What are the uses of using in C

Question

User kokos answered the wonderful Hidden Features of C  question by mentioning the using keyword  Can you elaborate on that  What are the uses of using

User · Answer

I ve used it a lot in the past to work with input and output streams   You can nest them nicely and it takes away a lot of the potential problems you usually run into  by automatically calling dispose    For example           using  FileStream fs   new FileStream  c  file txt   FileMode Open                         using  BufferedStream bs   new BufferedStream fs                                 using  System IO StreamReader sr   new StreamReader bs                                         string output   sr ReadToEnd

User · Answer

The reason for the using statement is to ensure that the object is disposed as soon as it goes out of scope  and it doesn t require explicit code to ensure that this happens   As in Understanding the  using  statement in C   codeproject  and Using objects that implement IDisposable  microsoft   the C  compiler converts  using  MyResource myRes   new MyResource          myRes DoSomething        to       Limits scope of myRes     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              C  8 introduces a new syntax  named  using declarations       A using declaration is a variable declaration preceded by the using keyword  It tells the compiler that the variable being declared should be disposed at the end of the enclosing scope    So the equivalent code of above would be   using var myRes   new MyResource    myRes DoSomething      And when control leaves the containing scope  usually a method  but it can also be a code block   myRes will be disposed

User · Answer

Everything outside the curly brackets is disposed  so it is great to dispose your objects if you are not using them  This is so because if you have a SqlDataAdapter object and you are using it only once in the application life cycle and you are filling just one dataset and you don t need it anymore  you can use the code   using SqlDataAdapter adapter object   new SqlDataAdapter sql command parameter           do stuff      here adapter object is disposed automatically

User · Answer

Things like this   using  var conn   new SqlConnection  connection string         conn Open            Execute SQL statement here on the connection you created     This SqlConnection will be closed without needing to explicitly call the  Close   function  and this will happen even if an exception is thrown  without the need for a try catch finally

User · Answer

The using keyword defines the scope for the object and then disposes of the object when the scope is complete   For example   using  Font font2   new Font  Arial   10 0f            use font2     See here for the MSDN article on the C  using keyword

User · Answer

Just adding a little something that I was surprised did not come up  The most interesting feature of using  in my opinion  is that no mater how you exit the using block  it will always dispose the object  This includes returns and exceptions   using  var db   new DbContext          if db State    State Closed  throw new Exception  Database connection is closed         return db Something ToList        It doesn t matter if the exception is thrown or the list is returned  The DbContext object will always be disposed

User · Answer

using is used when you have a resource that you want disposed after it s been used   For instance if you allocate a File resource and only need to use it in one section of code for a little reading or writing  using is helpful for disposing of the File resource as soon as your done   The resource being used needs to implement IDisposable to work properly   Example   using  File file   new File  parameters          code to do stuff with the file

User · Answer

Just adding a little something that I was surprised did not come up  The most interesting feature of using  in my opinion  is that no mater how you exit the using block  it will always dispose the object  This includes returns and exceptions   using  var db   new DbContext          if db State    State Closed  throw new Exception  Database connection is closed         return db Something ToList        It doesn t matter if the exception is thrown or the list is returned  The DbContext object will always be disposed

User · Answer

The reason for the using statement is to ensure that the object is disposed as soon as it goes out of scope  and it doesn t require explicit code to ensure that this happens   As in Understanding the  using  statement in C   codeproject  and Using objects that implement IDisposable  microsoft   the C  compiler converts  using  MyResource myRes   new MyResource          myRes DoSomething        to       Limits scope of myRes     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              C  8 introduces a new syntax  named  using declarations       A using declaration is a variable declaration preceded by the using keyword  It tells the compiler that the variable being declared should be disposed at the end of the enclosing scope    So the equivalent code of above would be   using var myRes   new MyResource    myRes DoSomething      And when control leaves the containing scope  usually a method  but it can also be a code block   myRes will be disposed

User · Answer

The Rhino Mocks Record-playback Syntax makes an interesting use of using

User · Answer

I ve used it a lot in the past to work with input and output streams   You can nest them nicely and it takes away a lot of the potential problems you usually run into  by automatically calling dispose    For example           using  FileStream fs   new FileStream  c  file txt   FileMode Open                         using  BufferedStream bs   new BufferedStream fs                                 using  System IO StreamReader sr   new StreamReader bs                                         string output   sr ReadToEnd

