The timeout period elapsed prior to obtaining a connection from the pool

If you are working with .NET applications, at some time you might get the below (or similar) error message: 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.


Why you get the connection pool timeout error

The most probable reason you are getting the above error message, is that your application opens connection to the database without closing them. This situation is then repeated and after a number of times, the application throws an exception because it cannot open more than a given number (max pool) of connections to the database.

The best practice is to let .NET do the connection handling for you by running your SQL queries with the “using” statement. Here’s a C# example:
using (SqlConnection conn = new SqlConnection(connString))
      SqlCommand cmd = new SqlCommand("SELECT @@VERSION", conn);
      SqlDataReader dr = cmd.ExecuteReader();
      //process the result


There is a useful MSDN blog article  (a little old but the principle is the same), which has more info about this type of connection handling in .NET.


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