License Compatibility Guide

October 12, 2025 by .NET Foundation Board

.NET Foundation License Compatibility Guide

Understanding What's Compatible (and What's Not)

What Licenses ARE Compatible

The .NET Foundation accepts projects that use permissive open source licenses. We automatically approve any permissive license from the Open Source Initiative (OSI) approved list.

Examples of compatible permissive licenses:

  • MIT License
  • Apache License 2.0
  • BSD licenses (2-clause, 3-clause)
  • ISC License

Key requirements:

  • Your main codebase must use a permissive OSI-approved license
  • Mandatory dependencies must also use permissive licenses (with exceptions for platform-specific requirements like .NET Framework or hardware-specific libraries where no open source alternative exists)
  • Committers must be bound by a Contributor License Agreement (CLA) or willing to embrace the .NET Foundation's CLA
  • Copyright ownership must be clearly defined and documented

What Licenses Are NOT Compatible

The Foundation does not accept projects using copyleft licenses. These include:

  • GPL (GNU General Public License) - any version
  • AGPL (GNU Affero General Public License)
  • RPL (Reciprocal Public License)
  • MPL (Mozilla Public License) in some contexts
  • Other licenses with strong copyleft provisions

Why copyleft licenses aren't compatible: Copyleft licenses require derivative works to be licensed under the same terms, which can be less attractive to corporate users and doesn't align with the Foundation's goal of broad ecosystem compatibility.

Business Models We Support

The Foundation DOES Support These Business Strategies:

1. Dual Licensing

You can offer your project under TWO licenses simultaneously:

  • One permissive OSI-approved license (for the open source community)
  • One commercial license (offering additional warranties, guarantees, or terms)

This is perfectly acceptable! The Foundation operates on a per-repository basis, so as long as the source code is available under a permissive license, you're good to go.

2. Commercial Services Around Open Source

  • Premium support packages
  • Consulting services
  • Training and certification
  • Managed hosting
  • Enterprise warranties and SLAs

Important: The source code must remain free and open, but services built around it don't have to be.

3. GitHub Sponsors

We actively encourage projects to use GitHub Sponsors to build sustainable funding for maintainers.

4. Company Formation

You can:

  • Set up a company around your project
  • Offer commercial editions with additional features
  • Provide paid support tiers
  • Keep premium packages proprietary (as long as the core remains permissive)

Our Core Principle

"As long as the source is free, that is our condition."

You don't have to make everything available as free packages. Premium support, commercial editions, and value-added services are all compatible with Foundation membership.

Addressing the AutoMapper Misconception

Recently, AutoMapper graduated from the .NET Foundation, and we want to clarify what happened to prevent misconceptions.

What Actually Happened

AutoMapper transitioned to the Reciprocal Public License (RPL 1.5), which is a copyleft license rather than a permissive one. While RPL 1.5 is technically an open source license, it doesn't align with the Foundation's requirement for permissive licenses.

What Did NOT Happen

  • AutoMapper was NOT forced out for wanting to monetize
  • AutoMapper was NOT punished for going commercial
  • The Foundation does NOT prohibit dual licensing or commercial models

The Real Reason

The departure occurred on friendly, amicable terms because AutoMapper's new license model (using a copyleft license) no longer met our eligibility criteria. The Foundation maintains that member projects must use permissive open source licenses.

If AutoMapper had chosen a dual-license model with:

  • One permissive license (like MIT or Apache 2.0) for open source
  • One commercial license for enterprises

...it would have remained fully compatible with Foundation membership!

For Maintainers: Your Options

As a .NET Foundation project maintainer, you have flexibility:

You CAN:

  • Implement dual licensing (permissive + commercial)
  • Form a company around your project
  • Charge for premium support and services
  • Create paid tiers and enterprise offerings
  • Use GitHub Sponsors or other funding platforms
  • Keep some packages proprietary (as long as core is permissive)
  • Offer commercial warranties and guarantees

You CANNOT:

  • Switch to a copyleft license (GPL, AGPL, RPL, etc.)
  • Make mandatory dependencies use copyleft licenses

Why copyleft licenses aren't compatible:

Copyleft licenses require derivative works to be licensed under the same terms, which can be less attractive to corporate users and doesn't align with the Foundation's goal of broad ecosystem compatibility.

The .NET community has a significantly higher proportion of enterprise and corporate users compared to many other open source ecosystems. These organizations often have strict legal requirements around license compatibility and derivative works. Permissive licenses provide the flexibility these users need, which is why they're fundamental to the Foundation's mission of supporting the .NET ecosystem effectively.

Questions?

Feel free to contact us [email protected], join our discord or interact with us on social media.

Remember: We want you to succeed sustainably! Building commercial services, offering paid support, and creating business models around your open source work is not just allowed—it's encouraged.