NullableAnnotations

NullableAnnotations refers to the system of compiler and metadata annotations that encode nullability information for reference types in .NET languages, notably C#. It supports the nullable reference types feature introduced in C# 8.0, enabling the compiler and tooling to distinguish between nullable and non-nullable reference types and to propagate that information across assemblies.

The two main mechanisms are the attributes in System.Runtime.CompilerServices: NullableAttribute and NullableContextAttribute. NullableAttribute can be applied to individual members to indicate whether their type is annotated as nullable or not. NullableContextAttribute marks a scope with a default nullability context, allowing members to inherit the default unless overridden. When a project has nullable reference types enabled, the compiler emits NullableAttribute metadata on relevant symbols, so analyzers and editors can perform cross-assembly nullability analysis and provide warnings about potential null dereferences. Tools and libraries can read these annotations to improve interop safety and static analysis. Libraries compiled without nullable metadata are treated as oblivious or unaware of nullability.

Usage: in C# source, developers indicate intent by writing types like string? for a nullable reference. The emitted NullableAttribute data communicates these intents to downstream code, including libraries that consumers compile with nullable contexts enabled.

Limitations: annotations are metadata for static analysis and tooling; they do not change runtime behavior. They rely on modern compilers and tooling to interpret and propagate across assemblies, and reflection or older runtimes may not surface all information.

This term is primarily discussed in the context of C# and .NET tooling, and is a facet of broader null-safety practices in modern languages.