This is an experiment that replaces DEFLATE with Zstandard in single-file published applications for compression of bundled managed assemblies.

Edit: This has been quite complicated to figure out how to test, documenting the steps below for future reference.

• `build.cmd -s Clr+Libs+Packs -c Release
• extract "artifacts\packages\Release\Shipping\dotnet-runtime-11.0.0-dev-win-x64.zip" somewhere
• copy sdk folder from local .dotnet to the folder above
• replace all sdk**\11.0.100-alpha.1.26064.118\Microsoft.NET.HostModel.dll with the dll from artifacts\bin\Microsoft.NET.HostModel\Release\net11.0\Microsoft.NET.HostModel.dll
  • This file contains the Bundler which does the compression during publish process.
• follow steps to dogfood locally built packages and singlefilehost:
  • https://github.com/dotnet/runtime/blob/99e94ce55d79ea7d702f4d1e0c5fc70f2677670f/c:/source/dotnet/runtime/docs/workflow/testing/host/using-apphost.md
  • https://github.com/dotnet/runtime/blob/6936d807ab02592e582c999fcfa39bff39c632b6/c:/source/dotnet/runtime/docs/workflow/testing/using-dev-shipping-packages.md

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net11.0</TargetFramework>
    <RootNamespace>test_app</RootNamespace>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>

    <!-- Compression is opt-in -->
    <EnableCompressionInSingleFile>true</EnableCompressionInSingleFile>

    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
    <PublishSingleFile>true</PublishSingleFile>
    <SingleFileHostSourcePath>C:\source\dotnet\runtime\artifacts\bin\win-x64.Release\corehost\singlefilehost.exe</SingleFileHostSourcePath>
  </PropertyGroup>

<ItemGroup>
  <FrameworkReference Update="Microsoft.NETCore.App" RuntimeFrameworkVersion="11.0.0-dev" />
</ItemGroup>

</Project>

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <config>
    <!-- use local nuget cache which you can delete between builds -->
    <add key="globalPackagesFolder" value="C:\source\repros\2026-02-02-zstd-bundling\test-app\nuget_cache" />
  </config>

  <packageSources>
    <!--To inherit the global NuGet package sources remove the <clear/> line below -->
    <clear />
    <!-- <add key="nuget" value="https://api.nuget.org/v3/index.json" /> -->
    <add key="dotnet11" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet11/nuget/v3/index.json" />
    <add key="local runtime" value="D:\dotnet\runtime\artifacts\packages\Release\Shipping" />
  </packageSources>
</configuration>

• If changes to the singlefilehost are made (e.g. peimagelayout.cpp), entire Clr subset
• If you make changes to the bundler (Microsoft.NET.HostModel.dll) delete the bin/obj folders of the test application to force repacking.

The testing application was a simple ASP.NET hello world with the weather API, with immediate exit after startup

if (args.Length == 0)
{
    return;
}

Startup speed impact

I temporarily put extra logs in the apphost to verify that we decompress at least some images, and there seem to have been about 20 of them. Startup times themselves had quite a multimodal distribution (probably due to filesystem caching), the times below show the lowest modality:

I would expect similar diffs on higher modalities, there was no difference across different compression level options that I could measure, so I did not include more lines in the table above

Build time and binary size impact:

For measuring build times and actual binary sizes I measured the execution of dotnet publish ... after deleting bin and obj folders (keeping the nuget_cache intact).

Looks like for Quality values between 5 and 11, we start getting small gains in both startup time and binary size, but they seem rather small, we can squeeze some more gains by training a dictionary on the usual dll files (runtime+aspnet?), but this shaves only about additional 1MB at most, so is likely not worth the extra logistics around embedding the decompression dictionary in the singlefile exe