Builds and applications stopped working when I upgraded to.NET 8.0.11, and I began to receive errors. I was seeing errors like "error MSB4018: …" and "hostpolicy.dll...not found." I was unable to locate singlefilehost.exe.

1. Problem description
Build project settings all worked well somewhere around .NET 8.0.0. Later, with the upgrade of .NET runtime to later versions of .NET 8.0 and .NET 9.0 and an upgrade to Visual Studio, some of the projects stopped working. It looks like they introduced breaking changes in the build tools. Logic is still sound, and build types are the same; just the build tools started to behave a bit differently. New build configurations and build scripts are needed. I think I can locate the change in behavior somewhere between (.NET Framework 8.0.0/.NET SDK 8.0.100) and (.NET Framework 8.0.11/.NET SDK 8.0.404). Not all, but some project builds failed.

1.1. The environment
The typical environment to which this article applies is C#/VS2022.

.NET version 8.0.11 or later
And you are building project type SelfContained or SIngleFile.

1.2. Problem manifestation
You get Errors/Exceptions

• The library 'hostpolicy.dll' required to execute the application was not found.
• error MSB4018: …. Could not find file ….. singlefilehost.exe.

+++Error1, When running the application:++++++++++++++++++++

A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application
was not found in 'C:\Program Files\dotnet\'.
Failed to run as a self-contained app.
  - The application was run as a self-contained app because
  'C:\tmpNetBundle\BundleExample02_NET90\ConsoleApp2C\
  SelfContained_SingleFile_win-x64\ConsoleApp2C.runtimeconfig.json'
  was not found.
  - If this should be a framework-dependent app, add the 'C:\tmpNetBundle\BundleExample02_NET90\ConsoleApp2C\
  SelfContained_SingleFile_win-x64\ConsoleApp2C.runtimeconfig.json'
  file and specify the appropriate framework.

PS C:\tmpNetBundle\BundleExample02_NET90\ConsoleApp2C\
SelfContained_SingleFile_win-x64>

+++Error2, During build++++++++++++++++++++

error MSB4018: The "GenerateBundle" task failed unexpectedly.
[C:\tmpNetBundle\BundleExample01_NET_8.0.0_SDK_8.0.100\ConsoleApp2\ConsoleApp2.csproj]
error MSB4018: System.IO.FileNotFoundException: Could not find file
'C:\tmpNetBundle\BundleExample01_NET_8.0.0_SDK_8.0.100\ConsoleApp2\
obj\Release\net8.0-windows\win-x64\singlefilehost.exe'.

1.3. Cause of problem and resolution
It looks like the flag <PublishSingleFile>true</PublishSingleFile> in the project file .csproj stopped to work in some cases. I was relying on that flag in my build scripts after that. No problem, we can anticipate that thing, just when we know what to expect.

It looks like the build process invoked “dotnet publish” from .NET SDK 9.* for projects that I was building for .NET 8.* framework. So, I decided to use a global.json file to explicitly specify which SDK I want to use.

2. Code Samples
These are old build settings that worked in NET_8.0.0/SDK_8.0.100 but stopped working in later versions.
// NET_8.0.0/SDK_8.0.100
// NOTE: These project settings all worked well somewhere around .NET 8.0.0.
// Later, with the upgrade of .NET runtime to later versions of .NET 8.0 and .NET 9.0
// and an upgrade to Visual Studio, some of projects stopped working. It looks like
// they introduced breaking changes in the build tools. Logic is still sound and build
// types are the same, just the build tools started to behave a bit differently. A new
// build configurations and build scripts are needed.

<!--ConsoleApp2C.csproj +++++++++++++++++++++++++++++++++++++-->
<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net8.0-windows</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <PlatformTarget>x64</PlatformTarget>
        <Platforms>AnyCPU;x64</Platforms>
        <RuntimeIdentifier>win-x64</RuntimeIdentifier>
        <DebugType>embedded</DebugType>
        <PublishSingleFile>true</PublishSingleFile>
        <PublishTrimmed>true</PublishTrimmed>
        <IsTrimmable>true</IsTrimmable>
        <SelfContained>true</SelfContained>
        <IncludeAllContentForSelfExtract>true</IncludeAllContentForSelfExtract>
        <EnableCompressionInSingleFile>true</EnableCompressionInSingleFile>
    </PropertyGroup>

    <ItemGroup>
        <ProjectReference Include="..\ClassLibrary1\ClassLibraryA.csproj" />
    </ItemGroup>

    <Target Name="PostBuild" AfterTargets="PostBuildEvent">
        <Exec Command="echo +++Post-Build+++++&#xD;&#xA;
        if $(ConfigurationName) == Debug (
        &#xD;&#xA;echo +++++Debug+++++                    &#xD;&#xA;) &#xD;&#xA;&#xD;&#xA;
        if $(ConfigurationName) == Release (&#xD;&#xA;
        echo +++++SelfContained_SingleFile_win-x64.cmd+++++    &#xD;&#xA;
        call SelfContained_SingleFile_win-x64.cmd         &#xD;&#xA;
        echo +++++SelfContained_SingleFile_win-x64_Trimmed.cmd+++++    &#xD;&#xA;
        call SelfContained_SingleFile_win-x64_Trimmed.cmd         &#xD;&#xA;)    " />
    </Target>

</Project>

+++++Script: SelfContained_SingleFile_win-x64.cmd
dotnet publish ConsoleApp2C.csproj --no-build --runtime win-x64 --configuration Release
   -p:PublishSingleFile=true -p:SelfContained=true -p:PublishReadyToRun=false
   -p:PublishTrimmed=false --output ./SelfContained_SingleFile_win-x64

