I have a method which return an instance of resource class, how can use the "using" statement to avoid resource leak?
public ResourceClass method()
{
return new ResourceClass ();
}
I hope my question is clear.
Thanks in advance
You can relegate the responsibility of disposing it to your caller by declaring your using block around the entire usage of the resource
public ResourceClass method()
{
return new ResourceClass();
}
then
using(var s = method())
{
//do something with s
}
This is as simple as just utilising the method method to create the instance in your using statement:
public void YourMethod()
{
using (ResourceClass Foo = method())
{
...
}
}
This will only work of course if ResourceClass implements IDisposable. Otherwise, your program won't compile.
Related
Let's imagine I have a simple class that is disposable:
class CanDispose : IDisposable
{
...
}
Now I can obviously put this in a "using" block to make it dispose:
using (var myDisposable = new CanDispose())
{
...
}
But what if I have a factory method that returns the disposable via an out parameter? In my example there can be multiple returns, so multiple outs are clean:
public bool CreateDisposable(out CanDispose canDispose)
{
canDispose = new CanDispose();
...
return ret;
}
How do I put this in a using statement? Simply putting the function call into a using block doesn't seem to work. Do I have to resort to returning a Tuple (will that work?), or is there a simpler way?
Note, this doesn't appear to work. At least it doesn't shut up the warning about disposing before going out of scope:
using (CreateDisposable(out CanDispose myDispose))
{
....
}
A using statement doesn't have to declare a variable. Having a disposable as an out paramter is not ideal, and you should avoid it in your own code, but if you're consuming a library that does it, you can simply put the variable in the using block's expression after calling the method.
CreateDisposable(out CanDispose myDispose);
using (myDispose)
{
}
How do I put this in a using statement?
You do not. Those two patterns -- out parameter, and automatic dispose via using -- do not compose well.
Do I have to resort to returning a Tuple (will that work?)
I note that questions of the form "will this work?" can be answered by trying it. But to save you those keystrokes: tuples are not disposable.
is there a simpler way?
Just use a using block on the out parameter; make it two statements, not one.
But your question indicates that there is a more fundamental problem here. Presumably the Boolean that is returned is meaningful. It sounds like you are planning on ignoring that Boolean and using the out parameter regardless; if that is safe to do, then just make a version that returns the value without the Boolean, since it is apparently not useful.
More generally -- the fact that you are returning two values from a factory is a code smell. Can you say more about what you are doing? There might be a better pattern to use altogether.
Put your using on the next line
CreateDisposable(out CanDispose myDispose);
using (myDispose)
{
}
You need to declare it first and then use it on the using statement.
CreateDisposable(out CanDispose myDispose)
using (myDispose)
{
....
}
You can look into this links to get more information about using
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement
https://www.c-sharpcorner.com/article/the-using-statement-in-C-Sharp/
Sinceusing statement is just an implementation of 'try / finally' statement, so you can just use this statement directly as the following:
CanDispose canDispose = null;
try
{
CreateDisposable(out CanDispose canDispose);
}
finally
{
canDispose?.Dispose();
}
Wrapping your factory method with an adapter will let you keep the semantics that you're used to. Consistency is generally a Good Thing™
public void SomeFunction()
{
var getInstance = () => {
CreateDisposable( out CanDispose instance );
return instance;
};
using ( var instance = getInstance() ) {
// do something with your instance before it gets disposed.
}
}
If you need multiple results
My suggestion is to use a Tuple, but it looks like you want to know if the object was created successfully, so you may want to be checking the results. In any case, using the out is going to require you to define the var on a different line.
using System;
public class Program
{
public static void Main()
{
Console.WriteLine("Hello World");
var factory = new DisposableFactory();
IDisposable d;
var success = factory.TryGetDisposable(out d);
if (success){
using (d) {
}
}
using (factory.GetDisposableResults().Item2){
}
}
}
public class CanDispose : IDisposable
{
void IDisposable.Dispose(){
}
}
public class DisposableFactory
{
public bool TryGetDisposable(out IDisposable d)
{
d = new CanDispose();
return true;
}
public (bool, IDisposable) GetDisposableResults()
{
var tup = (true, new CanDispose());
return tup;
}
}
In C++, it's fairly easy to write a Guard class which takes a reference to a variable (usually a bool) and when the instance object exits scope and gets destructed, the destructor resets the variable to the original value.
void someFunction() {
if(!reentryGuard) {
BoolGuard(&reentryGuardA, true);
// do some stuff that might cause reentry of this function
// this section is both early-exit and exception proof, with regards to restoring
// the guard variable to its original state
}
}
I'm looking for a graceful way to do this in C# using the disposal pattern (or maybe some other mechanism?) I'm thinking that passing a delegate to call might work, but seems a bit more error-prone than the guard above. Suggestions welcome!
