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Python as a post-build step to C#

I'm looking into ways to execute python as part of a C# build.
Specifically, I want to create a Python package based on a C# project through python.net. My general idea was to build the C# project first. And then, as some sort of post-build step, invoke python to build a package based on the newly generated NET assemblies.
I can't presume python will installed on the build host, so ideally I want to include a "portable" - even more ideally, nuget-based - python distribution.
I have found a promising nuget package, but am not entirely sure of its usage. It incldues no C# code, but has all python binaries included, and has build props as copy/pasted below for reference.
Given on that package's props - can I somehow reference its binaries from my own project as a post-build step?
Say, for example, I want to add a post-build step to my own project, that simply just invokes "python.exe" after the build. How could I do that?
My own project:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp2.1</TargetFramework>
<GenerateProgramFile>false</GenerateProgramFile>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="python" Version="3.10.0-a6" />
</ItemGroup>
<Target Name="MyCustomStep" AfterTargets="Build">
<!-- .. now what? I can't seem to access. e.g. #(PythonHome) or $(PythonHome) from here --/>
<Target>
</Project>
Props of the python package from nuget:
<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup Condition="$(Platform) == 'X64'">
<PythonHome Condition="$(PythonHome) == ''">$([System.IO.Path]::GetFullPath("$(MSBuildThisFileDirectory)\..\..\tools"))</PythonHome>
<PythonInclude>$(PythonHome)\include</PythonInclude>
<PythonLibs>$(PythonHome)\libs</PythonLibs>
<PythonTag>3.10</PythonTag>
<PythonVersion>3.10.0-a6</PythonVersion>
<IncludePythonExe Condition="$(IncludePythonExe) == ''">true</IncludePythonExe>
<IncludeDistutils Condition="$(IncludeDistutils) == ''">false</IncludeDistutils>
<IncludeLib2To3 Condition="$(IncludeLib2To3) == ''">false</IncludeLib2To3>
<IncludeVEnv Condition="$(IncludeVEnv) == ''">false</IncludeVEnv>
<GetPythonRuntimeFilesDependsOn>_GetPythonRuntimeFilesDependsOn310_None;$(GetPythonRuntimeFilesDependsOn)</GetPythonRuntimeFilesDependsOn>
</PropertyGroup>
<ItemDefinitionGroup Condition="$(Platform) == 'X64'">
<ClCompile>
<AdditionalIncludeDirectories>$(PythonInclude);%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
<RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
</ClCompile>
<Link>
<AdditionalLibraryDirectories>$(PythonLibs);%(AdditionalLibraryDirectories)</AdditionalLibraryDirectories>
</Link>
</ItemDefinitionGroup>
<Target Name="GetPythonRuntimeFiles" Returns="#(PythonRuntime)" DependsOnTargets="$(GetPythonRuntimeFilesDependsOn)" />
<Target Name="_GetPythonRuntimeFilesDependsOn310_None" Returns="#(PythonRuntime)">
<ItemGroup>
<_PythonRuntimeExe Include="$(PythonHome)\python*.dll" />
<_PythonRuntimeExe Include="$(PythonHome)\python*.exe" Condition="$(IncludePythonExe) == 'true'" />
<_PythonRuntimeExe>
<Link>%(Filename)%(Extension)</Link>
</_PythonRuntimeExe>
<_PythonRuntimeDlls Include="$(PythonHome)\DLLs\*.pyd" />
<_PythonRuntimeDlls Include="$(PythonHome)\DLLs\*.dll" />
<_PythonRuntimeDlls>
<Link>DLLs\%(Filename)%(Extension)</Link>
</_PythonRuntimeDlls>
<_PythonRuntimeLib Include="$(PythonHome)\Lib\**\*" Exclude="$(PythonHome)\Lib\**\*.pyc;$(PythonHome)\Lib\site-packages\**\*" />
<_PythonRuntimeLib Remove="$(PythonHome)\Lib\distutils\**\*" Condition="$(IncludeDistutils) != 'true'" />
<_PythonRuntimeLib Remove="$(PythonHome)\Lib\lib2to3\**\*" Condition="$(IncludeLib2To3) != 'true'" />
<_PythonRuntimeLib Remove="$(PythonHome)\Lib\ensurepip\**\*" Condition="$(IncludeVEnv) != 'true'" />
<_PythonRuntimeLib Remove="$(PythonHome)\Lib\venv\**\*" Condition="$(IncludeVEnv) != 'true'" />
<_PythonRuntimeLib>
<Link>Lib\%(RecursiveDir)%(Filename)%(Extension)</Link>
</_PythonRuntimeLib>
<PythonRuntime Include="#(_PythonRuntimeExe);#(_PythonRuntimeDlls);#(_PythonRuntimeLib)" />
</ItemGroup>
<Message Importance="low" Text="Collected Python runtime from $(PythonHome):%0D%0A#(PythonRuntime->' %(Link)','%0D%0A')" />
</Target>
</Project>

