To fix suspected database connection leaks in ASP.NET Core, first determine whether connections are actually being leaked or are simply being reused by a pool. Keep each EF Core DbContext scoped to a bounded unit of work, dispose manually opened ADO.NET connections and related objects, and check query duration, transactions, concurrency, pool fragmentation, and database capacity before changing pool limits.
First identify what is running out
“Connection leak” can describe different symptoms: a growing number of SQL Server sessions, a SqlClient connection-pool timeout, or a limit imposed by another database provider. These are not interchangeable. EF Core’s DbContext pool, when enabled, reuses context objects; the ADO.NET provider’s connection pool separately reuses physical database connections.
EF Core generally opens the provider connection immediately before a database operation and closes it afterward. Closing the logical connection can return its physical connection to the driver pool, where it may remain open for reuse. Therefore, seeing database sessions that outlast an individual request does not by itself prove an application leak. Microsoft explains EF Core connection management and context pooling.
For SqlClient, inspect its diagnostic counters—including active and free pooled connections, pool groups, stasis, hard and soft connects or disconnects, and reclaimed connections—and compare them over the same time window with server sessions, waits, blocking, and application load. A reclaimed-connection signal can point to logical connections that were not disposed, but a pool timeout alone does not establish that cause. Microsoft lists slow queries, blocked transactions, high concurrency, pool fragmentation, and database capacity as other possibilities. See Microsoft’s SQL Server connection-pooling guidance.
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Give each DbContext a bounded lifetime
In a typical ASP.NET Core application, register the context with AddDbContext and use the injected instance for a request or other bounded unit of work. AddDbContext<TContext> uses scoped lifetime by default, so dependency injection disposes the context when its scope ends.
builder.Services.AddDbContext<AppDbContext>(options =>
options.UseSqlServer(connectionString));Do not keep a context in a singleton or another object that outlives its intended work. If you create a context yourself or obtain one from a factory, dispose it explicitly; use await using when appropriate for an async-disposable context. Microsoft’s DbContext lifetime guidance states that a context should be disposed after use.
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Do not share one context across overlapping operations
DbContext is not thread-safe. Await an EF Core async operation before starting another operation on the same context, and do not run parallel operations through one instance. Use separate contexts for genuinely concurrent work. Some concurrent-use errors can leave a context unrecoverable, so do not attempt to keep using that instance after such an error. Microsoft documents these context-use constraints.
Dispose manually managed connections, commands, and readers
When application code explicitly creates or opens a SqlConnection, the code owns its cleanup. Put the connection in a using or, where supported and appropriate, an await using scope. Dispose commands, readers, and transactions as well, and keep their scopes no longer than the work requires. This ensures cleanup also happens when execution exits through an exception.
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await using var connection = new SqlConnection(connectionString);
await connection.OpenAsync();
// Execute commands and consume or dispose readers here.With pooling enabled, closing or disposing the logical connection returns it to the pool; it does not necessarily close the underlying physical connection. A connection variable simply going out of scope is not a substitute for explicit cleanup. Microsoft’s SqlConnection documentation cautions that a connection going out of scope is not thereby closed.
Use evidence to distinguish likely causes
| Possible cause | Evidence to inspect |
|---|---|
| Connection, command, or reader not disposed | Review code paths and exception handling for missing disposal; for SqlClient, check reclaimed-connection counters. SqlConnection documentation and pooling guidance. |
| Slow query or long transaction | Compare query duration, transaction duration, server waits, and blocking. A connection can remain occupied while work or a transaction is still active. Microsoft’s pooling guidance. |
| High concurrency or insufficient capacity | Compare active and free pooled-connection counts with request concurrency and database connection capacity, including demand across all app instances. Microsoft’s pooling guidance. |
| Pool fragmentation | Check for distinct connection strings and active pool groups. For SQL Server ADO.NET, pools are associated with exact connection-string matches, so text differences such as keyword order can split workloads across pools. Microsoft’s pooling guidance. |
| Normal pooling mistaken for a leak | Compare logical close or dispose activity with physical server-session lifetime; pooled physical connections can remain open for reuse. EF Core connection-management guidance. |
| Concurrent use of a DbContext | Look for unawaited async operations or parallel operations sharing the same context. DbContext configuration guidance. |
Counters help prioritize investigation; they do not prove root cause on their own. Correlate a specific period of client and server measurements with the application’s connection, query, and transaction scopes.
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Keep connection strings consistent and treat pool defaults carefully
For SQL Server ADO.NET, a pool is tied to an exact connection-string match. Textually different strings—including differences in keyword order—can create separate pools and divide available capacity. Use consistent connection strings for workloads intended to share a pool.
Microsoft’s SQL Server provider documentation gives a default maximum pool size of 100 and a default connection-request wait of 15 seconds before timeout. These are provider defaults, not universal EF Core settings; confirm the actual provider, version, connection string, and deployed configuration before relying on them. Other providers, including PostgreSQL, MySQL, and SQLite, have their own pooling behavior and diagnostics.
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Do not mistake DbContext pooling for connection-pool repair
AddDbContextPool reuses DbContext instances; it does not repair leaked database connections. EF Core resets its own context state, but generally does not reset driver state that application code changed directly. If code manually opens a connection or changes driver state while using a pooled context, restore that state—such as by closing the connection—before the context returns to its pool. Otherwise, later use of the reused context may inherit that state. Microsoft’s EF Core performance guidance distinguishes context pooling from connection pooling.
Change pool settings only after diagnosing the cause
Do not disable connection pooling as a leak fix: when pooling is off, disposing a connection closes the underlying server connection, which can add connection-setup overhead. Nor should increasing Max Pool Size be the first response. First verify prompt disposal and establish that the database can support the resulting connection demand across all application instances. A larger limit can mask a lifetime or capacity problem without resolving it.
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