User · Answer

It also can be used for creating scopes for Example   class LoggerScope IDisposable      static ThreadLocal lt LoggerScope gt  threadScope            new ThreadLocal lt LoggerScope gt        private LoggerScope previous      public static LoggerScope Current  gt  threadScope Value      public bool WithTime get       public LoggerScope bool withTime          previous   threadScope Value         threadScope Value   this         WithTime withTime           public void Dispose           threadScope Value   previous           class Program      public static void Main params string   args          new Program   Run             public void Run          log  something happend           using new LoggerScope false              log  the quick brown fox jumps over the lazy dog               using new LoggerScope true                  log  nested scope                                  void log string message         if LoggerScope Current  null             Console WriteLine message             if LoggerScope Current WithTime                Console WriteLine DateTime Now

User · Answer

Thanks to the comments below  I will clean this post up a bit  I shouldn t have used the words  garbage collection  at the time  apologies    When you use using  it will call the Dispose   method on the object at the end of the using s scope   So you can have quite a bit of great cleanup code in your Dispose   method    A bullet point here which will hopefully maybe get this un-markeddown  If you implement IDisposable  make sure you call GC SuppressFinalize   in your Dispose   implementation  as otherwise automatic garbage collection will try to come along and Finalize it at some point  which at the least would be a waste of resources if you ve already Dispose  d of it

User · Answer

using  in the sense of using  var foo   new Bar        Baz       Is actually shorthand for a try finally block  It is equivalent to the code  var foo   new Bar    try     Baz      finally     foo Dispose       You ll note  of course  that the first snippet is much more concise than the second and also that there are many kinds of things that you might want to do as cleanup even if an exception is thrown  Because of this  we ve come up with a class that we call Scope that allows you to execute arbitrary code in the Dispose method  So  for example  if you had a property called IsWorking that you always wanted to set to false after trying to perform an operation  you d do it like this  using  new Scope      gt  IsWorking   false       IsWorking   true    MundaneYetDangerousWork       You can read more about our solution and how we derived it here

User · Answer

Things like this   using  var conn   new SqlConnection  connection string         conn Open            Execute SQL statement here on the connection you created     This SqlConnection will be closed without needing to explicitly call the  Close   function  and this will happen even if an exception is thrown  without the need for a try catch finally

User · Answer

In conclusion  when you use a local variable of a type that implements IDisposable  always  without exception  use using1   If you use nonlocal IDisposable variables  then always implement the IDisposable pattern   Two simple rules  no exception1  Preventing resource leaks otherwise is a real pain in the  ss     1   The only exception is      when you re handling exceptions  It might then be less code to call Dispose explicitly in the finally block

User · Answer

Interestingly  you can also use the using IDisposable pattern for other interesting things  such as the other point of the way that Rhino Mocks uses it    Basically  you can take advantage of the fact that the compiler will always call  Dispose on the  used  object   If you have something that needs to happen after a certain operation     something that has a definite start and end     then you can simply make an IDisposable class that starts the operation in the constructor  and then finishes in the Dispose method   This allows you to use the really nice using syntax to denote the explicit start and end of said operation   This is also how the System Transactions stuff works

User · Answer

Another great use of using is when instantiating a modal dialog   Using frm as new Form1  Form1 ShowDialog    do stuff here  End Using

User · Answer

Not that it is ultra important  but using can also be used to change resources on the fly  Yes disposable as mentioned earlier  but perhaps specifically you don t want the resources they mismatch with other resources during the rest of your execution  So you want to dispose of it so it doesn t interfere elsewhere

User · Answer

The reason for the using statement is to ensure that the object is disposed as soon as it goes out of scope  and it doesn t require explicit code to ensure that this happens   As in Understanding the  using  statement in C   codeproject  and Using objects that implement IDisposable  microsoft   the C  compiler converts  using  MyResource myRes   new MyResource          myRes DoSomething        to       Limits scope of myRes     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              C  8 introduces a new syntax  named  using declarations       A using declaration is a variable declaration preceded by the using keyword  It tells the compiler that the variable being declared should be disposed at the end of the enclosing scope    So the equivalent code of above would be   using var myRes   new MyResource    myRes DoSomething      And when control leaves the containing scope  usually a method  but it can also be a code block   myRes will be disposed

User · Answer

Another great use of using is when instantiating a modal dialog   Using frm as new Form1  Form1 ShowDialog    do stuff here  End Using

User · Answer

When using ADO NET you can use the keywork for things like your connection object or reader object  That way when the code block completes it will automatically dispose of your connection