That is for the use of internal nuget rather than your main project. You cannot get that property under main project.
You have to use my function:
1) edit csproj file and set this for your PackageReference python
<GeneratePathProperty>true</GeneratePathProperty>
Like this:
<ItemGroup>
<PackageReference Include="python" Version="3.10.0-a6">
<GeneratePathProperty>true</GeneratePathProperty>
</PackageReference>
</ItemGroup>
2) Then, you can use $(Pkgpython) to get that path.
<Target Name="MyCustomStep" AfterTargets="Build">
<Exec Command="$(Pkgpython)\tools\python.exe" />
</Target>

.NET Azure Functions - Dependency Injection Issue

On Startup.cs in my Azure Function v2 project:
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection;
using MyCompany.MyLib.Contracts; //namespace from external library
[assembly: FunctionsStartup(typeof(Startup))]
namespace Test
{
public class Startup : FunctionsStartup
{
public override void Configure(IFunctionsHostBuilder builder)
{
builder.Services.AddTransient(typeof(Logging.ILogger<>), typeof(Logging.Logger<>));
builder.Services.AddTransient<IUserLogic, UserLogic>();
builder.Services.AddTransient<IBillingLogic, BillingLogic>(); //---> loads up from above referenced "MyCompany.MyLib.Contracts" namespace and this namespace is from externally referenced class library but with in same solution
}
}
}
The above code with my own custom classes within function app project like "EmailLogic", "Logger" works fine.
But the moment I added up custom classes to services container like "BillingLogic" from external C# library project which is added as reference project from the existing visual studio solution it throws up below issue:
"A host error has occurred during startup operation '945918c0-af3a-4d50-ab1d-ac405d4f1c7b'. [2/3/2020 2:11:02 PM] MyFunction.FunctionApp: Could not load file or assembly 'MyCompany.MyLib.Contracts, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. Could not find or load a specific file. (Exception from HRESULT: 0x80131621). System.Private.CoreLib: Could not load file or assembly ''MyCompany.MyLib.Contracts, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
If these lines from "referenced external projects" are removed,
using MyCompany.MyLib.Contracts;
builder.Services.AddTransient<IBillingLogic, BillingLogic>();
startup.cs works as expected but referring this external class from referenced project is must for my solution.
My Azure function csproj file:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<AzureFunctionsVersion>v2</AzureFunctionsVersion>
<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
<AutoGenerateBindingRedirects>false</AutoGenerateBindingRedirects>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.Azure.Functions.Extensions" Version="1.0.0" />
<PackageReference Include="Microsoft.Azure.Storage.Queue" Version="11.1.2" />
<PackageReference Include="Microsoft.Azure.WebJobs.Extensions.Storage" Version="3.0.8" />
<PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="2.2.0" />
<PackageReference Include="Microsoft.NET.Sdk.Functions" Version="1.0.29" />
<PackageReference Include="NLog" Version="4.6.8" />
<PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
<PackageReference Include="NLog.Extensions.AzureStorage" Version="1.1.4" />
<PackageReference Include="Microsoft.Azure.DocumentDB.Core" Version="2.9.2" />
</ItemGroup>
<ItemGroup>
<ProjectReference Include="..\MyCSharpLib.DataStore\MyCSharpLib.DataStore.csproj">
<Private>true</Private>
</ProjectReference>
</ItemGroup>
<ItemGroup>
<None Update="appsettings.json">
<CopyToOutputDirectory>Always</CopyToOutputDirectory>
<CopyToPublishDirectory>Always</CopyToPublishDirectory>
</None>
<None Update="host.json">
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</None>
<None Update="local.settings.json">
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
<CopyToPublishDirectory>Never</CopyToPublishDirectory>
</None>
<None Update="nlog.config">
<CopyToOutputDirectory>Always</CopyToOutputDirectory>
<CopyToPublishDirectory>Always</CopyToPublishDirectory>
</None>
</ItemGroup>
</Project>
MyCSharpLib.DataStore.csproj file:
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<RuntimeIdentifier>win7-x64</RuntimeIdentifier>
<Platforms>x64</Platforms>
</PropertyGroup>
<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" />
<ItemGroup>
<PackageReference Include="Microsoft.Azure.Cosmos.Table" Version="1.0.6" />
<PackageReference Include="Microsoft.Azure.DocumentDB.Core" Version="2.9.2" />
<PackageReference Include="Microsoft.Azure.Storage.Blob" Version="11.1.1" />
<PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
<PackageReference Include="Newtonsoft.Json.Bson" Version="1.0.1" />
<PackageReference Include="Polly" Version="5.3.1" />
<PackageReference Include="StackExchange.Redis" Version="1.2.6" />
</ItemGroup>
<ItemGroup>
<ProjectReference Include="..\MyContractLib.Contracts\MyContractLib.Contracts.csproj" />
</ItemGroup>
</Project>

