment.OSVersion.Platform == PlatformID.Unix;
+#elif NETSTANDARD
             return RuntimeInformation.IsOSPlatform(OSPlatform.Linux);
+#endif
         }
 
         /// <summary>
@@ -107,6 +119,9 @@ private static string GetOSType()
 
         private static string GetCpuArchitecture()
         {
+#if NETFRAMEWORK
+            return Environment.Is64BitOperatingSystem ? "x86-64" : "x86";
+#elif NETSTANDARD
             switch (RuntimeInformation.OSArchitecture)
             {
                 case Architecture.Arm:
@@ -120,11 +135,16 @@ private static string GetCpuArchitecture()
                 default:
                     return RuntimeInformation.OSArchitecture.ToString();
             }
+#endif
         }
 
         private static string GetClrVersion()
         {
+#if NETFRAMEWORK
+            return $".NET Framework {Environment.Version}";
+#elif NETSTANDARD
             return RuntimeInformation.FrameworkDescription;
+#endif
         }
 
         #endregion