User · Answer

The reason for the using statement is to ensure that the object is disposed as soon as it goes out of scope  and it doesn t require explicit code to ensure that this happens   As in Understanding the  using  statement in C   codeproject  and Using objects that implement IDisposable  microsoft   the C  compiler converts  using  MyResource myRes   new MyResource          myRes DoSomething        to       Limits scope of myRes     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              C  8 introduces a new syntax  named  using declarations       A using declaration is a variable declaration preceded by the using keyword  It tells the compiler that the variable being declared should be disposed at the end of the enclosing scope    So the equivalent code of above would be   using var myRes   new MyResource    myRes DoSomething      And when control leaves the containing scope  usually a method  but it can also be a code block   myRes will be disposed

User · Answer

In conclusion  when you use a local variable of a type that implements IDisposable  always  without exception  use using1   If you use nonlocal IDisposable variables  then always implement the IDisposable pattern   Two simple rules  no exception1  Preventing resource leaks otherwise is a real pain in the  ss     1   The only exception is      when you re handling exceptions  It might then be less code to call Dispose explicitly in the finally block

User · Answer

Not that it is ultra important  but using can also be used to change resources on the fly  Yes disposable as mentioned earlier  but perhaps specifically you don t want the resources they mismatch with other resources during the rest of your execution  So you want to dispose of it so it doesn t interfere elsewhere

User · Answer

Not that it is ultra important  but using can also be used to change resources on the fly  Yes disposable as mentioned earlier  but perhaps specifically you don t want the resources they mismatch with other resources during the rest of your execution  So you want to dispose of it so it doesn t interfere elsewhere

User · Answer

Thanks to the comments below  I will clean this post up a bit  I shouldn t have used the words  garbage collection  at the time  apologies    When you use using  it will call the Dispose   method on the object at the end of the using s scope   So you can have quite a bit of great cleanup code in your Dispose   method    A bullet point here which will hopefully maybe get this un-markeddown  If you implement IDisposable  make sure you call GC SuppressFinalize   in your Dispose   implementation  as otherwise automatic garbage collection will try to come along and Finalize it at some point  which at the least would be a waste of resources if you ve already Dispose  d of it

User · Answer

Microsoft documentation states that using has a double function  https   msdn microsoft com en-us library zhdeatwt aspx   both as a directive and in statements  As a statement  as it was pointed out here in other answers  the keyword is basically syntactic sugar to determine a scope to dispose an IDisposable object  As a directive  it is routinely used to import namespaces and types  Also as a directive  you can create aliases for namespaces and types  as pointed out in the book  C  5 0 In a Nutshell  The Definitive Guide   http   www amazon com 5-0-Nutshell-The-Definitive-Reference-ebook dp B008E6I1K8    by Joseph and Ben Albahari  One example   namespace HelloWorld       using AppFunc   Func lt IDictionary lt DateTime  string gt   List lt string gt  gt       public class Startup               public static AppFunc OrderEvents                          AppFunc appFunc    IDictionary lt DateTime  string gt  events    gt                                if   events    null   amp  amp   events Count  gt  0                                         List lt string gt  result   events OrderBy ev   gt  ev Key                           Select ev   gt  ev Value                           ToList                        return result                                    throw new ArgumentException  Event dictionary is null or empty                                return appFunc                      This is something to adopt wisely  since the abuse of this practice can hurt the clarity of one s code  There is a nice explanation on C  aliases  also mentioning pros and cons  in DotNetPearls  http   www dotnetperls com using-alias

User · Answer

Microsoft documentation states that using has a double function  https   msdn microsoft com en-us library zhdeatwt aspx   both as a directive and in statements  As a statement  as it was pointed out here in other answers  the keyword is basically syntactic sugar to determine a scope to dispose an IDisposable object  As a directive  it is routinely used to import namespaces and types  Also as a directive  you can create aliases for namespaces and types  as pointed out in the book  C  5 0 In a Nutshell  The Definitive Guide   http   www amazon com 5-0-Nutshell-The-Definitive-Reference-ebook dp B008E6I1K8    by Joseph and Ben Albahari  One example   namespace HelloWorld       using AppFunc   Func lt IDictionary lt DateTime  string gt   List lt string gt  gt       public class Startup               public static AppFunc OrderEvents                          AppFunc appFunc    IDictionary lt DateTime  string gt  events    gt                                if   events    null   amp  amp   events Count  gt  0                                         List lt string gt  result   events OrderBy ev   gt  ev Key                           Select ev   gt  ev Value                           ToList                        return result                                    throw new ArgumentException  Event dictionary is null or empty                                return appFunc                      This is something to adopt wisely  since the abuse of this practice can hurt the clarity of one s code  There is a nice explanation on C  aliases  also mentioning pros and cons  in DotNetPearls  http   www dotnetperls com using-alias

