Microsoft CsWin32: generating P/Invoke bindings from Win32 metadata
A source generator to add a user-defined set of Win32 P/Invoke methods and supporting types to a C# project.
At a glance
- What is it?
- CsWin32 is a C# source generator that emits only the Win32 P/Invoke methods and supporting types your project declares. It removes hand-written DllImport declarations, but it is Windows-only and its configuration lives in a NativeMethods.txt file.
- Who is it for?
- Adopt CsWin32 if you maintain hand-written DllImport declarations in a Windows-only C# project and want them generated from Microsoft.Windows.SDK.Win32Metadata at compile time. Do not adopt it if you need cross-platform builds, or if you cannot accept that the generated API surface changes with each package release.
- Can I use it commercially?
- Yes. MIT is a permissive licence: you can use, modify and sell software built on it, as long as you keep its copyright and licence notices.
- Is it still maintained?
- Yes. The repository last received commits 5 days ago.
- What is it written in?
- Mainly C#, according to GitHub's language statistics.
Answers come from the project's GitHub data, last synced on September 25, 2026, and from our analysis. They are not legal advice.
Editorial analysis
The hand-written DllImport problem CsWin32 addresses
A C# application that calls Windows APIs normally carries a file of DllImport declarations. Each one names a library, a function, a calling convention and a set of parameter types, and each one is a place where a wrong type or a missing attribute produces a runtime failure rather than a compile error. The declarations are also partial: a project that needs one function from user32.dll often copies a block of neighbouring declarations because that is how the snippet was found.
CsWin32 replaces that file with a source generator. The README describes the project as providing "P/Invoke and COM Interop projection support for C#" that "generates strongly-typed, source-generated bindings from CsWin32-compatible .winmd metadata files". The audience is narrow and identifiable: C# developers building Windows desktop software, services or tooling who already call native APIs and want the signatures to come from a metadata source instead of from memory.
The README also states that CsWin32 supports Microsoft.Windows.SDK.Win32Metadata as its first-party metadata, and that third-party metadata is supported. That second point matters for anyone calling an API that is not in the Windows SDK metadata set.
How the generator decides what to emit
The mechanism is a Roslyn source generator driven by a text file. A project references the Microsoft.Windows.CsWin32 package and includes a NativeMethods.txt file; each line names a method, type or constant the project wants. At compilation the generator reads that list, looks the names up in the .winmd metadata, and emits C# source into the compilation. Nothing is emitted for names that are not listed, which is why the README says the package "ships no bulky assemblies alongside your application".
The generator does more than transcribe signatures. The README lists friendly overloads and extensions, including SafeHandle support, and XML documentation that links back to learn.microsoft.com. The first point is the more consequential design choice: a raw metadata signature is often not the shape a C# caller wants, so the generator produces convenience forms in addition to the direct one. That is convenient, and it is also the part of the output most likely to shift between releases, because overload shape is a generator decision rather than a metadata fact.
Related search terms around this project include cswin32 nativemethods json and cswin32 nativemethods txt, which reflects the fact that the input file is the main thing a new user has to understand. The README points to a getting-started page, an examples page and a third-party metadata page rather than documenting the file format inline.
Installing CsWin32 and generating a first binding
The README links to a getting-started page and an examples page rather than printing install steps, so the exact commands are not reproduced here. What the README does establish is the shape of the setup: the package is Microsoft.Windows.CsWin32, published on nuget.org, and the generator is driven by a NativeMethods.txt file in the project. The third-party metadata page covers the case where the API you need is not in the first-party metadata.
Start from the getting-started page at https://microsoft.github.io/CsWin32/docs/getting-started.html, which the README lists first under Getting started. Follow it in a Windows-targeting C# project, and use the Examples page at https://microsoft.github.io/CsWin32/docs/Examples.html to see the generated members in use. If the API you want is not part of Microsoft.Windows.SDK.Win32Metadata, the third-party metadata page at https://microsoft.github.io/CsWin32/docs/3rdPartyMetadata.html is the one to read before you add anything.
The one thing worth understanding before you start is that nothing is generated for a name you have not listed. A misspelled entry in NativeMethods.txt produces no member and a compile error at the call site, which is a more useful failure than a runtime entry-point exception.
Where the generator model costs you
The generated surface is tied to the metadata version that ships with the package. When you update Microsoft.Windows.CsWin32, the signatures and the friendly overloads can change, and a project that pinned an older package may fail to compile against the newer output. That is normal for a source generator, but it means package updates are not free, and the release cadence visible in the repository (v0.3.298 on 2026-06-22, v0.3.296 on 2026-06-15, v0.3.287 on 2026-06-11) suggests frequent small releases rather than occasional large ones.