MyCSharpLib.DataStore
.\MyContractLib.Contracts\MyContractLib.Contracts.csproj
My Azure function csproj file:
<ProjectReference Include="..\MyCSharpLib.DataStore\MyCSharpLib.DataStore.csproj">
so
using MyCompany.MyLib.Contracts;
is coming through the ref to DataStore which then has ref to MyContractLib.Contracts
But it is not coping the dll as its silly, so either get Azure function csproj to ref MyLib.Contracts
or do this
How to set dependencies when I use .NET Standard 2.0 DLL libraries with a .NET Framework console application?
which is on all your std libs add
<RestoreProjectStyle>PackageReference</RestoreProjectStyle>
so on both your standard libs
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<RestoreProjectStyle>PackageReference</RestoreProjectStyle>
</PropertyGroup>
if this does not work i will delete

Change assembly name for build settings in .csproj

I have an application which I want to publish with ClickOnce via command line. I have a test and a live version. It should be allowed to have both installed at the same time, which means that I need to change the assemble name (and preferably also the product name) for one of the builds. I would like to do this in the build settings.
I have managed to make some build settings, which works fine, but I cannot figure out how to change the assembly and product name, for just one of them.
I have added the following code to my .csproj file, which I call with the command msbuild /target:Test or msbuild /target:Live. But where do I implement the assembly and product name change?
<PropertyGroup>
<ProjLocation>$(ProjectDir)</ProjLocation>
<ProjLocationReleaseDir>$(ProjLocation)\bin\Debug</ProjLocationReleaseDir>
<ProjPublishLocation>$(ProjLocationReleaseDir)\app.publish</ProjPublishLocation>
<DeploymentFolder>C:\MyProjects\Software\Publish\</DeploymentFolder>
</PropertyGroup>
<!-- Build settings for live version -->
<Target Name="Live" DependsOnTargets="Clean">
<MSBuild Projects="$(ProjLocation)\$(ProjectName).csproj"
Properties="$(DefaultBuildProperties)"
Targets="Publish"/>
<ItemGroup>
<SetupFiles Include="$(ProjPublishLocation)\*.*"/>
<UpdateFiles Include="$(ProjPublishLocation)\Application Files\**\*.*"/>
</ItemGroup>
<Copy SourceFiles="#(SetupFiles)" DestinationFolder="$(DeploymentFolder)\Live\" />
<Copy SourceFiles="#(UpdateFiles)" DestinationFolder="$(DeploymentFolder)\Live\Application Files\%(RecursiveDir)"/>
</Target>
<!-- Build settings for test version -->
<Target Name="Test" DependsOnTargets="Clean">
<MSBuild Projects="$(ProjLocation)\$(ProjectName).csproj"
Properties="$(DefaultBuildProperties)"
Targets="Publish"/>
<ItemGroup>
<SetupFiles Include="$(ProjPublishLocation)\*.*"/>
<UpdateFiles Include="$(ProjPublishLocation)\Application Files\**\*.*"/>
</ItemGroup>
<Copy SourceFiles="#(SetupFiles)" DestinationFolder="$(DeploymentFolder)\Public Test\" />
<Copy SourceFiles="#(UpdateFiles)" DestinationFolder="$(DeploymentFolder)\Public Test\Application Files\%(RecursiveDir)"/>
</Target>