User · Answer

The using keyword defines the scope for the object and then disposes of the object when the scope is complete   For example   using  Font font2   new Font  Arial   10 0f            use font2     See here for the MSDN article on the C  using keyword

User · Answer

Another great use of using is when instantiating a modal dialog   Using frm as new Form1  Form1 ShowDialog    do stuff here  End Using

User · Answer

For me the name  using  is a little bit confusing  because is can be a directive to import a Namespace or a statement  like the one discussed here  for error handling   A different name for error handling would ve been nice  and maybe a somehow more obvious one

User · Answer

public class ClassA IDisposable        region IDisposable Members             public void Dispose                             GC SuppressFinalize this              endregion       public void fn Data               using  ClassA ObjectName   new ClassA                                    use objectName

User · Answer

The using statement tells  NET to release the object specified in the using block once it is no longer needed  So you should use  using  block for classes that require cleaning up after them  like System IO Types

User · Answer

The using statement provides a convenience mechanism to correctly use IDisposable objects  As a rule  when you use an IDisposable object  you should declare and instantiate it in a using statement  The using statement calls the Dispose method on the object in the correct way  and  when you use it as shown earlier  it also causes the object itself to go out of scope as soon as Dispose is called  Within the using block  the object is read-only and cannot be modified or reassigned   This comes from  here

User · Answer

using  can also be used to resolve name space conflicts  See http   www davidarno org c-howtos aliases-overcoming-name-conflicts  for a short tutorial I wrote on the subject

User · Answer

Everything outside the curly brackets is disposed  so it is great to dispose your objects if you are not using them  This is so because if you have a SqlDataAdapter object and you are using it only once in the application life cycle and you are filling just one dataset and you don t need it anymore  you can use the code   using SqlDataAdapter adapter object   new SqlDataAdapter sql command parameter           do stuff      here adapter object is disposed automatically

User · Answer

Another example of a reasonable use in which the object is immediately disposed   using  IDataReader myReader   DataFunctions ExecuteReader CommandType Text  sql ToString    dp Parameters  myConnectionString          while  myReader Read                   MyObject theObject   new MyObject            theObject PublicProperty   myReader GetString 0           myCollection Add theObject

User · Answer

When using ADO NET you can use the keywork for things like your connection object or reader object  That way when the code block completes it will automatically dispose of your connection

User · Answer

It also can be used for creating scopes for Example   class LoggerScope IDisposable      static ThreadLocal lt LoggerScope gt  threadScope            new ThreadLocal lt LoggerScope gt        private LoggerScope previous      public static LoggerScope Current  gt  threadScope Value      public bool WithTime get       public LoggerScope bool withTime          previous   threadScope Value         threadScope Value   this         WithTime withTime           public void Dispose           threadScope Value   previous           class Program      public static void Main params string   args          new Program   Run             public void Run          log  something happend           using new LoggerScope false              log  the quick brown fox jumps over the lazy dog               using new LoggerScope true                  log  nested scope                                  void log string message         if LoggerScope Current  null             Console WriteLine message             if LoggerScope Current WithTime                Console WriteLine DateTime Now

User · Answer

In conclusion  when you use a local variable of a type that implements IDisposable  always  without exception  use using1   If you use nonlocal IDisposable variables  then always implement the IDisposable pattern   Two simple rules  no exception1  Preventing resource leaks otherwise is a real pain in the  ss     1   The only exception is      when you re handling exceptions  It might then be less code to call Dispose explicitly in the finally block

User · Answer

Thanks to the comments below  I will clean this post up a bit  I shouldn t have used the words  garbage collection  at the time  apologies    When you use using  it will call the Dispose   method on the object at the end of the using s scope   So you can have quite a bit of great cleanup code in your Dispose   method    A bullet point here which will hopefully maybe get this un-markeddown  If you implement IDisposable  make sure you call GC SuppressFinalize   in your Dispose   implementation  as otherwise automatic garbage collection will try to come along and Finalize it at some point  which at the least would be a waste of resources if you ve already Dispose  d of it

