CLScompliant

CLScompliant refers to adherence to the Common Language Specification (CLS) within the .NET framework. The CLS defines a set of rules intended to ensure that public APIs are usable from any language that targets the .NET platform. When code is CLS-compliant, its public surface can be consumed reliably by languages such as C#, VB.NET, and F#.

In .NET, compliance is indicated by the CLSCompliantAttribute in the System namespace. An assembly can declare itself CLS-compliant by applying [assembly: CLSCompliant(true)]. Individual types or members can override this with [CLSCompliant(false)]. If an assembly is declared CLS-compliant, the compiler will warn about public members that are not; developers can then mark those members as non-compliant to suppress warnings.

Common sources of non-compliance include exposing unsigned integral types (for example, uint or ulong) in public parameters or return types, since not all CLS languages guarantee support for unsigned types. Other issues can arise from language-specific features or identifiers that behave differently across languages. Non-compliant members should be annotated with [CLSCompliant(false)] to indicate their exclusion from the CLS surface.

CLS conformance affects only public and protected surfaces; private implementation details may freely use non-CLS constructs. Achieving CLS compliance promotes interoperability across .NET languages, but it does not guarantee identical semantics or portability to non-.NET runtimes. Tools and compilers can emit warnings to help enforce compliance, aiding library authors who aim for broad language compatibility. See also the Common Language Specification and the System.CLSCompliantAttribute type for more details.