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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsChoose Fody when you want build-integrated weaving through an add-in; PostSharp when you want a higher-level commercial aspect framework and ready-made patterns; and Mono.Cecil when you need to inspect or rewrite assemblies directly. All three work around compiled .NET assemblies, but they sit at different levels: Fody provides an extensible build workflow, PostSharp provides a more complete aspect product, and Cecil is a library for custom tooling.
What IL weaving does
IL weaving transforms a managed .NET assembly after the C# or Visual Basic compiler has produced it. A build tool reads the compiler output, changes method bodies or metadata, validates the transformation, and writes the resulting assembly. The application then uses that transformed build artifact.
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That makes weaving a build-time technique, not simply a runtime interception mechanism. Depending on the design, weaving can avoid runtime proxy or interception costs, but the result is still generated code: the transformed assembly needs to be validated, tested, and debugged as part of the build.
The Tool Desk
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| Tool | What it is | Best fit | What to evaluate |
|---|---|---|---|
| Fody | An open-source, extensible build tool whose add-ins perform specific transformations. Its project description highlights the plumbing involved in manipulating assembly IL during a build. (Fody project) | Projects that want package-based, build-integrated weaving and can assess each add-in on its own merits. | Whether the required add-in is maintained and compatible with your target .NET and Visual Studio versions; its behavior and support profile. |
| PostSharp | A commercial MSIL-rewriting and aspect framework. Its documented workflow post-processes compiler output, performs transformations and validations, and writes the final assembly. (PostSharp workflow and product documentation) | Teams seeking a higher-level aspect workflow, ready-made implementations, vendor documentation, and enterprise-oriented tooling. | Licensing and support, available aspects, build integration, and compatibility with the project’s toolchain. |
| Mono.Cecil | A library for loading managed assemblies, examining their types, modifying them, and saving the result. It is not a turnkey aspect product. (Mono.Cecil documentation) | Engineers building custom inspection, rewriting, instrumentation, obfuscation, or migration tools that need direct control over metadata and CIL. | The engineering effort required to implement and validate the transformation yourself, along with target-runtime and build compatibility. |
When Fody is the right choice
Fody is useful when the transformation you need already exists as an appropriate add-in and you want it integrated into the build rather than implemented from scratch. A Fody project normally discovers weavers through package references and configuration, with the framework handling much of the build-system plumbing.
#1 Best Overall
Think of Fody as the extensible engine, not as a guarantee that every transformation is included or equally maintained. The add-in determines what is woven. Before adopting one, check its current maintenance, license and support terms, compatibility with your .NET and Visual Studio versions, and whether it behaves as your build expects.
When PostSharp is the right choice
PostSharp is the higher-level option when the team wants an aspect-oriented workflow with ready-made patterns rather than assembling the solution from lower-level pieces. Its documented areas include logging, contracts, INotifyPropertyChanged, caching, multithreading, weak events, and architecture validation.
The workflow is explicitly post-compilation: the compiler emits binaries, then PostSharp analyzes them and injects aspect implementations. Consider it when vendor documentation and a supported product are important alongside the aspect features, and assess licensing and compatibility for the exact edition and toolchain you plan to use.
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When Mono.Cecil is the right choice
Mono.Cecil is the lower-level choice for developers who need to build their own assembly-processing tool. It can load existing managed assemblies, browse types, modify them, and save the altered assembly. It can also extract CIL and inspect assembly images without loading compatible runtime assemblies, which can help when the task is inspection or transformation tooling rather than adding a ready-made application aspect.
This control comes with responsibility: your code must define the transformation, preserve the assembly’s intended behavior, and validate the saved result. Use Cecil when those requirements are central and a turnkey weaver does not provide the needed control.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Specialized Fody add-ins: check the exact package
Some add-ins address narrower jobs. CompileTimeWeaver.Fody demonstrates compile-time AOP weaving across methods, properties, constructors, and extension methods. MixedIL.Fody demonstrates injecting an IL method body supplied from an IL file. These examples illustrate the range of the add-in model; they do not establish current maintenance or compatibility for a particular project.
Rank #4
For any specialized package, verify its current release and compatibility against your target .NET, Visual Studio, and build setup before relying on it. A package’s existence is not evidence that it supports every current toolchain.
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How to choose for a project
- Define the transformation. Decide whether you need a common aspect such as logging or property-change notification, a specific custom method-body change, or general-purpose assembly inspection and rewriting.
- Check for a ready-made fit. Look for a PostSharp feature or a Fody add-in that implements the required behavior. Confirm licensing, support, maintenance, and compatibility for the exact product or package.
- Choose the implementation level. Prefer a higher-level framework when its supported patterns match the requirement; choose Cecil when direct control is necessary and the team can own the transformation.
- Validate the build artifact. Treat the rewritten assembly as production code. Test the woven output and ensure the team can diagnose behavior in the transformed build, not only in the source before weaving.
What to compare before adopting a weaver
- Build integration: how the tool participates in your build system and whether the workflow is clear to everyone producing the artifact.
- Capability level: ready-made aspects, add-ins, or access to raw CIL and metadata.
- Licensing and support: particularly important when comparing a commercial framework, an open-source engine, and a library.
- Compatibility: verify support for the exact .NET, Visual Studio, and build-tool versions in your project; do not assume compatibility from the tool category alone.
- Runtime and debugging implications: build-time weaving can avoid runtime proxy or interception costs for some designs, but the transformed output still needs testing and debugging.
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