Something like:
void someFunction() {
if(!reentryGuard) {
using(var guard = new BoolGuard(ref reentryGuard, true)) {
// do some stuff that might cause reentry of this function
// this section is both early-exit and exception proof, with regards to restoring
// the guard variable to its original state
}
}
}
With the understanding that the above code won't work.
You are correct…without unsafe code, you can't save the address of a by-ref parameter. But, depending on how much you can change the overall design, you can create a "guardable" type, such that it's a reference type containing the value to actually guard.
For example:
class Program
{
class Guardable<T>
{
public T Value { get; private set; }
private sealed class GuardHolder<TGuardable> : IDisposable where TGuardable : Guardable<T>
{
private readonly TGuardable _guardable;
private readonly T _originalValue;
public GuardHolder(TGuardable guardable)
{
_guardable = guardable;
_originalValue = guardable.Value;
}
public void Dispose()
{
_guardable.Value = _originalValue;
}
}
public Guardable(T value)
{
Value = value;
}
public IDisposable Guard(T newValue)
{
GuardHolder<Guardable<T>> guard = new GuardHolder<Guardable<T>>(this);
Value = newValue;
return guard;
}
}
static void Main(string[] args)
{
Guardable<int> guardable = new Guardable<int>(5);
using (var guard = guardable.Guard(10))
{
Console.WriteLine(guardable.Value);
}
Console.WriteLine(guardable.Value);
}
}
Here's a functional (as in lambda-based) way to do it. Pluses are, no need to use a using:
(note: This is not thread-safe. If you are looking to keep different threads from running the same code simultaneously, look at the lock statement, the monitor, and the mutex)
// usage
GuardedOperation TheGuard = new GuardedOperation() // instance variable
public void SomeOperationToGuard()
{
this.TheGuard.Execute(() => TheCodeToExecuteGuarded);
}
// implementation
public class GuardedOperation
{
public bool Signalled { get; private set; }
public bool Execute(Action guardedAction)
{
if (this.Signalled)
return false;
this.Signalled = true;
try
{
guardedAction();
}
finally
{
this.Signalled = false;
}
return true;
}
}
EDIT
Here is how you could use the guarded with parameters:
public void SomeOperationToGuard(int aParam, SomeType anotherParam)
{
// you can pass the params to the work method using closure
this.TheGuard.Execute(() => TheMethodThatDoesTheWork(aParam, anotherParam);
}
private void TheMethodThatDoesTheWork(int aParam, SomeType anotherParam) {}
You could also introduce overloads of the Execute method that accept a few different variants of the Action delegate, like Action<T> and Action<T1, T2>
If you need return values, you could introduce overloads of Execute that accept Func<T>
Sounds like the sort of thing you'd have to implement yourself - there are no such mechanisms built into C# or the .NET framework, though I did locate a deprecated class Guard on MSDN.
This sort of functionality would likely need to use a Using statement to operate without passing around an Action block, which as you said could get messy. Note that you can only call using against and IDisposable object, which will then be disposed - the perfect trigger for resetting the value of the object in question.
You can derive your object from IDisposable interface and implement it.
In specific case you are presenting here Dispose will be called as soon as you leave using scope.
Example:
public class BoolGuard : IDisposable
{
....
...
public void Dispose()
{
//DISPOSE IMPLEMANTATION
}
}
Can we use the using statement in a constructor to declare an instance of an object for later usage. For example.
public class TestClass {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
using (this._dataContext = DataClassesDataContext(connString));
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
}
You must implement IDisposable yourself and call Dispose on the data context from you Dispose method.
public class TestClass : IDisposable {
private DataClassesDataContext _dataContext;
public TestClass(string connString){
_dataContext = new DataClassesDataContext(connString);
}
private bool someMethod(){
_dataContext.instanceMethod(); // i want to use instance methods wherever needed and define once
}
public void Dispose(){
_dataContext.Dispose();
}
}
It's not clear what you expect the using statement to do here. All it does is make sure that Dispose is called at the end of the block.
So basically you'd be creating a DataClassesDataContext (I assume you missed the new keyword...), storing a reference in a field, and then immediately disposing of it. That's not going to work well - you should get rid of the using statement, but quite possibly make your class implement IDisposable so that when the instance of TestClass is disposed, you dispose of the data context.
According to MSDN:
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.
