Experimental: Reducing the size of .NET Core applications with Mono’s Linker

The .NET team has built a linker to reduce the size of .NET Core applications. It is built on top of the excellent and battle-tested mono linker. The Xamarin tools also use this linker so it makes sense to try it out and perhaps use it everywhere!

"In trivial cases, the linker can reduce the size of applications by 50%. The size wins may be more favorable or more moderate for larger applications. The linker removes code in your application and dependent libraries that are not reached by any code paths. It is effectively an application-specific dead code analysis." - Using the .NET IL Linker

I recently updated a 15 year old .NET 1.1 application to cross-platform .NET Core 2.0 so I thought I'd try this experimental linker on it and see the results.

The linker is a tool one can use to only ship the minimal possible IL code and metadata that a set of programs might require to run as opposed to the full libraries. It is used by the various Xamarin products to extract only the bits of code that are needed to run an application on Android, iOS and other platforms.

I'll add this line to a nuget.config in my project's folder. Note that NuGet will inherit global settings and ADD this line.

<?xml version="1.0" encoding="utf-8"?>

<configuration>

 <packageSources>

    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />

 </packageSources>

</configuration>

Then I'll add the IL Linker's NuGet package to my project with this command line command (or from Visual Studio):

dotnet add package ILLink.Tasks -v 0.1.4-preview-906439

The assemblies will automatically be "trimmed" when they are published (not built) so I'll build it twice, disabling it with a switch:

D:githubTinyOSOS Project>dotnet publish -c release -r win-x64 -o notlinked /p:LinkDuringPublish=false

Microsoft (R) Build Engine version 15.3 for .NET Core


  TinyOSCore -> D:githubTinyOSOS ProjectbinReleasenetcoreapp2.0win-x64TinyOSCore.dll

  TinyOSCore -> D:githubTinyOSOS Projectnotlinked


D:githubTinyOSOS Project>dotnet publish -c release -r win-x64 -o linked

Microsoft (R) Build Engine version 15.3 for .NET Core


  TinyOSCore -> D:githubTinyOSOS ProjectbinReleasenetcoreapp2.0win-x64TinyOSCore.dll

  TinyOSCore -> D:githubTinyOSOS Projectlinked

And here's the results:

You can also run it with  /p:ShowLinkerSizeComparison=true and get a nice table. I've trimmed the table as it's super long.

  TinyOSCore -> D:githubTinyOSOS ProjectbinReleasenetcoreapp2.0win-x64TinyOSCore.dll

                                                             Before linking (B)   After linking (B)    Size decrease

-----------                                                  -----------          -----------          -----------

Total size of assemblies                                     48,025,824           16,740,056           65.14%

-----------                                                  -----------          -----------          -----------

TinyOSCore.dll                                               36,352               36,352               0.00%

Microsoft.Extensions.Configuration.dll                       24,584               24,584               0.00%

Microsoft.Extensions.Configuration.Abstractions.dll          20,480               20,480               0.00%

Microsoft.Extensions.Configuration.Binder.dll                24,064               24,064               0.00%

Microsoft.Extensions.Configuration.FileExtensions.dll        22,528               22,528               0.00%

Microsoft.Extensions.Configuration.Json.dll                  24,072               24,072               0.00%

Microsoft.Extensions.DependencyInjection.dll                 46,600               46,600               0.00%

Microsoft.Extensions.DependencyInjection.Abstractions.dll    35,336               35,336               0.00%

Microsoft.Extensions.FileProviders.Abstractions.dll          17,920               17,920               0.00%

Microsoft.Extensions.FileProviders.Physical.dll              31,240               31,240               0.00%

Microsoft.Extensions.FileSystemGlobbing.dll                  39,432               39,432               0.00%

Microsoft.Extensions.Options.dll                             26,120               26,120               0.00%

Microsoft.Extensions.Options.ConfigurationExtensions.dll     16,904               16,904               0.00%

Microsoft.Extensions.Primitives.dll                          33,800               33,800               0.00%

Newtonsoft.Json.dll                                          639,488              639,488              0.00%

Microsoft.CSharp.dll                                         1,092,096            392,192              64.09%

Microsoft.VisualBasic.dll                                    465,416              0                    100.00%

Microsoft.Win32.Primitives.dll                               18,968               4,608                75.71%

Microsoft.Win32.Registry.dll                                 85,008               0                    100.00%

SOS.NETCore.dll                                              54,264               0                    100.00%

System.AppContext.dll                                        14,336               2,560                82.14%

System.Buffers.dll                                           14,336               2,560                82.14%

System.Collections.Concurrent.dll                            206,360              31,744               84.62%

System.Collections.Immutable.dll                             2,378,264            0                    100.00%

System.Collections.NonGeneric.dll                            96,792               24,576               74.61%

System.Collections.Specialized.dll                           88,608               15,360               82.67%

System.Collections.dll                                       326,664              52,224               84.01%


  TinyOSCore -> D:githubTinyOSOS ProjectbinReleasenetcoreapp2.0win-x64publish

You can see in some places where there's no size decrease. That's because I'm using those assemblies completely. Some see a 100% decrease - they've been removed entirely - because I'm not using the Registry, for example. And some see a fractional decrease because I'm using some methods but not others.

You can check out the full instructions and try this yourself at https://github.com/dotnet/core/blob/master/samples/linker-instructions.md. Again, it's a work in progress.

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