What is SpecFlow/Reqnroll, and how does it relate to Cucumber?

SpecFlow (now continued under the community-driven fork Reqnroll, after SpecFlow's commercial development wound down) is a BDD framework for .NET that uses the same Gherkin syntax and feature-file format as Cucumber, but with its own separate implementation, test runner integration, and step-definition binding mechanism built for the .NET ecosystem.

Because both tools speak the same Gherkin grammar, a feature file written for one is largely readable and reusable in concept by the other, but the actual step definition code (C# attributes and methods in SpecFlow/Reqnroll versus Java/Ruby/JS methods in Cucumber's own implementations) isn't directly portable between them; they're separate projects that happen to share a common specification language rather than one project extended across languages.

For .NET teams specifically, Reqnroll (or SpecFlow, for teams still on it) is generally the natural choice over trying to run genuine Cucumber-JVM or Cucumber.js in a .NET-centric codebase, since it integrates directly with .NET's own test runners and tooling.