Max Pool Size exhaustion behavior and diagnostic counter reliability

Answer

SqlClient begins to queue new connection requests the instant the number of active (in‑use) connections equals the Max Pool Size. At that point the pool is 100 % utilized; any further Open() call blocks until an idle connection becomes available or the connection timeout expires, at which point a TimeoutExpired exception is thrown.

Confirmed facts

1. Queuing threshold: When NumberOfActiveConnections reaches the Max Pool Size (default 100), the pool is exhausted and subsequent Open() calls are queued.
2. ClearPool behavior: SqlConnection.ClearPool (or ClearAllPools) removes only idle connections from the pool; it does not affect connections that are currently in use, so it cannot release leaked connections or alter the exhaustion state.
3. Counter reliability: The NumberOfActiveConnections performance counter increments on each successful Open() and decrements when a connection is returned. When it hits the Max Pool Size and stays there, the pool is exhausted before any timeout exception is raised, providing an early‑warning window equal to the remaining connection timeout (subject to the counter’s reporting latency, typically a few hundred milliseconds).

Steps to verify the behavior

1. Create a console app that opens SqlConnection objects in a loop without disposing them, using a connection string with the default Max Pool Size (=100) and a known connection timeout (e.g., 15 seconds).
2. Instantiate a PerformanceCounter for the category ".NET Data Provider for SqlServer" and the instance matching your connection string, monitoring NumberOfActiveConnections and NumberOfPooledConnections.
3. Log the counter values after each Open() call. You will see NumberOfActiveConnections increase until it reaches 100 and then stop increasing, while subsequent Open() calls begin to block.
4. Measure the elapsed time from the moment the counter hits 100 until a TimeoutExpired exception is thrown; it should approximate the remaining connection timeout.
5. Optionally call SqlConnection.ClearPool after exhaustion and verify that NumberOfActiveConnections remains at 100 and that further Open() calls still block until timeout, confirming that only idle connections are cleared.