Concept quick reference

Start with the small answer, then follow the link to the precise rule. Code fragments on this page illustrate an expression or declaration in a larger program; complete runnable programs are linked alongside them.

Nullable values

T? can hold nil. T and T? are distinct types; use nullable flow rather than assuming a value is present.

Null conditional access

?. accesses a member only when its receiver is present:

let length = name?.Length ?? 0

Here name is a string? supplied by the surrounding program. The result is zero when the name is absent.

Null coalescing

?? supplies a fallback. !! asserts presence and can fail; it is not the safe alternative to a fallback. if let, guard let, and while let introduce non-null bindings in the appropriate region.

Read nullable control flow or the complete program in the Kotlin bridge.

Data models

Choice	Use it for
struct	Value-typed aggregates.
class	Reference-typed objects and their identity or behavior.
data struct	Value-typed structural data with equality and copy/update.
data class	Reference-typed structural data with equality and copy/update.
inline struct	A nominal one-field value wrapper.

let prevents rebinding; it does not recursively freeze a collection or object graph. A counter that changes needs var. Copy/update does not imply a deep copy of nested mutable state.

Read types and values, binding conventions, or build the data-model tutorial.

Concurrency

Start owned child work with go call(...) inside scope. The scope joins its children and observes failures. Merely constructing an unrelated .NET task inside the block is not a lifetime guarantee.

Channels carry values. in chan[T] can receive, out chan[T] can send, and chan[T] is bidirectional. An empty-capacity channel is a rendezvous; a buffered channel has a bounded number of queued values, not an automatic worker-count limit.

These are language features, not an opt-in Gsharp.Extensions.Go namespace. async func and await provide task-based interoperability, while async sequence[T] supports asynchronous iteration.

Read the concurrency guide, channel operations, or the complete Trail pipeline.

.NET interop

Import CLR namespaces, reference .NET libraries through your project, and call their members. Generic arguments use brackets, such as List[int32]. G# emits ordinary managed assemblies.

The runtime and library contracts still matter: collection mutability, nullable annotations, disposal, exception behavior, and overload resolution do not disappear behind a new syntax.

Read the interop Tour for examples and CLR interop for boundaries. Check the feature matrix before relying on an advanced capability.

Find a diagnostic

Search an exact GSxxxx code or use the diagnostics reference. Check the documentation version before applying a remedy: retired diagnostics and current diagnostics have different meanings.