The using statement is basically syntactic sugar for try/finally.
try
{
_dataContext = new DataClassesDataContext(connString);
}
finally
{
if (_dataContext != null)
((IDisposable)dataContext).Dispose();
}
Looking at it in this way it should become obvious that datacontext is no longer in scope and therefore can't be used by other methods as you desire. To solve the problem, you should make the class implement IDisposeable.
using (this._dataContext = DataClassesDataContext(connString));
is the same as
try
{
this._dataContext = DataClassesDataContext(connString);
}
catch
{
}
finally
{
if(this._dataContext!=null)
((IDisposable)this._dataContext).Dispose();
}
So you'll get _dataContext disposed in you constructor and it will be no longer available. You should implement IDisposable interface and you'll be able to employ using statement where you want like this:
using (TestClass test = new TestClass("conn"))
{
//your code
}
Just a quick sanity check here!
If I have a static method in an instance class e.g:
public class myClass
{
public static void AMethod()
{
//do somit
}
}
Would it cause problems if I was making reference to IDisposable resources in the method body, an object context for example?
public static void AMethod()
{
ObjectContext context = new ObjectContext();
// do somit
}
By would it cause problems, I mean, would it retain the object context behind the scenes after the end of the method body due to the fact it was a static method?
The class as can be seen, isn't static, and the variable is local to the method.
I'm aware that I should be using 'using' here, just curious as to whether this particular combo of events could/would cause memory leaks.
In order to avoid any problems it is recommended to dispose IDisposable resources as soon as you have finished using them. This could happen by wrapping them in a using statement:
public static void AMethod()
{
using (ObjectContext context = new ObjectContext())
{
// do something
}
}
After leaving the scope of your method AMethod, your context object can't be used anymore so it'll be garbage collected eventually.
But as it implements IDisposable, you should use the using statement:
using (ObjectContext context = new ...)
{
// Use the context here
}
If I am using the using keyword, do I still have to implement IDisposable?
You can't have one without the other.
When you write :
using(MyClass myObj = new MyClass())
{
myObj.SomeMthod(...);
}
Compiler will generate something like this :
MyClass myObj = null;
try
{
myObj = new MyClass();
myObj.SomeMthod(...);
}
finally
{
if(myObj != null)
{
((IDisposable)myObj).Dispose();
}
}
So as you can see while having using keyword it is assumed/required that IDisposable is implemented.
If you use the using statement the enclosed type must already implement IDisposable otherwise the compiler will issue an error. So consider IDisposable implementation to be a prerequisite of using.
If you want to use the using statement on your custom class, then you must implement IDisposable for it. However this is kind of backward to do because there's no sense to do so for the sake of it. Only if you have something to dispose of like an unmanaged resource should you implement it.
// To implement it in C#:
class MyClass : IDisposable {
// other members in you class
public void Dispose() {
// in its simplest form, but see MSDN documentation linked above
}
}
This enables you to:
using (MyClass mc = new MyClass()) {
// do some stuff with the instance...
mc.DoThis(); //all fake method calls for example
mc.DoThat();
} // Here the .Dispose method will be automatically called.
Effectively that's the same as writing:
MyClass mc = new MyClass();
try {
// do some stuff with the instance...
mc.DoThis(); //all fake method calls for example
mc.DoThat();
}
finally { // always runs
mc.Dispose(); // Manual call.
}
You are confusing things. You can only use the "using" keyword on something that implements IDisposable.
Edit: If you use the using keyword, you don't have to explicity invoke Dispose, it will be called automatically at the end of the using block. Others have already posted examples of how the using statement is translated into a try - finally statement, with Dispose invoked within the finally block.
Yes, the using keyword is syntactic sugar for this type of pattern...(from msdn)
Font font1 = new Font("Arial", 10.0f);
try
{
byte charset = font1.GdiCharSet;
}
finally
{
if (font1 != null)
((IDisposable)font1).Dispose();
}
Edit: a useful example.
When you find you are doing things in a finally section consistantly, for example resetting a Cursor back to its default after having set it to a wait cursor, this is a candidate for this pattern...
public class Busy : IDisposable
{
private Cursor _oldCursor;
private Busy()
{
_oldCursor = Cursor.Current;
}
public static Busy WaitCursor
{
get
{
Cursor.Current = Cursors.WaitCursor;
return new Busy();
}
}
#region IDisposable Members
public void Dispose()
{
Cursor.Current = _oldCursor;
}
#endregion
}
Called like...
using(Busy.WaitCursor)
{
// some operation that needs a wait cursor.
}
Using will only dispose of disposable objects. So wraping a using block around an object that does not implement IDisposable is rather useless will in fact cause a compiler error.
http://msdn.microsoft.com/en-us/library/yh598w02.aspx
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 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.
You must implement IDisposable to use using. If you try to use using() on a type that does not implement IDisposable you get the following compile time error:
error CS1674: 'SomeType': type used in a using statement must be implicitly convertible to 'System.IDisposable'
The using keyword already implements, so if you use the using keyword, you do not have to invoke IDisposable