Using statement in C#

Introduction

C#, through the .NET Framework common language runtime (CLR), automatically releases the memory used to store objects that are no longer required. The release of memory is non-deterministic; memory is released whenever the CLR decides to perform garbage collection. However, it is usually best to release limited resources such as file handles and network connections as quickly as possible.

The using statement allows the programmer to specify when objects that use resources should release them. The object provided to the using statement must implement the IDisposable interface. This interface provides the Dispose method, which should release the object's resources.

A using statement can be exited either when the end of the using statement is reached or if an exception is thrown and control leaves the statement block before the end of the statement.

This article is an introduction to the using statement in c# and also provides some insight into the actual implementation of the statement.

The Code

When you are using an object that encapsulates any resource, you have to make sure that when you are done with the object, the object's Dispose method is called. This can be done more easily using the using statement in C#. The using statement simplifies the code that you have to write to create and then finally clean up the object. The using statement obtains the resource specified, executes the statements and finally calls the Dispose method of the object to clean up the object. The following piece of code illustrates its use.

using (TextWriter w = File.CreateText("log.txt"))
{
    w.WriteLine("This is line one");
}

Now that's cool. But before we can start using this code, let us try to understand what happens behind the screen. Lets have a look at the IL code for the above code section.

.locals init ([0] class [mscorlib]System.IO.TextWriter w)
  IL_0000:  ldstr      "log.txt"
  IL_0005:  call       class [mscorlib]System.IO.StreamWriter 
      [mscorlib]System.IO.File::CreateText(string)
  IL_000a:  stloc.0
 
 .try
  {
    IL_000b:  ldloc.0
    IL_000c:  ldstr      "This is line one"
    IL_0011:  callvirt   instance void [mscorlib]
      System.IO.TextWriter::WriteLine(string)
    IL_0016:  leave.s    IL_0022
  }  // end .try
  finally
  {
    IL_0018:  ldloc.0
    IL_0019:  brfalse.s  IL_0021
    IL_001b:  ldloc.0
    IL_001c:  callvirt   instance void [mscorlib]
      System.IDisposable::Dispose()
    IL_0021:  endfinally
  }  // end handler

Hmmmm.... Well doesn't look like this is my code. That's because I see a try and a finally in the IL code (something that I haven't implemented). Wait a minute. IT IS MY CODE....

Waaaaaah... Somebody changed my code...

Well the truth is, somebody did change your code. The CLR. The CLR converts your code into MSIL. And the using statement gets translated into a try and finally block. This is how the using statement is represented in IL. A using statement is translated into three parts: acquisition, usage, and disposal. The resource is first acquired, then the usage is enclosed in a try statement with a finally clause. The object then gets disposed in the finally clause. For example the following lines of code using the using statement,

using (MyResource myRes = new MyResource())
{
    myRes.DoSomething();

}

gets translated to,

    
MyResource myRes= new MyResource();
try
{
    myRes.DoSomething();
}
finally
{
    // Check for a null resource.
    if (myRes!= null)
        // Call the object's Dispose method.
        ((IDisposable)myRes).Dispose();
}

Hmmm... That explains it.

The above code that uses the using statement corresponds to one of the two possible expansions. When MyResource is a value type, the expansion in the finally block will be
finally{((IDisposable)myRes).Dispose();}
If MyResource is of reference type, the expansion becomes
finally
{
if(myRes != null)
((IDisposable)myRes).Dispose();
}
This way, if a null resource is acquired, then no call will be made to Dispose, thus avoiding any exception that occurs.
Well, that explains everything.

Using 'using'

A typical scenario where we could use the using statement is :

string connString = "Data Source=localhost;Integrated " + 
  "Security=SSPI;Initial Catalog=Northwind;";

using (SqlConnection conn = new SqlConnection(connString))
{
  SqlCommand cmd = conn.CreateCommand();
  cmd.CommandText = "SELECT ID, Name FROM Customers";
  
  conn.Open();

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

Note

The using statement is only useful for objects with a lifetime that does not extend beyond the method in which the objects are constructed. Remember that the objects you instantiate must implement the System.IDisposable interface.
There is no equivalent for the using statement in vb.net. You have to use the try finally block.

Declaring Multiple variable

You can instance several objects (Of same type)in the same using statement block:

using (SqlConnection conn, SqlConnection conn2, SqlConnection conn3...) { 

     //Do Something... 

}

As a final note, remember that any object instanced in this statement must implement the System.IDisposable interface.

References:

 http://www.codeproject.com/Articles/6564/Understanding-the-using-statement-in-C