You can add "AssemblyName" property to the PropertyGroup. like this:
<PropertyGroup>
<AssemblyName>YourAppName</AssemblyName>
</PropertyGroup>
or you can use the MSBuild command line switch. like this :
msbuild /property:AssemblyName=YourAppName

I had almost the same task (need to distinguish between Staging and Production) and I solved it with the following MSBuild-Target:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<!-- call "nuget restore Deployment\packages.config" before executing this script -->
<Import Project="..\packages\MSBuildTasks.1.5.0.235\build\MSBuildTasks.targets" />
<PropertyGroup>
<Configuration>Release</Configuration>
<ClientProject>..\MyProject\MyProject.vbproj</ClientProject>
<ClientPublishDir Condition="$(Environment) == 'Staging'">\\deployment-staging\MyProject\</ClientPublishDir>
<ClientPublishDir Condition="$(Environment) == 'Production'">\\deployment\MyProject\</ClientPublishDir>
</PropertyGroup>
<ItemGroup>
<ClientModifiedAppConfig Include="$(ClientProject)\..\App.$(Environment).config" />
</ItemGroup>
<Target Name="DeployClient">
<Error Condition="$(Environment) == ''" Text="The Property 'Environment' has not been set." />
<Error Condition="$(Environment) != 'Staging' AND $(Environment) != 'Production'" Text="The Property 'Environment' has not been set properly. Valid values are 'Staging' and 'Production'." />
<!-- Sets different assembly names for INT and PRD applications. Due to this, both INT and PRD applications with the
same version number can be installed on the same system. -->
<XmlUpdate Condition="$(Environment) == 'Staging'" Prefix="n" Namespace="http://schemas.microsoft.com/developer/msbuild/2003" XmlFileName="$(ClientProject)" Xpath="/n:Project/n:PropertyGroup/n:AssemblyName" Value="MyProjectStaging" />
<XmlUpdate Condition="$(Environment) == 'Staging'" Prefix="n" Namespace="http://schemas.microsoft.com/developer/msbuild/2003" XmlFileName="$(ClientProject)" Xpath="/n:Project/n:PropertyGroup/n:ProductName" Value="MyProject Staging" />
<XmlUpdate Condition="$(Environment) == 'Production'" Prefix="n" Namespace="http://schemas.microsoft.com/developer/msbuild/2003" XmlFileName="$(ClientProject)" Xpath="/n:Project/n:PropertyGroup/n:AssemblyName" Value="MyProject" />
<XmlUpdate Condition="$(Environment) == 'Production'" Prefix="n" Namespace="http://schemas.microsoft.com/developer/msbuild/2003" XmlFileName="$(ClientProject)" Xpath="/n:Project/n:PropertyGroup/n:ProductName" Value="MyProject" />
<!-- Overwrites the original App.config with the environment-dependent App.config.
Reason: ClickOnce only uses App.config and does not apply transformations, as it is done in Web Projects. -->
<Copy
SourceFiles="#(ClientModifiedAppConfig)"
DestinationFiles="$(ClientProject)\..\App.config"
OverwriteReadOnlyFiles="true"
/>
<!-- Publish -->
<MSBuild
Projects="$(ClientProject)"
Targets="Publish"
Properties="
PublishDir=$(ClientPublishDir);
Configuration=$(Configuration);
Platform=x86" />
</Target>
</Project>
To make this work, you have to restore the NuGet package MSBuildTasks.
Finally, all I have to call is msbuild Deployment.targets /t:DeployClient /p:Environment=Staging
Hope that helps!