User · Answer

public class ClassA IDisposable        region IDisposable Members             public void Dispose                             GC SuppressFinalize this              endregion       public void fn Data               using  ClassA ObjectName   new ClassA                                    use objectName

User · Answer

using  can also be used to resolve name space conflicts  See http   www davidarno org c-howtos aliases-overcoming-name-conflicts  for a short tutorial I wrote on the subject

User · Answer

Another great use of using is when instantiating a modal dialog   Using frm as new Form1  Form1 ShowDialog    do stuff here  End Using

User · Answer

using  in the sense of using  var foo   new Bar        Baz       Is actually shorthand for a try finally block  It is equivalent to the code  var foo   new Bar    try     Baz      finally     foo Dispose       You ll note  of course  that the first snippet is much more concise than the second and also that there are many kinds of things that you might want to do as cleanup even if an exception is thrown  Because of this  we ve come up with a class that we call Scope that allows you to execute arbitrary code in the Dispose method  So  for example  if you had a property called IsWorking that you always wanted to set to false after trying to perform an operation  you d do it like this  using  new Scope      gt  IsWorking   false       IsWorking   true    MundaneYetDangerousWork       You can read more about our solution and how we derived it here

User · Answer

Using Clause is used to define the scope for the particular variable   For example        Using SqlConnection conn new SqlConnection ConnectionString                                Conn Open                  Execute sql statements here                You do not have to close the connection explicitly here as  USING  will close the connection once the object Conn becomes out of the defined scope

User · Answer

using  in the sense of using  var foo   new Bar        Baz       Is actually shorthand for a try finally block  It is equivalent to the code  var foo   new Bar    try     Baz      finally     foo Dispose       You ll note  of course  that the first snippet is much more concise than the second and also that there are many kinds of things that you might want to do as cleanup even if an exception is thrown  Because of this  we ve come up with a class that we call Scope that allows you to execute arbitrary code in the Dispose method  So  for example  if you had a property called IsWorking that you always wanted to set to false after trying to perform an operation  you d do it like this  using  new Scope      gt  IsWorking   false       IsWorking   true    MundaneYetDangerousWork       You can read more about our solution and how we derived it here

User · Answer

Since a lot of people still do   using  System IO StreamReader r   new System IO StreamReader      using  System IO StreamReader r2   new System IO StreamReader             code     I guess a lot of people still don t know that you can do   using  System IO StreamReader r   new System IO StreamReader      r2   new System IO StreamReader             code

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You can make use of the alias namespace by way of the following example   using LegacyEntities   CompanyFoo CoreLib x86 VBComponents CompanyObjects    This is called a using alias directive as as you can see  it can be used to hide long-winded references should you want to make it obvious in your code what you are referring to e g   LegacyEntities Account   instead of  CompanyFoo CoreLib x86 VBComponents CompanyObjects Account   or simply  Account      It is not obvious this is a legacy entity

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using as a statement automatically calls the dispose on the specified   object  The object must implement the IDisposable interface  It is   possible to use several objects in one statement as long as they are   of the same type    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

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Another example of a reasonable use in which the object is immediately disposed   using  IDataReader myReader   DataFunctions ExecuteReader CommandType Text  sql ToString    dp Parameters  myConnectionString          while  myReader Read                   MyObject theObject   new MyObject            theObject PublicProperty   myReader GetString 0           myCollection Add theObject

User · Answer

The using keyword defines the scope for the object and then disposes of the object when the scope is complete   For example   using  Font font2   new Font  Arial   10 0f            use font2     See here for the MSDN article on the C  using keyword

User · Answer

I ve used it a lot in the past to work with input and output streams   You can nest them nicely and it takes away a lot of the potential problems you usually run into  by automatically calling dispose    For example           using  FileStream fs   new FileStream  c  file txt   FileMode Open                         using  BufferedStream bs   new BufferedStream fs                                 using  System IO StreamReader sr   new StreamReader bs                                         string output   sr ReadToEnd

