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LINQ to DB connection pool depletion

This project demonstrates an issue with connection pooling in a specific scenario when using LINQ to DB. When a query is executed using new DataContext(...), using LoadWith and FirstOrDefault (or SingleOrDefault) together in same the query it looks like connections are not correctly closed/disposed, thus not returned to the connection pool. This quickly results in a connection pool depletion and eventually the exception "Connect Timeout expired. All pooled connections are in use.". The problem occurs with both the MySqlConnector and the MySql.Official data provider.

Running the project

First run the Docker Compose file which will start a MariaDB container and create a database with the necessary tables and columns:

docker compose up

After that, just start the project by selecting Debug in your editor, or run the command:

dotnet run --project LinqToDbConnectionIssue/LinqToDbConnectionIssue.csproj

Set maxExecutionCount to 20 (a low number) and leave the application running to see that connections are not correctly returned to the connection pool. When using the command SHOW PROCESSLIST; MariaDB will show the open connections, which all will be in Sleep state and will not be terminated when the configured Connection Idle Timeout expires.