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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?
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.
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
}
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.
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;
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(() => IsWorking = false))
{
IsWorking = true;
MundaneYetDangerousWork();
}
You can read more about our solution and how we derived it here.
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<IDictionary<DateTime, string>, List<string>>;
public class Startup
{
public static AppFunc OrderEvents()
{
AppFunc appFunc = (IDictionary<DateTime, string> events) =>
{
if ((events != null) && (events.Count > 0))
{
List<string> result = events.OrderBy(ev => ev.Key)
.Select(ev => 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).
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();
}
}
}
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 matter 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.
Another great use of using is when instantiating a modal dialog.
Using frm as new Form1
Form1.ShowDialog
' Do stuff here
End Using
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
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.
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.
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.
"using" can also be used to resolve namespace conflicts.
See http://www.davidarno.org/c-howtos/aliases-overcoming-name-conflicts/ for a short tutorial I wrote on the subject.
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
}
}
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#.
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
}
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);
}
}
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
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.
It also can be used for creating scopes for Example:
class LoggerScope:IDisposable {
static ThreadLocal<LoggerScope> threadScope =
new ThreadLocal<LoggerScope>();
private LoggerScope previous;
public static LoggerScope Current=> 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);
}
}
}
}
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:
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.
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.
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.
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.
The using statement tells .NET to release the object specified in the using block once it is no longer needed.
So you should use the 'using' block for classes that require cleaning up after them, like System.IO types.
The Rhino Mocks Record-playback Syntax makes an interesting use of using.
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 CIL. And the using statement gets translated into a try and finally block. This is how the using statement is represented in CIL. 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.
The 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 goes out of the defined scope.
}
I am the third generation to work on a system within my organization and of course there are differences in programming styles. I was wondering what the correct way of connecting to a database is as there are two different styles being used within the system. So whats the "correct" way?
Method 1:
try
{
using (SqlConnection con = new SqlConnection(connectionString))
{
con.Open();
using (SqlCommand command = new SqlCommand("StoredProcedure", con))
{
command.CommandType = CommandType.StoredProcedure;
command.Parameters.Add(new SqlParameter("foo", bar));
}
using (SqlDataReader reader = command.ExecuteReader())
{
//do stuff
}
}
}
catch (Exception ex)
{
//do stuff
}
Method 2:
// Variables
SqlCommand loComm;
SqlConnection loConn = null;
SqlDataReader loDR;
try
{
// Init command
loComm = new SqlCommand("foo");
loComm.CommandType = CommandType.StoredProcedure;
loComm.Parameters.AddWithValue("foo", bar);
// Init conn
loConn = new SqlConnection(String);
loConn.Open();
loComm.Connection = loConn;
// Run command
loDR = loComm.ExecuteReader();
//do stuff
}
catch (Exception ex)
{
//do stuff
}
While both methods work I am not sure which one is the most appropriate to use. Is there a reason to use one over the other? The second method is cleaner and easier to understand to me, but it doesn't automatically run the iDispose() function when it is finished.
Edit: My question is different then the one suggested, because one approach uses the "using" statement while the other doesn't. So my question is directly related to whether or not to utilize the "using" statements when making a database connection.
Thanks, for all the responses.
Method 2 is simply incorrect and should be repaired.
When completed, you will be left with IDisposable instances that have not been disposed. Most likely, this will play havoc with the connection management that goes on behind the scenes with SqlConnection, especially if this code gets thrashed a lot before GC decides to step in.
If an instance is IDisposable, then it needs to be Disposed of, preferably in the smallest scope possible. Using using will ensure that this happens, even if the code malfunctions and exceptions are thrown.
A using statement:
using(var disposableInstance = new SomeDisposableClass())
{
//use your disposable instance
}
is (give or take a few minor details) syntactic sugar for:
var disposableInstance = new SomeDisposableClass();
try
{
//use your disposable instance
}
finally
{
//this will definitely run, even if there are exceptions in the try block
disposableInstance.Dispose();
}
Even if something goes wrong in the try block, you can be assured that the finally block will execute, thereby ensuring that your disposables are disposed, no matter what happens.
In the first example, if there is an exception thrown, the objects wrapped in the using blocks will be properly closed and disposed.
In the second, you will manually need to dispose of your objects.
One other thing worth mentioning is that if you were planning on disposing of your objects only when an exception is thrown, you will have objects that are not disposed of, so you would need to implement a try..catch..finally and dispose the objects within the finally block.
The first is the better option.
The first one will close the connection if an exception is thrown.
The second one won't, so I'd say go with the first choice.