User · Answer

There are two usages of the using keyword in C  as follows    As a directive  Generally we use the using keyword to add namespaces in code-behind and class files  Then it makes available all the classes  interfaces and abstract classes and their methods and properties in the current page   Example   using System IO   As a statement  This is another way to use the using keyword in C   It plays a vital role in improving performance in Garbage Collection   The using statement ensures that Dispose   is called even if an exception occurs when you are creating objects and calling methods  properties and so on  Dispose   is a method that is present in the IDisposable interface that helps to implement custom Garbage Collection  In other words if I am doing some database operation  Insert  Update  Delete  but somehow an exception occurs then here the using statement closes the connection automatically  No need to call the connection Close   method explicitly   Another important factor is that it helps in Connection Pooling  Connection Pooling in  NET helps to eliminate the closing of a database connection multiple times  It sends the connection object to a pool for future use  next database call   The next time a database connection is called from your application the connection pool fetches the objects available in the pool  So it helps to improve the performance of the application  So when we use the using statement the controller sends the object to the connection pool automatically  there is no need to call the Close   and Dispose   methods explicitly   You can do the same as what the using statement is doing by using try-catch block and call the Dispose   inside the finally block explicitly  But the using statement does the calls automatically to make the code cleaner and more elegant  Within the using block  the object is read-only and cannot be modified or reassigned   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 preceding code I am not closing any connection  it will close automatically  The using statement will call conn Close   automatically due to the using statement  using  SqlConnection conn   new SqlConnection connString   and the same for a SqlDataReader object  And also if any exception occurs it will close the connection automatically   For more information  see Usage and Importance of Using in C

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Things like this   using  var conn   new SqlConnection  connection string         conn Open            Execute SQL statement here on the connection you created     This SqlConnection will be closed without needing to explicitly call the  Close   function  and this will happen even if an exception is thrown  without the need for a try catch finally

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using can be used to call IDisposable   It can also be used to alias types   using  SqlConnection cnn   new SqlConnection        code    using f1   System Windows Forms Form

User · Answer

Everything outside the curly brackets is disposed  so it is great to dispose your objects if you are not using them  This is so because if you have a SqlDataAdapter object and you are using it only once in the application life cycle and you are filling just one dataset and you don t need it anymore  you can use the code   using SqlDataAdapter adapter object   new SqlDataAdapter sql command parameter           do stuff      here adapter object is disposed automatically

User · Answer

The using statement provides a convenience mechanism to correctly use IDisposable objects  As a rule  when you use an IDisposable object  you should declare and instantiate it in a using statement  The using statement calls the Dispose method on the object in the correct way  and  when you use it as shown earlier  it also causes the object itself to go out of scope as soon as Dispose is called  Within the using block  the object is read-only and cannot be modified or reassigned   This comes from  here

User · Answer

using can be used to call IDisposable   It can also be used to alias types   using  SqlConnection cnn   new SqlConnection        code    using f1   System Windows Forms Form

User · Answer

using can be used to call IDisposable   It can also be used to alias types   using  SqlConnection cnn   new SqlConnection        code    using f1   System Windows Forms Form

User · Answer

Interestingly  you can also use the using IDisposable pattern for other interesting things  such as the other point of the way that Rhino Mocks uses it    Basically  you can take advantage of the fact that the compiler will always call  Dispose on the  used  object   If you have something that needs to happen after a certain operation     something that has a definite start and end     then you can simply make an IDisposable class that starts the operation in the constructor  and then finishes in the Dispose method   This allows you to use the really nice using syntax to denote the explicit start and end of said operation   This is also how the System Transactions stuff works

User · Answer

using can be used to call IDisposable   It can also be used to alias types   using  SqlConnection cnn   new SqlConnection        code    using f1   System Windows Forms Form

User · Answer

Since a lot of people still do   using  System IO StreamReader r   new System IO StreamReader      using  System IO StreamReader r2   new System IO StreamReader             code     I guess a lot of people still don t know that you can do   using  System IO StreamReader r   new System IO StreamReader      r2   new System IO StreamReader             code

User · Answer

Not that it is ultra important  but using can also be used to change resources on the fly  Yes disposable as mentioned earlier  but perhaps specifically you don t want the resources they mismatch with other resources during the rest of your execution  So you want to dispose of it so it doesn t interfere elsewhere

User · Answer

Interestingly  you can also use the using IDisposable pattern for other interesting things  such as the other point of the way that Rhino Mocks uses it    Basically  you can take advantage of the fact that the compiler will always call  Dispose on the  used  object   If you have something that needs to happen after a certain operation     something that has a definite start and end     then you can simply make an IDisposable class that starts the operation in the constructor  and then finishes in the Dispose method   This allows you to use the really nice using syntax to denote the explicit start and end of said operation   This is also how the System Transactions stuff works

