C# Language Highlight: Using Keyword

Welcome to this installment of the .NET Nuts & Bolts column. The focus of this article will be on highlighting the C# using keyword. The using keyword has been in the language since the initial release. Something many developers do not realize is that the using keyword is a dual-purpose keyword. You'll touch on the common usage that C# developers should be familiar with, add a twist to it, and focus on the lesser known use that is valuable to understand.

Using Directive

Many readers will recognize the C# using keyword for its most common usage, which is to import types defined in namespaces into the scope of a code file. It is a directive when employed in this fashion. It adorns the top of a code file to import a namespace so that it can be used throughout the code file in more of a shorthand notation. Example code without the using directive:

namespace CodeGuru.Example
{
   class Program
   {
      static void Main(string[] args)
      {
         Program.TestCfg();
      }

      public static string TestCfg()
      {
         return System.Configuration
            .ConfigurationSettings.AppSettings["MySetting"];
      }
   }
)

Here is the example from above refactored to demonstrate the using directive to import types:

using System.Configuration;

namespace CodeGuru.Example
{
   class Program
   {
      static void Main(string[] args)
      {
         Program.TestCfg();
      }

      public static string TestCfg()
      {
         return ConfigurationSettings.AppSettings["MySetting"];
      }
   }
}

In addition to importing a namespace, the using directive can be used to create what is known as an alias. Aliasing allows you to assign an alternate name to use in place of a particular namespace. It looks syntactically similar to a variable declaration and assignment except it just denotes an alternate name to replace the namespace, not unlike the act of defining a synonym. The following example demonstrates a refactoring of the examples above to employ an alias for the System.Configuration.ConfigurationSettings namespace:

using cfg = System.Configuration.ConfigurationSettings;

namespace CodeGuru.Example
{
   class Program
   {
      static void Main(string[] args)
      {
         Program.TestCfg();
      }

      public static string TestCfg()
      {
         // use of the cfg alias
         return cfg.AppSettings["MySetting"];
      }
   }
}

I personally will typically employ the using directive to import types and not bother with aliasing, but I know others who enjoy aliasing.

Using Statement

The dual purpose of the using keyword of which many C# developers are painfully unaware is employing using as a statement. When used as a statement, it defines a scope at the end of which a specific object will be disposed once the scope is exited. This may not seem earth shattering, but the first time that your application runs out of available database connections because a developer unknowingly wasn't closing and disposing a DataReader object it will become pretty clear pretty fast. The following sample code demonstrates a code mistake that I see all too often around use of DataReaders:

using System.Data;
using System.Data.SqlClient;
using cfg = System.Configuration.ConfigurationSettings;

namespace CodeGuru.Example
{
   class Program
   {
      static void Main(string[] args)
      {
         Program.TestCfg();
      }

      public static void TestCfg()
      {
         string connectString =
            cfg.AppSettings["ConnectionString"];
         string query =
            "Select * from Northwind";
         SqlConnection connection =
            new SqlConnection(connectString);
         SqlCommand command = new SqlCommand(query, connection);
         IDataReader reader = null;
         try
         {
            reader = command.ExecuteReader();
            while (reader.Read())
            {
               // Some processing stuff here
            }

            // Works great as long as we are on a happy path
            // with no errors
            reader.Close();
         }
         catch (SqlException e)
         {
            // Some error handling stuff here
         }
      }
   }
}

Mark Strawmyer is a Senior Architect of .NET applications for large and mid-size organizations. He specializes in architecture, design and development of Microsoft-based solutions. Mark was honored to be named a Microsoft MVP for application development with C# for the fifth year in a row. You can reach Mark at mark.strawmyer@crowehorwath.com.