Spans and stack allocation

Spans are specialized, allocation-free views over contiguous memory. System.Span<T> provides mutable access, while System.ReadOnlySpan<T> provides read-only access. Both support indexed reads, slicing, and for iteration; read-only span indexers are not settable.

The following implicit conversions are available:

• a single-dimensional T[] to Span<T> or ReadOnlySpan<T>;
• Span<T> to ReadOnlySpan<T>;
• string to ReadOnlySpan<char>;
• an array, Span<T>, or ReadOnlySpan<T> to ReadOnlySpan<U> when the element reference conversion from T to U is covariant.

Span conversions participate in generic type inference and overload resolution. When neither target exactly matches the source expression, a span conversion is preferred over a non-span conversion. Between equivalent mutable and read-only span targets, ReadOnlySpan<T> is preferred.

Collection expressions can directly target either span type:

val values: System.ReadOnlySpan<int> = [1, 2, 3]

stackalloc T[count] reserves contiguous storage in the current stack frame. It naturally produces Span<T> and can be explicitly targeted to ReadOnlySpan<T>:

val buffer = stackalloc byte[256]
val readOnly: System.ReadOnlySpan<byte> = stackalloc byte[16]

Stack-allocated storage cannot escape the declaring function. The restriction follows pointer, span, and read-only-span locals and aliases backed by the allocation.

System.Memory<T> and System.ReadOnlyMemory<T> retain their normal .NET conversion behavior. Their Span properties interoperate with the same indexed and iterative span operations.

See Ref structs and ref safety for the lifetime rules shared by spans and other ref-like values. Pointer-targeted stack allocation is covered by Unsafe code and interoperability.