The second constraint is platform. CsWin32 projects Windows metadata, so it is the wrong tool for a library that must build on Linux or macOS, and it is the wrong tool for a cross-platform abstraction that already has a managed implementation. It is also unnecessary if you call one or two functions once; the configuration file and the generator add moving parts that a single DllImport does not.
Third, the README does not document a rollback path for a bad generator release beyond changing the package version. There is no described mechanism for pinning the generated output itself, so the practical control is the package version in your project file. Treat that version as part of your build contract.
CsWin32 compared with direct DllImport and LibraryImport
The alternative most C# developers already use is a hand-written DllImport declaration. The difference is where the signature comes from: with DllImport you write it and you own every mistake in it, while CsWin32 reads it from Microsoft.Windows.SDK.Win32Metadata and emits it. The trade is control for accuracy. A hand-written declaration never changes underneath you; a generated one can.
LibraryImport is the other comparison point, and it is a different layer of the problem. It is a source-generator-based marshalling mechanism in the runtime, and it still needs a declaration to generate from. CsWin32 can emit declarations that use it, but LibraryImport by itself does not know which Windows APIs exist or what their parameters are. The two are complementary rather than competing.
CsWinRT occupies a third position: it projects Windows Runtime types into C#, while CsWin32 projects Win32 and COM metadata into P/Invoke and COM interop. A project that touches both WinRT and raw Win32 APIs may end up referencing both generators, and the two produce different styles of binding.
Maintenance, licence and what the repository shows
The last push to the default branch was on 2026-06-22, and the repository is not archived. The release list shows three releases in June 2026, so the project is being published regularly, though the gap between that date and the present is long enough that you should check the current release before assuming a fix has shipped.
The licence is MIT, stated in the LICENSE file at the repository root. For adopters the practical consequence is that generated code and the package can be used in closed-source and commercial products, subject to the usual requirement to preserve the copyright and permission notice. The repository also carries NOTICE.txt and COPYRIGHT files, which are worth reading if you redistribute the generated output, though this is not legal advice and your own counsel should review distribution terms.
The upgrade cost is the package version. Because the generator reads metadata shipped with the package, moving that version forward can change emitted signatures and overloads, so plan to rebuild and review the diff in generated files after each bump rather than assuming a patch release is inert.
Editorial conclusion
Adopt CsWin32 if you maintain hand-written DllImport declarations in a Windows-only C# project and want them generated from Microsoft.Windows.SDK.Win32Metadata at compile time. Do not adopt it if you need cross-platform builds, or if you cannot accept that the generated API surface changes with each package release. Before committing, verify that your project targets a supported .NET version, that your NativeMethods.txt lists the exact method names you use, and that you have reviewed the MIT-licensed generated output for the methods that matter to you.
Frequently asked questions
What is Win32 used for?
Win32 is the native Windows API surface that CsWin32 projects into C#. The project generates P/Invoke and COM interop bindings from Microsoft.Windows.SDK.Win32Metadata so a managed application can call those APIs with strongly typed signatures.
Which is better, Win32 or Win64?
CsWin32 does not present them as alternatives: it generates bindings from Windows SDK metadata for the platform your project targets. The choice of target architecture is a project setting, not a CsWin32 setting.
What is the purpose of the USER32.dll file?
USER32.dll is one of the native libraries that Win32 functions are exported from, and it is the kind of library a hand-written DllImport declaration names. With CsWin32 you list the method name in NativeMethods.txt and the generator emits the binding instead of you naming the library yourself.
Is Win32 still used?
The README does not make a claim about Win32 adoption over time. It does show that Microsoft maintains Win32 metadata and publishes CsWin32 releases against it, with the most recent release listed as v0.3.298 on 2026-06-22.
how to use cswin32
Add the Microsoft.Windows.CsWin32 package to a Windows-targeting C# project, create a NativeMethods.txt file listing the API names you want, and build. The generator emits the bindings during compilation, and the README links to a getting-started page and an examples page for the details.
Official sources
Add this badge to your README
If you maintain this project, the badge below links readers to this analysis and shows its maintenance status from the daily GitHub snapshot. Paste the markdown into your README; add ?metric=license or ?metric=stars to the image URL for a different field.
[](https://hysenlabs.com/projects/microsoft-cswin32)