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The short answer: value types do not always live on the stack, and reference types do not make every related value live on the heap. In .NET, location depends on where a value is stored and how it is used. A struct can be stored inline inside a heap object, while boxing copies a value type into a new heap object. The more useful questions are what contains the data, how long it remains valid, and whether an operation allocates an object.
What the stack-versus-heap rule gets wrong
“Structs live on the stack; classes live on the heap” is a rough mnemonic, not a rule for every value. A value type contains its value directly, so it may be stored as a local or inline inside another value or object. A reference-type variable holds a reference to an object, and that object is allocated on the managed heap. Microsoft’s value types documentation describes these storage distinctions.
For example, a struct field inside a class instance is stored as part of that heap-allocated object; it is not a separate stack object. Conversely, a local variable’s apparent source-code location does not by itself establish where all data associated with it lives. Think in terms of the containing storage, rather than assigning every type to a single memory region.
Reference variables and the objects they refer to
A class variable holds a reference, not the class object itself. The reference may be a local, while the referenced object is on the managed heap. Copying that reference copies the way to reach the object; it does not create another object. The object remains available while it is reachable, subject to the runtime’s garbage-collection rules.
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Boxing puts a copy of a value type in a heap object
Boxing occurs when a value type is converted to object or to an interface it implements. The runtime allocates a managed-heap object and copies the value into it. Unboxing retrieves a value from that boxed object. Microsoft explains the allocation and conversion in its boxing and unboxing guide.
int i = 123;
object o = i; // Boxes i: o refers to a heap object containing a copy.
i = 456; // The boxed copy remains 123.The boxed value is a copy, so changing i after the conversion does not change the value held by o. Boxing allocates and constructs an object, which adds work compared with a simple assignment. Avoid unnecessary boxing in performance-sensitive code, but do not infer a universal speed advantage for one memory region from the type alone.
stackalloc creates method-scoped stack storage
The stackalloc expression reserves a block of memory on the stack for the method execution. Microsoft’s C# reference puts its lifetime plainly: “A stack-allocated memory block created during the method execution is automatically discarded when that method returns.” The block is not managed by the garbage collector.
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Span<int> numbers = stackalloc int[3];
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;Keep stack allocations small and bounded. Available stack size depends on the execution environment; large or repeated allocations can exhaust it. Microsoft recommends avoiding stackalloc inside loops and using an array for larger buffers. The allocated contents are undefined until initialized, so write values before reading them. See the stackalloc expression reference for the language details and cautions.
Span<T> is a view, not a promise about location
Span<T> represents a contiguous region of memory. It can view an array, a stackalloc buffer, or unmanaged memory, so the presence of a span does not mean its backing storage is on the stack. The span is a lightweight view over that storage, and the storage location is a separate question.
Span<T> is a ref struct with lifetime restrictions that prevent it from escaping to the managed heap. For example, it cannot be boxed or stored in a class field. Its use across async and iterator boundaries is also constrained; exact allowances depend on the C# language version. Consult Microsoft’s ref struct documentation and async-method rules for the applicable version.
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Use Memory<T> when the wrapper must persist
If a memory wrapper needs to be stored on the heap or retained for work that crosses an async boundary, Memory<T> is often the appropriate alternative. Unlike Span<T>, a Memory<T> value can be stored on the managed heap. It is still a view: its backing memory may be an array or another supported memory source. Microsoft compares the two types in its Memory<T> and Span<T> guidance.
- Use
Span<T>for synchronous, short-lived access when its ref-struct restrictions fit the code. - Use
Memory<T>when the wrapper must be retained, including common async workflows.
What the garbage collector manages
When the application creates a managed object, the CLR allocates space for it on the managed heap. The garbage collector tracks allocation activity, identifies objects that are no longer in use, and reclaims their memory. An object that remains reachable is not eligible for collection simply because a particular method has returned. Microsoft outlines this process in its garbage collection fundamentals.
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Quick Recap
A practical way to reason about location
| Case | Where the data is | Lifetime or allocation point |
|---|---|---|
| Value-type local | Stored in its local context; do not assume every value type has one universal location. | Depends on the containing context and how the value is used. |
| Value-type field in a class | Inline within the containing heap object. | Part of that object’s lifetime. |
| Reference-type object | Managed heap; a variable holds a reference to it. | Reclaimed by the GC when no longer in use. |
| Boxed value type | A copy inside a managed-heap object. | Object allocated during boxing; later reclaimed by the GC when no longer in use. |
stackalloc buffer |
Stack memory. | Discarded when the method returns; not reclaimed by the GC. |
Span<T> or Memory<T> |
The wrapper is a view; backing storage may be an array, stack buffer, or other supported memory. | Span<T> is ref-struct restricted; Memory<T> can be stored on the heap. |
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