Why dispose c

This article discusses both methods and provides code examples and tips on how and when to use them. NET garbage collector manages the memory of managed objects native. NET objects but it does not manage, nor is it directly able to clean up unmanaged resources. Managed resources are those that are cleaned up implicitly by the garbage collector.

You do not have to write code to release such resources explicitly. In contrast, you must clean up unmanaged resources file handles, database collections, etc.

There are situations when you might need to allocate memory for unmanaged resources from managed code. As an example, suppose you have to open a database connection from within a class. The database connection instance is an unmanaged resource encapsulated within this class and should be released as soon as you are done with it. In such cases, you'll need to free the memory occupied by the unmanaged resources explicitly, because the GC doesn't free them implicitly.

Briefly, the GC works as shown below:. The GC maintains lists of managed objects arranged in "generations. The generation number indicates to which generation an object belongs. Recently created objects are stored in lower generations compared to those created earlier in the application's life cycle. Longer-lived objects get promoted to higher generations.

Because applications tend to create many short-lived objects compared to relatively few long-lived objects, the GC runs much more frequently to clean up objects in the lower generations than in the higher ones. Keep this information about the.

NET garbage collector in mind as you read the remainder of the article. I'll walk you through the Finalize method first, and then discuss the Dispose method. Finalization is the process by which the GC allows objects to clean up any unmanaged resources that they're holding, before actually destroying the instance. An implementation of the Finalize method is called a "finalizer.

The GC attempts to call finalizers on objects when it finds that the object is no longer in use—when no other object is holding a valid reference to it. In other words, finalizers are methods that the GC calls on "seemingly dead objects" before it reclaims memory for that object. The GC calls an object's finalizer automatically, typically once per instance—although that's not always the case.

The framework calls finalizers on a secondary thread handled by the GC. You should never rely on finalizers to clean up managed resources. A class that has no finalizer implemented but is holding references to unmanaged objects can cause memory leaks, because the resources might become orphaned if a class instance is destroyed before releasing the unmanaged objects. You must implement finalizers very carefully; it's a complex operation that can carry considerable performance overhead.

An object with a previously called Dispose method may not be reused. Dispose is recommended only for the management of native resource objects and Component Object Model COM objects exposed to the. NET Framework. Dispose may not be concurrently invoked from multiple threads, due to unpredictable results. Value types should not be created as disposable types or with unmanaged resource members.

When using unmanaged resources, it is considered best practice to apply the source code's Using statement, which automatically invokes the object's Dispose method after completing the object code. Share this Term. Programming Programming Language. Tech moves fast! Stay ahead of the curve with Techopedia! This should happen for both managed and unmanaged resources. The Dispose bool disposing runs in two distinct scenarios:. To give your base classes a chance to cleanup resources, you should always make a call to your base class' Dispose bool disposing if it is available as the last operation in your own Dispose bool disposing method.

Make sure to pass the same value for disposing to the base class' Dispose bool disposing method. If your object is responsible for at least one resource that does not have its own finalizer, you should implement a finalizer in your object. If your base class already overrides Finalize to follow this pattern, you shouldn't override it yourself.

Your Finalize method should make a single virtual call to Dispose false. All of the finalization cleanup logic should be in the Dispose bool disposing method. If any of the base classes in your inheritance chain implements the IDisposable interface, overrides void Dispose , or overrides Finalize , you should simply override Dispose bool disposing , adding your cleanup logic and making sure to call base.

Dispose disposing as the last statement. Dispose should be considered a reserved word in order to help prevent confusion, so you shouldn't create any other variations other than:. The simplest implementation of the Dispose Pattern does not include a Finalize method.

This is the pattern that you will follow for the majority of types. A more complex implementation of the pattern is a base-class implementation that implements a Finalizer.

The ComplexCleanupExtender shows how you would hook into the Dispose and Finalize cycle from a subclass. Notice that it does not re-implement Dispose or Finalize.