User · Answer

There are two usages of the using keyword in C  as follows    As a directive  Generally we use the using keyword to add namespaces in code-behind and class files  Then it makes available all the classes  interfaces and abstract classes and their methods and properties in the current page   Example   using System IO   As a statement  This is another way to use the using keyword in C   It plays a vital role in improving performance in Garbage Collection   The using statement ensures that Dispose   is called even if an exception occurs when you are creating objects and calling methods  properties and so on  Dispose   is a method that is present in the IDisposable interface that helps to implement custom Garbage Collection  In other words if I am doing some database operation  Insert  Update  Delete  but somehow an exception occurs then here the using statement closes the connection automatically  No need to call the connection Close   method explicitly   Another important factor is that it helps in Connection Pooling  Connection Pooling in  NET helps to eliminate the closing of a database connection multiple times  It sends the connection object to a pool for future use  next database call   The next time a database connection is called from your application the connection pool fetches the objects available in the pool  So it helps to improve the performance of the application  So when we use the using statement the controller sends the object to the connection pool automatically  there is no need to call the Close   and Dispose   methods explicitly   You can do the same as what the using statement is doing by using try-catch block and call the Dispose   inside the finally block explicitly  But the using statement does the calls automatically to make the code cleaner and more elegant  Within the using block  the object is read-only and cannot be modified or reassigned   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 preceding code I am not closing any connection  it will close automatically  The using statement will call conn Close   automatically due to the using statement  using  SqlConnection conn   new SqlConnection connString   and the same for a SqlDataReader object  And also if any exception occurs it will close the connection automatically   For more information  see Usage and Importance of Using in C

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Everything outside the curly brackets is disposed  so it is great to dispose your objects if you are not using them  This is so because if you have a SqlDataAdapter object and you are using it only once in the application life cycle and you are filling just one dataset and you don t need it anymore  you can use the code   using SqlDataAdapter adapter object   new SqlDataAdapter sql command parameter           do stuff      here adapter object is disposed automatically

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using is used when you have a resource that you want disposed after it s been used   For instance if you allocate a File resource and only need to use it in one section of code for a little reading or writing  using is helpful for disposing of the File resource as soon as your done   The resource being used needs to implement IDisposable to work properly   Example   using  File file   new File  parameters          code to do stuff with the file

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Since a lot of people still do   using  System IO StreamReader r   new System IO StreamReader      using  System IO StreamReader r2   new System IO StreamReader             code     I guess a lot of people still don t know that you can do   using  System IO StreamReader r   new System IO StreamReader      r2   new System IO StreamReader             code

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using as a statement automatically calls the dispose on the specified   object  The object must implement the IDisposable interface  It is   possible to use several objects in one statement as long as they are   of the same type    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

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I ve used it a lot in the past to work with input and output streams   You can nest them nicely and it takes away a lot of the potential problems you usually run into  by automatically calling dispose    For example           using  FileStream fs   new FileStream  c  file txt   FileMode Open                         using  BufferedStream bs   new BufferedStream fs                                 using  System IO StreamReader sr   new StreamReader bs                                         string output   sr ReadToEnd

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using  can also be used to resolve name space conflicts  See http   www davidarno org c-howtos aliases-overcoming-name-conflicts  for a short tutorial I wrote on the subject

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using is used when you have a resource that you want disposed after it s been used   For instance if you allocate a File resource and only need to use it in one section of code for a little reading or writing  using is helpful for disposing of the File resource as soon as your done   The resource being used needs to implement IDisposable to work properly   Example   using  File file   new File  parameters          code to do stuff with the file

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Interestingly  you can also use the using IDisposable pattern for other interesting things  such as the other point of the way that Rhino Mocks uses it    Basically  you can take advantage of the fact that the compiler will always call  Dispose on the  used  object   If you have something that needs to happen after a certain operation     something that has a definite start and end     then you can simply make an IDisposable class that starts the operation in the constructor  and then finishes in the Dispose method   This allows you to use the really nice using syntax to denote the explicit start and end of said operation   This is also how the System Transactions stuff works

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The Rhino Mocks Record-playback Syntax makes an interesting use of using

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You can make use of the alias namespace by way of the following example   using LegacyEntities   CompanyFoo CoreLib x86 VBComponents CompanyObjects    This is called a using alias directive as as you can see  it can be used to hide long-winded references should you want to make it obvious in your code what you are referring to e g   LegacyEntities Account   instead of  CompanyFoo CoreLib x86 VBComponents CompanyObjects Account   or simply  Account      It is not obvious this is a legacy entity

