C# Interlocked CompareExchange: Lock-Free Counters

Interlocked is a .NET class whose static methods update one variable in one uninterruptible step (atomic), so two threads cannot corrupt it mid-update. Compare-and-swap (CAS) means “write this new value only if the location still holds the value I just read.”

Race on i++ (not atomic)

  Thread A: read 5 ── add 1 ── write 6
  Thread B: read 5 ── add 1 ── write 6
  Lost an increment. Both thought they won.

New to this → stay here. Merging a PR → wrong vs right. On-call / interview → CAS loop · if an interviewer asks.

Terms used here: Atomic = one uninterruptible CPU step. CAS / compare-and-swap = write only if the location still holds the value you read. volatile = a visibility hint, not a counter.

If you are new to threading, start with lock. Default for two fields: lock. Default for one counter: Interlocked.Increment.

Smallest example

Interlocked.Increment(ref _inFlight);
Interlocked.Decrement(ref _inFlight);
Interlocked.Add(ref _bytes, length);

if (Interlocked.CompareExchange(ref _started, 1, 0) == 0)
{
    // we won — start the hosted loop once
}

CompareExchange stores to only if the location still equals comparand. It returns the original value.

Wrong vs right

I would reject i++ on a shared counter and a CAS loop on the request path “so we don’t need lock.”

// Wrong
lock (_gate) { _count++; } // fine, but coarse on every request

volatile int _flag;
_flag++; // still a race — volatile is not a counter

// Right for one integer
Interlocked.Increment(ref _count);

A metrics helper under a load test showed the lock next to real work. Replacing the increment with Interlocked.Increment dropped contention. The dashboard object (count and last-timestamp) still needed a lock.

Comparison: volatile versus Interlocked versus lock.

Tool	Prevents torn increment	Publishes value to other cores	Multi-field atomicity
Nothing	No	No	No
volatile	No	Helps visibility	No
Interlocked.*	Yes (that location)	Yes	No
lock / Monitor	Yes (inside the section)	Yes	Yes

I almost never put volatile on new code; Volatile.Read / Write or Interlocked are harder to misuse.

CAS loop (only when Add is not enough)

int snapshot;
int computed;
do
{
    snapshot = Volatile.Read(ref _max);
    computed = Math.Max(snapshot, candidate);
} while (Interlocked.CompareExchange(ref _max, computed, snapshot) != snapshot);

If another thread changed _max, you retry. That is lock-free: some thread makes progress. It is not wait-free. A hot CAS loop can burn CPU. Do not invent a lock-free queue — use Channel / ConcurrentQueue.

CompareExchange on double exists on modern .NET; on older runtimes check your .NET version for the overload.

Memory model, short

Skip this unless you are writing publication protocols. The CLR does not promise a plain int write is visible immediately to another core. lock, Interlocked, and Volatile insert barriers. You do not need to recite MESI in an ASP.NET interview.

Flags and state machines

const int Idle = 0, Running = 1, Stopping = 2;

public bool TryStart() =>
    Interlocked.CompareExchange(ref _state, Running, Idle) == Idle;

Good for a hosted service gate. Bad for “transfer money”: debit and credit are two locations. That is a lock (or a database).

Common mistakes

• Interlocked on a field, then unsynchronized read of a second field
• Using CAS to avoid learning lock and then spinning on the request path (CPU high this time)
• lazyInit with a bool instead of Lazy<T>

What this is not

Multi-field coordination: lock vs Monitor. Maps: ConcurrentDictionary. Topic map: async & threading hub.

If an interviewer asks

Lock-free / CAS; race on i++; volatile; thread-safe singleton (atomic flag vs Lazy<T>).

Strong answer: one word, Interlocked; two fields, lock; don’t write a lock-free list.

. If you are inventing a lock-free ring buffer in an API, we should talk before that ships.