+++++Script: SelfContained_SingleFile_win-x64_Trimmed.cmd
dotnet publish ConsoleApp2C.csproj --no-build --runtime win-x64 --configuration Release
   -p:PublishSingleFile=true -p:SelfContained=true -p:PublishReadyToRun=false
   -p:PublishTrimmed=true --output ./SelfContained_SingleFile_win-x64_Trimmed

These are new build settings that work in NET_8.0.11/SDK_8.0.404.
// NET_8.0.11/SDK_8.0.404

<!--ConsoleApp2C.csproj +++++++++++++++++++++++++++++++++++++-->
<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net8.0-windows</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <PlatformTarget>x64</PlatformTarget>
        <Platforms>AnyCPU;x64</Platforms>
        <RuntimeIdentifier>win-x64</RuntimeIdentifier>
        <DebugType>embedded</DebugType>
        <PublishSingleFile>true</PublishSingleFile>
        <PublishTrimmed>true</PublishTrimmed>
        <IsTrimmable>true</IsTrimmable>
        <SelfContained>true</SelfContained>
        <IncludeAllContentForSelfExtract>true</IncludeAllContentForSelfExtract>
        <EnableCompressionInSingleFile>true</EnableCompressionInSingleFile>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="9.0.0" />
    </ItemGroup>

    <ItemGroup>
        <ProjectReference Include="..\ClassLibrary1\ClassLibraryA.csproj" />
    </ItemGroup>

    <Target Name="RunAfterBuild1" AfterTargets="Build">
        <Exec Command="call SelfContained_SingleFile_win-x64.cmd"
        Condition=" '$(BuildingInsideVisualStudio)' == 'true' "/>
    </Target>

    <Target Name="RunAfterBuild2" AfterTargets="Build">
        <Exec Command="call SelfContained_SingleFile_win-x64_Trimmed.cmd"
        Condition=" '$(BuildingInsideVisualStudio)' == 'true' "/>
    </Target>

</Project>

+++++Script: SelfContained_SingleFile_win-x64.cmd
echo .NET SDK version:
dotnet --version
dotnet publish ConsoleApp2C.csproj --nologo --no-restore --runtime win-x64
--configuration Release   -p:PublishSingleFile=true -p:SelfContained=true
-p:PublishReadyToRun=false  -p:PublishTrimmed=false
--output ./SelfContained_SingleFile_win-x64

+++++Script: SelfContained_SingleFile_win-x64_Trimmed.cmd
echo .NET SDK version:
dotnet --version
dotnet publish ConsoleApp2C.csproj --nologo --no-restore --runtime win-x64
 --configuration Release   -p:PublishSingleFile=true -p:SelfContained=true
 -p:PublishReadyToRun=false  -p:PublishTrimmed=true
 --output ./SelfContained_SingleFile_win-x64_Trimmed

+++++Configfile: global.json
{
  "sdk": {
    "version": "8.0.404"
  }
}

These are new build settings that work in NET_9.0.0/SDK_9.0.101.
// NET_9.0.0/SDK_9.0.101

<!--ConsoleApp3C.csproj +++++++++++++++++++++++++++++++++++++-->
<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net9.0-windows7.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <PlatformTarget>x64</PlatformTarget>
        <Platforms>AnyCPU;x64</Platforms>
        <RuntimeIdentifier>win-x64</RuntimeIdentifier>
        <DebugType>embedded</DebugType>
        <PublishSingleFile>true</PublishSingleFile>
        <PublishTrimmed>true</PublishTrimmed>
        <IsTrimmable>true</IsTrimmable>
        <SelfContained>true</SelfContained>
        <PublishReadyToRun>true</PublishReadyToRun>
        <IncludeAllContentForSelfExtract>true</IncludeAllContentForSelfExtract>
        <EnableCompressionInSingleFile>true</EnableCompressionInSingleFile>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="9.0.0" />
    </ItemGroup>

    <ItemGroup>
        <ProjectReference Include="..\ClassLibrary1\ClassLibraryA.csproj" />
    </ItemGroup>

    <Target Name="RunAfterBuild1" AfterTargets="Build">
        <Exec Command="call SelfContained_SingleFile_win-x64_ReadyToRun.cmd"
        Condition=" '$(BuildingInsideVisualStudio)' == 'true' " />
    </Target>

    <Target Name="RunAfterBuild2" AfterTargets="Build">
        <Exec Command="call SelfContained_SingleFile_win-x64_Trimmed_ReadyToRun.cmd"
        Condition=" '$(BuildingInsideVisualStudio)' == 'true' " />
    </Target>

</Project>

+++++Script: SelfContained_SingleFile_win-x64_ReadyToRun.cmd
echo .NET SDK version:
dotnet --version
dotnet publish ConsoleApp3C.csproj --nologo --no-restore --runtime win-x64
--configuration Release   -p:PublishSingleFile=true -p:SelfContained=true
-p:PublishTrimmed=false -p:PublishReadyToRun=true
--output ./SelfContained_SingleFile_win-x64_ReadyToRun

+++++Script: SelfContained_SingleFile_win-x64_Trimmed_ReadyToRun.cmd
echo .NET SDK version:
dotnet --version
dotnet publish ConsoleApp3C.csproj --nologo --no-restore --runtime win-x64
--configuration Release   -p:PublishSingleFile=true -p:SelfContained=true
-p:PublishReadyToRun=true  -p:PublishTrimmed=true
--output ./SelfContained_SingleFile_win-x64_Trimmed_ReadyToRun

+++++Configfile: global.json
{
  "sdk": {
    "version": "9.0.101"
  }
}