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A pinned memory object is a normally movable memory object whose address must remain unchanged during a specified operation. Pinning permits a pointer to remain usable when the code consuming it cannot participate in the runtime's relocation of references. The pinning lifetime is part of that guarantee: an address retained beyond the protected operation need not remain valid. Microsoft's language reference specifies that a fixed statement prevents relocation during its body and restricts the declared pointer to that statement.^[ref]

For an original layout example, consider96 bytes with a pinned16-byte object at offsets $[32,48)$ and one other16-byte live object. The remaining64 bytes are free. Even if the other object can move anywhere, the pinned object divides the address range into regions of32 and48 bytes. A56-byte contiguous allocation cannot fit. Moving the other object into $[0,16)$ gives free intervals $[16,32)$ and $[48,96)$, whose largest size is48 bytes. If both live objects could move, placing them in $[0,32)$ would instead leave a64-byte interval. Pinning can therefore limit memory compaction despite ample total free capacity.

The numerical layout omits allocation headers and memory alignment; a real allocator must include them. Pinning an object's address also does not by itself specify how its virtual memory pages are backed. The garbage collector's object-relocation guarantee and an operating system's physical-page residency rules concern different layers. Likewise, a pinned buffer in a storage cache may mean protection from eviction rather than this object-relocation guarantee.

Once the protected operation is finished, releasing its pin can restore the runtime's freedom to move the object during a later collection. The program still has to obey the language's reference-lifetime rules: releasing a pin does not convert an old unmanaged address into a relocatable reference. The general garbage collection process must update references when it moves surviving objects.^[ref]

The pinning lifetime is the interval during which a pinned memory object must retain its address for all consumers entitled to use a non-relocatable pointer into it. The interval must cover actual memory access, rather than merely the act of handing out the address. Microsoft's fixed statement guarantees address stability during its body and restricts its declared pointer to that statement; a pointer retained for later work needs a guarantee covering that later work too.^[ref]

For an original timing example, suppose a pin starts at0ms, an external operation receives the address at0.5ms, its submission function returns at1ms, and its final memory access finishes at5ms. Releasing the pin at the function's return leaves4ms of possible access after address stability has ended. Releasing it only after completion at5ms can cover the whole access interval. These times describe an illustrative protocol, not a timing claim about a particular runtime or device.

If the pin is active on $[t_p,t_r)$, every access time $t_a$ must satisfy $t_p\le t_a<t_r$. With several users, the release point must follow the last access by all of them. A count of outstanding operations can enforce this rule: after two operations start, one completion changes the count from2 to1 and does not permit release; the last completion changes1 to0. Asynchronous pin release therefore depends on a completion guarantee. Requesting that unfinished work stop does not by itself demonstrate that its final access has ended.

The pinning lifetime and data synchronization solve different problems. Stable addresses do not prevent two users from changing the same bytes concurrently; a mutex or another synchronization mechanism may still be needed. Nor should a pin be held indefinitely just because longer protection seems convenient: pinning can obstruct memory compaction, as the pinned memory object layout example shows. The useful interval is long enough for the access contract and ends when that contract permits release.