Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached."

Message from the system: "Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached."

Always close your connection in the final block:

or

Increase pool size like in your connection string 

string connectionString = "Data Source=localhost; Initial Catalog=Northwind;" + "Integrated Security=SSPI; Min Pool Size=10; Max Pool Size=100";

Don't use pooling

string connectionString = "Data Source=localhost; Initial Catalog=Northwind;" + "Integrated Security=SSPI; Pooling=false;";