assembly-rewriter

Rewrites .NET assemblies with Mono.Cecil, to allow two different versions of the same assembly to be referenced within an application.

It assumes that the assembly DLL name is the top level namespace and rewrites

1. the top level namespace for all types within the assembly
2. assemblies in the order of dependencies first
3. IL ldstr op codes if they start with the namespace
4. compiler generated backing fields

This small program was written to allow different versions Elasticsearch .NET clients to be rewritten for benchmark comparisons. Your mileage may vary rewriting other assemblies :)

Installation

Distributed as a .NET tool so install using the following

dotnet tool install assembly-rewriter

Run

dotnet assembly-rewriter

You can omit dotnet if you install this as a global tool

GitHub Action

- uses: nullean/assembly-rewriter@main
  with:
    args: -i Nest.dll -o Nest620.dll

Runs assembly-rewriter from a pre-built, distroless container (ghcr.io/nullean/assembly-rewriter) — no .NET SDK install needed in the workflow. args is the full command line, since every input and output path is passed as an -i/-o pair (see below). Mount your working directory's DLLs where the container can see them; a container action's default working directory already maps to the workflow's checkout. Linux runners only (ubuntu-latest and similar) — container actions can't run on Windows or macOS runners.

Container image

ghcr.io/nullean/assembly-rewriter also works as a general-purpose container, outside GitHub Actions — GitLab CI, a local machine without the .NET SDK, anywhere docker run works:

docker run --rm -v "$(pwd)":/workspace ghcr.io/nullean/assembly-rewriter:edge -i /workspace/Nest.dll -o /workspace/Nest620.dll

Distroless: native-AOT, chiseled runtime-deps base, no shell, runs as a non-root user. Tags follow assembly-rewriter's own releases — edge tracks the latest commit on master, latest and a semver tag follow tagged releases.

Examples

Rewrite NEST, the Elasticsearch .NET high level client, version 6.2.0

assembly-rewriter -i C:/Nest.dll -o C:/Nest620.dll

Now, Nest620.dll and another version of Nest.dll can be referenced in the same project.

There's a small issue here however; both versions of NEST rely on Elasticsearch.Net.dll, so we should also rewrite this dependency at the same time, and update the references to Elasticsearch.Net within NEST to reference the new rewritten assembly

assembly-rewriter -i C:/Nest.dll -o C:/Nest620.dll -i C:/Elasticsearch.Net.dll -o C:/Elasticsearch.Net620.dll

Great! Now we can reference both in the same project.

If there are other direct dependencies that may version clash, these can be passed as well

assembly-rewriter -i C:/Nest.dll -o C:/Nest620.dll -i C:/Elasticsearch.Net.dll -o C:/Elasticsearch.Net620.dll -i C:/Newtonsoft.Json.dll -o C:/Newtonsoft.Json620.dll

Rewrite validation

You can check to see if everything expected has been rewritten using IL Disassembler

ildasm <rewritten>.dll /OUT=<rewritten>.il /NOBAR
Select-String -Path <rewritten>.il -Pattern '<original namespace>\.' -AllMatches | ft LineNumber,Line

License

Apache 2.0