During a recent leetcode exercise, I was struggling why my already optimal solution is still having 1ms time behind the top solvers. When I inspect their code, it’s a shame for me that I didn’t realize this keyword exists after writing C# for more than a decade. Documenting it here because I have learned something new. 😀

The keyword stackalloc is for creating small, short‑lived buffers without heap allocation.

It’s alternative to new, which allocate memory on heap instead.

Typical use cases:

• Temporary scratch buffers in hot paths (parsers, encoders, algorithms) to avoid GC pressure.
• Interop scenarios where you need a fixed buffer for native calls.
• High‑performance code where you want to avoid allocating a new array on each call.

Limits / Notes:

• Size should be small; stack is limited and too‑large allocations can cause stack overflow.
• Lifetime is the current method scope only.
• Works with Span<T>/ReadOnlySpan<T> in safe code; no need for unsafe in many cases now.
• Stack should be small, rule of thumb: A few K bytes maximum.

Alternatives

• Other choice of high performance data structure: ArrayPool<T>
  • Have to be careful on not accessing memory outside of rent boundary
  • Have to be careful on explicit requesting array clean up on return, otherwise it is not guaranteed to be cleared.

var pool = ArrayPool<byte>.Shared;
byte[] buffer = pool.Rent(4096);
try
{
    // use buffer
}
finally
{
    pool.Return(buffer, clearArray: true);
}

Next time when you need to frequently create a small data structure for your algorithm, try stackalloc or ArrayPool<T>. Happy coding. 👋