Multiple Errors in Razor -- Missing Include? -- Visual Studio for Mac - C# / MVC / Entity Framework

I am experimenting with Visual Studio on the Mac. I have a _layout.cshtml file that works fine in the Windows version of VS but I am getting multiple errors when executing it on the Mac.
The error is:
One or more compilation references are missing. Ensure that your project is referencing 'Microsoft.NET.Sdk.Web' and the 'PreserveCompilationContext' property is not set to false.
And it is flagging many common razor function such as
The type or namespace name 'HtmlString' could not be found (are you missing a using directive or an assembly reference?)
private HtmlString WriteOption(string Menu)
The name 'Styles' does not exist in the current context
#Styles.Render( "~/Content/css" )
The name 'Scripts' does not exist in the current context
#Scripts.Render( "~/bundles/modernizr" )
The name 'Session' does not exist in the current context
int CurrentCustomerId = (Session["CurrentCustomerId"] == null ? -1 : (int)Session["CurrentCustomerId"]);
I've tried numerous things such as
adding and removing Microsoft.AspNetCore
ensuring PreserveCompilationContext is set to true
adding Microsoft.AspNetCoreRazor packages
adding: #using HttpContext.Current;
adding: #using System.Web.Optimization;
rebooting, cleaning and rebuilding....
This is what my .csproj file looks like:
<Project Sdk="Microsoft.NET.Sdk.Web">
<PropertyGroup>
<TargetFramework>netcoreapp2.0</TargetFramework>
<PreserveCompilationContext>true</PreserveCompilationContext>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Newtonsoft.Json" Version="10.0.3" />
<PackageReference Include="jQuery" Version="3.3.1" />
<PackageReference Include="bootstrap" Version="4.0.0" />
<PackageReference Include="log4net" Version="2.0.8" />
<PackageReference Include="FontAwesome" Version="4.7.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite.Core" Version="2.0.1" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite.Design" Version="1.1.5" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="2.0.1" />
<PackageReference Include="EntityFramework" Version="6.2.0" />
<PackageReference Include="Microsoft.AspNetCore" Version="2.0.1" />
<PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.0.2" />
<PackageReference Include="Microsoft.AspNetCore.All" Version="2.0.5" />
<PackageReference Include="Microsoft.AspNetCore.Mvc.Razor" Version="2.0.2" />
<PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="2.0.2" />
<PackageReference Include="Microsoft.AspNet.Razor" Version="3.2.3" />
</ItemGroup>
<ItemGroup>
<DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="2.0.0" />
</ItemGroup>
<ItemGroup>
<Folder Include="Data\" />
<Folder Include="Views\Customer\" />
<Folder Include="Views\Contact\" />
<Folder Include="Views\User\" />
<Folder Include="wwwroot\img\" />
<Folder Include="Helpers\" />
</ItemGroup>
<ItemGroup>
<Compile Remove="Views\Customer\Index.cshtml.cs" />
</ItemGroup>
<ItemGroup>
<Content Remove="Views\Home\About.cshtml" />
<Content Remove="Views\Home\Contact.cshtml" />
<Content Remove="wwwroot\images\banner1.svg" />
<Content Remove="wwwroot\images\banner2.svg" />
<Content Remove="wwwroot\images\banner3.svg" />
<Content Remove="wwwroot\images\banner4.svg" />
</ItemGroup>
</Project>
Here's the IDE vitals:
Visual Studio Community 2017 for Mac
Version 7.3.3 (build 12)
Installation UUID: 0237be3a-3db7-4f5c-af82-a9699df5a1f9
Runtime:
Mono 5.8.0.108 (2017-10/9aa78573ee2) (64-bit)
GTK+ 2.24.23 (Raleigh theme)
Package version: 508000108
NuGet
Version: 4.3.1.4445
.NET Core
Runtime: /usr/local/share/dotnet/dotnet
Runtime Version: 2.0.5
SDK: /usr/local/share/dotnet/sdk/2.1.4/Sdks
SDK Version: 2.1.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.8.0/lib/mono/msbuild/15.0/bin/Sdks
Any suggestions?

