Use .NET Framework Reference Assemblies nuget package.

src/Discord.Net.Commands/Discord.Net.Commands.csproj

@@ -6,10 +6,14 @@
     <RootNamespace>Discord.Commands</RootNamespace>
     <Description>A Discord.Net extension adding support for bot commands.</Description>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
+    <!-- The reference assemblies package here can allow to build for .NET Framework when not using windows as well. -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" Condition="'$(TargetFramework)' == 'net461'">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
 

src/Discord.Net.Core/Discord.Net.Core.csproj

@@ -6,10 +6,14 @@
     <RootNamespace>Discord</RootNamespace>
     <Description>The core components for the Discord.Net library.</Description>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
+    <!-- The reference assemblies package here can allow to build for .NET Framework when not using windows as well. -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" Condition="'$(TargetFramework)' == 'net461'">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <!-- System.ValueTuple is in .NET Framework 4.7+, older versions need the package. -->
     <PackageReference Include="System.ValueTuple" Version="4.5.0" Condition="'$(TargetFramework)' == 'net461'" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />

src/Discord.Net.DebugTools/Discord.Net.DebugTools.csproj

@@ -5,10 +5,14 @@
     <RootNamespace>Discord</RootNamespace>
     <Description>A Discord.Net extension adding some helper classes for diagnosing issues.</Description>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net45;netstandard1.3</TargetFrameworks>
-    <TargetFramework Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.3</TargetFramework>
+    <TargetFrameworks>net45;netstandard1.3</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
+    <!-- The reference assemblies package here can allow to build for .NET Framework when not using windows as well. -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" Condition="'$(TargetFramework)' == 'net45'">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
 </Project>

src/Discord.Net.Rest/Discord.Net.Rest.csproj

@@ -6,10 +6,14 @@
     <RootNamespace>Discord.Rest</RootNamespace>
     <Description>A core Discord.Net library containing the REST client and models.</Description>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
+    <!-- The reference assemblies package here can allow to build for .NET Framework when not using windows as well. -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" Condition="'$(TargetFramework)' == 'net461'">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'net461' ">

src/Discord.Net.WebSocket/Discord.Net.WebSocket.csproj

@@ -6,11 +6,15 @@
     <RootNamespace>Discord.WebSocket</RootNamespace>
     <Description>A core Discord.Net library containing the WebSocket client and models.</Description>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <ItemGroup>
+    <!-- The reference assemblies package here can allow to build for .NET Framework when not using windows as well. -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" Condition="'$(TargetFramework)' == 'net461'">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
     <ProjectReference Include="..\Discord.Net.Rest\Discord.Net.Rest.csproj" />
   </ItemGroup>