Explain the importance of Using keyword and Using Statement in C#

Answer:

In C# parlance, Using can be use in two ways

a) As a directive where it's behavior will be as inclusion of a namespace. e.g.

using System; //import namespace

using System.IO;

using Item = System.Collections.Generic; //creating alias for namespace

namespace UsingNamespaceDemo

{	

  public class TestUsingClass

 {

     //observe Item.List where Item is an alias of System.Collections.Generic namespace

       var intItemList = new Item.List<int>(){1,2,3,4,5}; 

       foreach(var item in intItemList) Console.WriteLine(item);				

 }

}

b)As a statement that can only be used with types that implement IDisposable interface.
e.g.

string connString = "Data Source=abc;Integrated Security=SSPI;Initial Catalog=db;";

using (SqlConnection conn = new SqlConnection(connString))

{

	SqlCommand cmd = conn.CreateCommand();

	cmd.CommandText = "SELECT * FROM tblTest";  

	conn.Open();

	using (SqlDataReader dr = cmd.ExecuteReader())

	{

		while (dr.Read())

		Console.WriteLine("{0}", dr.GetString(0));

	}

}

It can also be consider as a syntactic sugar(short hand version) for try/finally block e.g.

using (SqlConnection conn = new SqlConnection(connString))

{

	//do something

}

is same as

SqlConnection conn = new SqlConnection(connString);

try

{

	//do something

}

finally

{

	if(conn != null)

	{

		conn.Dispose();

	}

}