There is a difference between "ASP.NET Web Application" and "ASP.NET Core Web Application". Your project is of the first type which uses .Net Framework and does not work on Mac. The "Core" type uses .Net Core and is supported on Mac.
The project types have some differences, like the absence of #Styles and #Scripts. More differences can be found here: http://www.mithunvp.com/difference-between-asp-net-mvc6-asp-net-mvc5/
If you would like to migrate your project, you can use this guide: https://learn.microsoft.com/en-us/aspnet/core/migration/mvc

Condition using File::Exists not working

I currently create my first MSBuild-script.
I've a tag "Folders" that findes all Directories in a given root path:
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
<Target Name="Build">
<PropertyGroup>
<RootFolder>tmp</RootFolder>
</PropertyGroup>
<ItemGroup>
<Folders Include="$([System.IO.Directory]::GetDirectories("$(RootFolder)"))"/>
</ItemGroup>
<Message Text="#(Folders -> '%(FullPath)\Bin\Debug\%(Filename)%(Extension).dll', ';')"/>
</Target>
</Project>
That works perfect.
My problem is that I only need directories where the specified file exists.
I tried a condition like that
Condition="$([System.IO.File]::Exists("%(FullPath)\\Bin\\Debug\\%(Filename)%(Extension).dll"))"
for the folder tag.
This script runs without any error but my list is empty.
Why?
Are there any other solutions to check for a file?
I used this solution because it uses C# and I'm a C#-developer.
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
<Target Name="Build">
<PropertyGroup>
<RootFolders>tmp</RootFolders>
</PropertyGroup>
<GetFiles rootFolders="$(RootFolders)">
<Output PropertyName="Files" TaskParameter="Files" />
</GetFiles>
<Message Text="$(Files)" />
</Target>
<UsingTask
TaskName="GetFiles"
TaskFactory="CodeTaskFactory"
AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
<ParameterGroup>
<rootFolders ParameterType="System.String" Required="true" />
<files ParameterType="System.String" Output="true" />
</ParameterGroup>
<Task>
<Using Namespace="System" />
<Using Namespace="System.IO" />
<Using Namespace="System.Linq" />
<Code Type="Fragment" Language="cs">
<![CDATA[
Func<string, string> BuildFilePath = path => path + #"\Bin\Debug\" + Path.GetFileName(path) + ".dll";
var dirs = Directory.GetDirectories(rootFolders).Where(x => File.Exists(BuildFilePath(x)));
files = string.Join("\n", dirs.Select(BuildFilePath));
]]>
</Code>
</Task>
</UsingTask>
</Project>

AFAIK, the thing is Condition is executed and checked for the whole declaration of Items (i.e. <Folders ..> tag).
I think, you need to loop through the collection (e.g. using target/task batching) and check the file to exist in every single folder folder in the collection. Then if the file exists - include it in the new <FoldersFiletered> items collection.
NB: I don't have time to test the code now, but this is the idea roughly:
<Target Name="FilterFolders"
Inputs="#(Folders)"
Outputs="%(FullPath)">
<ItemGroup>
<FoldersFiltered Include="#(Folders->'%(FullPath)')"
Condition="$([System.IO.File]::Exists("#(Folders->'%(FullPath)'\\Bin\\Debug\\YourFile.dll"))" />
</ItemGroup>
</Target>