What the using block does is to warranty that the object Dispose method will always be invoked, no matter if an exception is thrown or not.
Dispose is a method used to clean up resources. In the case of a DB connection, the connection is released, which is really important.
So, the correct way in this case is using the using pattern.
The object included between the using parents mus be IDisposable whic means that it has a Dispose method that should be invoked when the object is no longer needed. If you don't invoke dispose, it will be disposde at any indeterminate time, when the garbage collector destroys the object. That's undesirable in case like a DB connection that must be closed as soon as possible.
The equivalen of usin is a try finally, which includes a call to Dispose within the finally block.
Reading here, it says:
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. The using statement ensures that Dispose is called even if an exception occurs while you are calling methods on the object. You can achieve the same result by putting the object inside a try block and then calling Dispose in a finally block; in fact, this is how the using statement is translated by the compiler.
That sounds like the correct way is method 1.
The database access classes that implement IDbConnection, IDbCommand and IDataReader all implement IDisposable, but obviously the Command and the Reader are dependent on the Connection. My question is, do I have to Dispose() of each these objects individually or will disposing of the Connection object dispose of the others too ?
That is, can I do this and guarantee I'm not risking leaving any unmanaged resources not being freed:
using (IDbConnection conn = GetConnection())
{
IDbCommand cmd = conn.CreateCommand();
cmd.CommandText = " ..... ";
IDataReader rdr = cmd.ExecuteReader();
while (rdr.Read())
{
}
}
Or do I have to do this instead:
using (IDbConnection conn = GetConnection())
{
using (IDbCommand cmd = conn.CreateCommand())
{
cmd.CommandText = " ..... ";
using (IDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
}
}
}
}
Or is this implementation dependent, so it would potentially work using one database's provider but not for another's ?
Best policy is to use all ADO.NET objects in using blocks- full stop.
A little Reflector-ing on various ADO.NET objects shows that things will more or less be dropped on the floor if not Closed/Disposed. The effect of that will depend a lot on which providers you're using- if you're using something with unmanaged handles underneath (ODBC, OleDb, etc), you're probably going to leak memory, as I didn't see anything in the way of finalizers. If it's an all-managed provider (eg, SqlClient), it'll eventually get cleaned up, but depending on which objects you're holding onto, you could end up keeping resources in use on the DB server a lot longer than you want.
You should dispose them individually or you could also use the solution provide here.
Dispose of them individually. It's entirely possible that the Reader and Command objects do more in their dispose routines than just destroy the connection (either now or in a future version of ADO.NET). So to (a) be explicit and (b) be safe you should dispose of them all individually.
The fact that they implement IDisposable suggests that you should always run the Dispose routine when you're finished with the object, so it's best not to try and second guess the interface, it won't give you any benefit, and you'll probably end up leaking memory and handles.
There are things you can do though to make your code look cleaner when nesting using statements. For example:
(using SqlConnection conn = new SqlConnection())
(using SqlCommand comm = new SqlCommand())
{
//Do stuff
}
That looks neater (in my opinion) than nesting braces.
You should dispose them individually. Even if disposing the IDbConnection would dispose everything under the covers (I don't know if it actually does, nor do I care, for the reason that follow), you should program in the future tense. Tomorrow somebody will edit that code and issue a new command on the same connection and then it will wonder why is he getting an error that the connection is still in use (because the IDataReader was no disposed).
The main advantage of the using statement is resources releasing management. Usually, the .NET Framework Garbage Collector controls resources allocation and release. With the using statement, we can take the control of it. So a using statement implements an IDisposible interface which contains a public Dispose() method.
Also you can always call a Dispose() method manually to release a resource or an object from memory.
hope this helps.
If I have code with nested objects like this do I need to use the nested using statements to make sure that both the SQLCommand and the SQLConnection objects are disposed of properly like shown below or am I ok if the code that instantiates the SQLCommand is within the outer using statement.
using (var conn = new SqlConnection(sqlConnString))
{
using (var cmd = new SqlCommand())
{
cmd.CommandType = CommandType.Text;
cmd.CommandText = cmdTextHere;
conn.Open();
cmd.Connection = conn;
rowsAffected = cmd.ExecuteNonQuery();
}
}
Yes. You could clean it up a little like this
using (SqlConnection conn = new SqlConnection(sqlConnString))
using (System.Data.SqlClient.SqlCommand cmd = new System.Data.SqlClient.SqlCommand())
{
// code here
}
But you would still want a using for each IDisposable object.