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Using Clause is used to define the scope for the particular variable   For example        Using SqlConnection conn new SqlConnection ConnectionString                                Conn Open                  Execute sql statements here                You do not have to close the connection explicitly here as  USING  will close the connection once the object Conn becomes out of the defined scope

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using is used when you have a resource that you want disposed after it s been used   For instance if you allocate a File resource and only need to use it in one section of code for a little reading or writing  using is helpful for disposing of the File resource as soon as your done   The resource being used needs to implement IDisposable to work properly   Example   using  File file   new File  parameters          code to do stuff with the file

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For me the name  using  is a little bit confusing  because is can be a directive to import a Namespace or a statement  like the one discussed here  for error handling   A different name for error handling would ve been nice  and maybe a somehow more obvious one

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using  can also be used to resolve name space conflicts  See http   www davidarno org c-howtos aliases-overcoming-name-conflicts  for a short tutorial I wrote on the subject

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Another example of a reasonable use in which the object is immediately disposed   using  IDataReader myReader   DataFunctions ExecuteReader CommandType Text  sql ToString    dp Parameters  myConnectionString          while  myReader Read                   MyObject theObject   new MyObject            theObject PublicProperty   myReader GetString 0           myCollection Add theObject

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The Rhino Mocks Record-playback Syntax makes an interesting use of using

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The Rhino Mocks Record-playback Syntax makes an interesting use of using

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Things like this   using  var conn   new SqlConnection  connection string         conn Open            Execute SQL statement here on the connection you created     This SqlConnection will be closed without needing to explicitly call the  Close   function  and this will happen even if an exception is thrown  without the need for a try catch finally

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Thanks to the comments below  I will clean this post up a bit  I shouldn t have used the words  garbage collection  at the time  apologies    When you use using  it will call the Dispose   method on the object at the end of the using s scope   So you can have quite a bit of great cleanup code in your Dispose   method    A bullet point here which will hopefully maybe get this un-markeddown  If you implement IDisposable  make sure you call GC SuppressFinalize   in your Dispose   implementation  as otherwise automatic garbage collection will try to come along and Finalize it at some point  which at the least would be a waste of resources if you ve already Dispose  d of it

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In conclusion  when you use a local variable of a type that implements IDisposable  always  without exception  use using1   If you use nonlocal IDisposable variables  then always implement the IDisposable pattern   Two simple rules  no exception1  Preventing resource leaks otherwise is a real pain in the  ss     1   The only exception is      when you re handling exceptions  It might then be less code to call Dispose explicitly in the finally block

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Another example of a reasonable use in which the object is immediately disposed   using  IDataReader myReader   DataFunctions ExecuteReader CommandType Text  sql ToString    dp Parameters  myConnectionString          while  myReader Read                   MyObject theObject   new MyObject            theObject PublicProperty   myReader GetString 0           myCollection Add theObject

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When using ADO NET you can use the keywork for things like your connection object or reader object  That way when the code block completes it will automatically dispose of your connection

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The using statement tells  NET to release the object specified in the using block once it is no longer needed  So you should use  using  block for classes that require cleaning up after them  like System IO Types

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The using keyword defines the scope for the object and then disposes of the object when the scope is complete   For example   using  Font font2   new Font  Arial   10 0f            use font2     See here for the MSDN article on the C  using keyword

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When using ADO NET you can use the keywork for things like your connection object or reader object  That way when the code block completes it will automatically dispose of your connection

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using  in the sense of using  var foo   new Bar        Baz       Is actually shorthand for a try finally block  It is equivalent to the code  var foo   new Bar    try     Baz      finally     foo Dispose       You ll note  of course  that the first snippet is much more concise than the second and also that there are many kinds of things that you might want to do as cleanup even if an exception is thrown  Because of this  we ve come up with a class that we call Scope that allows you to execute arbitrary code in the Dispose method  So  for example  if you had a property called IsWorking that you always wanted to set to false after trying to perform an operation  you d do it like this  using  new Scope      gt  IsWorking   false       IsWorking   true    MundaneYetDangerousWork       You can read more about our solution and how we derived it here

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Since a lot of people still do   using  System IO StreamReader r   new System IO StreamReader      using  System IO StreamReader r2   new System IO StreamReader             code     I guess a lot of people still don t know that you can do   using  System IO StreamReader r   new System IO StreamReader      r2   new System IO StreamReader             code

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