What is connection pooling in C# and how to implement it?

They are used to import namespaces (or create aliases for namespaces or types).

They are located at the top of the file, before any declarations.

using System;
using System.IO;
using WinForms = global::System.Windows.Forms;
using WinButton = WinForms::Button;

The using statement ensures that Dispose() is called even if an exception occurs while creating an object, calling a method, property, etc. Dispose() is a method provided in the IDisposable interface that helps implement custom garbage collection. In other words, if we are performing some database operation (insert, update, delete) and for some reason an exception occurs, then the using statement here will automatically close the connection. There is no need to explicitly call the connection Close() method here.

Another important factor is that it helps with connection pooling. Connection pooling in .NET helps eliminate closing database connections multiple times. It sends the connection object to the pool for future use (next database call). The next time a database connection is called from the application, the connection pool will get the objects available in the pool. Hence it helps improve the performance of the application. So when we use the using statement, the controller automatically sends the object to the connection pool without explicitly calling the Close() and Dispose() methods.

using (Stream input = File.OpenRead(filename)) {
   ...
}

Example

string connString = "Data Source=localhost;Integrated Security=SSPI;Initial Catalog=Northwind;";
using (SqlConnection conn = new SqlConnection(connString)) {
   SqlCommand cmd = conn.CreateCommand();
   cmd.CommandText = "SELECT CustomerId, CompanyName FROM Customers";
   conn.Open();

   using (SqlDataReader dr = cmd.ExecuteReader()) {
      while (dr.Read())
      Console.WriteLine("{0}\t{1}", dr.GetString(0), dr.GetString(1));
   }
}

In the above code we have not closed any connection, it will be closed automatically. Since the using statement, the using statement will automatically call conn.Close()

(using (SqlConnection conn = new SqlConnection(connString)), also for the SqlDataReader object. And the connection will be automatically closed if any exception occurs.

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