Edit: Consider this example of not using an inner using statement.
class A : IDisposable
{
public void Dispose()
{
Console.WriteLine("A Disposed");
}
}
class B : IDisposable
{
public void Dispose()
{
Console.WriteLine("B Disposed");
}
}
Code
using (A a = new A())
{
B b = new B();
}
At the end of the block, A is properly disposed. What happens to B? Well, it's out of scope and simply waiting to be garbage collected. B.Dispose() is not called. On the other hand
using (A a = new A())
using (B b = new B())
{
}
When execution leaves the block (actually, blocks), the compiled code executes calls to the Dispose() method of each object.
You can omit the using around the SqlCommand. The GC will eventually clean it up for you. However, I strongly advice you not to do this. I will explain why.
SqlCommand indirectly inherits from System.ComponentModel.Component, and it therefore inherits its Finalizer method. Not calling dispose on a SqlCommand will ensure that the command is promoted at least one generation after it goes out of scope (the .NET garbage collector is a generational gc). For instance: when the command was in gen 1, it will move on to gen 2. Finalizable objects are kept longer in memory to ensure the finalizer can safely run. But not only the command itself is kept in memory, but all the objects it references go with it to that generation. Objects that it will reference are the SqlConnection, the list of SqlParameter objects, the possibly large CommandText string, and many other internal object it references. That memory can only be removed when that generation is collected, but the higher the generation the less often it is swept.
Not calling will therefore give extra memory pressure and extra work for the finalizer thread.
When .NET is unable to allocate new memory, the CLR will force a garbage collect of all generations. After this, the runtime will usually have again enough space to allocate new objects. However, when this forced collect happens when there are a lot of objects in memory that still have to be promoted to a next gen (because they are finalizable, or are referenced by a finalizable object) it is possible that the CLR is unable to free up enough memory. An OutOfMemoryException will be the result of this.
I must admit I have never seen this happen, because developers didn’t dispose just their SqlCommand objects. However, I have seen OOMs in production systems a lot, caused by not disposing objects properly.
I hope this gives a little bit of background on how the GC works and what the risk is of not disposing (finalizable) object properly. I always dispose all disposable objects. While a look in Reflector could prove that this not necessarily for a certain type, this kind of programming leads to code that is less maintainable and makes code depend on internal behavior of a type (and this behavior might change in the future).
You should call dispose on both, however, it is easier to read if you just do this:
using (SqlConnection conn = new SqlConnection(sqlConnString))
using (SqlCommand cmd = new SqlCommand())
{
cmd.CommandType = CommandType.Text;
cmd.CommandText = cmdTextHere;
conn.Open();
cmd.Connection = conn;
rowsAffected = cmd.ExecuteNonQuery();
}
since there's an implicit block created (like an if statement or for statement) and since using is an entire block, you don't need the braces and it looks neater in my opinion. Some will say you shoul always use braces because it's too easy to accidentally add a second statement and create a bug, but in this case i think that's unlikely.
To answer your question, yes, you can do that.
Since the SqlCommand object is limited by outer braces, it will be collected by GC when the execution goes out of the block.
Other answers are also okay, but they did not exactly answered your 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?
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.
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
}
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.
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;
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(() => IsWorking = false))
{
IsWorking = true;
MundaneYetDangerousWork();
}
You can read more about our solution and how we derived it here.
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<IDictionary<DateTime, string>, List<string>>;
public class Startup
{
public static AppFunc OrderEvents()
{
AppFunc appFunc = (IDictionary<DateTime, string> events) =>
{
if ((events != null) && (events.Count > 0))
{
List<string> result = events.OrderBy(ev => ev.Key)
.Select(ev => 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).
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();
}
}
}
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 matter 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.
Another great use of using is when instantiating a modal dialog.
Using frm as new Form1
Form1.ShowDialog
' Do stuff here
End Using
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
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.
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.
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.
"using" can also be used to resolve namespace conflicts.
See http://www.davidarno.org/c-howtos/aliases-overcoming-name-conflicts/ for a short tutorial I wrote on the subject.
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
}
}
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#.
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
}
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);
}
}
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
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.
It also can be used for creating scopes for Example:
class LoggerScope:IDisposable {
static ThreadLocal<LoggerScope> threadScope =
new ThreadLocal<LoggerScope>();
private LoggerScope previous;
public static LoggerScope Current=> 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);
}
}
}
}
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:
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.
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.
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.
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.
The using statement tells .NET to release the object specified in the using block once it is no longer needed.
So you should use the 'using' block for classes that require cleaning up after them, like System.IO types.
The Rhino Mocks Record-playback Syntax makes an interesting use of using.
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 CIL. And the using statement gets translated into a try and finally block. This is how the using statement is represented in CIL. 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.
The 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 goes out of the defined scope.
}