If your class has a finalizer, you should always implement Dispose. This gives users of your class the ability to explicitly clean up the resources which the finalizer is responsible for. You can also implement Dispose on classes that don't have finalizers. The only exception to this rule is for value types, which can't have either a finalizer or a destructor. It is important to remember that Dispose may be called more than once. If that happens, you should never throw an exception, but you may choose to ignore the subsequent calls.

The C automatically generated finalizer chaining does call Dispose bool multiple times, which is why it is especially important to implement the pattern only once in the inheritance chain. If your object controls any disposable types, it should call Dispose on those types in your own Dispose method. If this is done from inside the Dispose bool disposing method, you should only do this if disposing is true.

If your object doesn't control a disposable type, you shouldn't attempt to dispose off it as other code could still be referencing it. The destructor will automatically call reader. You should not throw exceptions from within Dispose except under critical situations.

If executing a Dispose bool disposing method, never throw an exception if disposing is false ; doing so will terminate the process if executing inside a finalizer context. Recreating an object that has already been disposed is usually very difficult, and generally should not be done.

Because of this, you should consider making your object unusable after calling Dispose by throwing an ObjectDisposedException. If you are able to reconstruct the object, you should warn users that they must potentially re-dispose of an object multiple times. If the semantics of your object allows for it, implement a Close method for your resource cleanup.

This method implementation should be identical to Dispose as most developers will not think to call both Close and Dispose. In these cases, the Dispose method should be a call to Close. There are times, however, when it is necessary to provide different implementations for Close and Dispose.

This is usually the case when an object can be opened and closed multiple times without recreating the instance. NET Framework uses this pattern with the System. SqlConnection class. This class allows you to open and close a connection multiple times, but the instance still needs to be disposed after you are done using it. As an alternative, you can release the resources in Close and lazily reacquire them later in a subsequent Open.

If you choose to do this, you should call GC. ReRegisterForFinalize this when reacquiring them. You will also potentially need to reconstruct the object state. If you have cyclic references in your objects, you should set those references to null before calling Dispose.

You should also consider setting any large and expensive managed objects that you own to null before calling Dispose. This is seldom necessary, but it can help reduce the lifetime of the object by making it eligible for garbage collection sooner.

Of course, the definition of large and expensive is subjective, and should be based on performance profiling and measurement. If you are creating a value type, you should avoid making it disposable, and it should never contain unmanaged resources directly. Finalizers are very difficult to implement correctly, mainly because you cannot make certain normally valid assumptions about the state of the system during their execution.

If you decide to implement a finalizer, you should make sure to do it properly. There is both a performance and code complexity cost associated when implementing a finalizer, so consider it very carefully before you do so.

When possible, you should use resource wrappers, such as SafeHandle , to encapsulate unmanaged resources. Finalization increases the cost and duration of your object's lifetime since each finalizable object must be placed on the finalization queue when it is allocated. This happens even if the finalizer is never called.

This has the side effect of making the GC work harder to dispose of your object, and causes it to be kept alive longer. If you must implement a finalizer, make the Finalize method protected , not public or private. These rules do not apply just to the Finalize method, but to any code that executes during finalization.

If you implement the Dispose Pattern described in this article, it also applies to the code that executes inside Dispose bool disposing when disposing is false. Since finalizers run non-deterministically, there is no ordering among them, so you can't rely on them being executing in a specific order. You should not access any finalizable objects your type may have a reference to, because they may have already been finalized. As a result, you should only free unmanaged resources that your object owns.

This caution also extends to static variables. Accessing a static variable that refers to a finalizable object is not safe. This also applies to calling a static method that may use values stored in static variables. HasShutdownStarted to detect if your finalizer is running. It is okay to access unboxed value types. You should never directly call the Finalize method. In fact, it is legal Intermediate Language IL syntax. Even though you should never directly call Finalize , it should still be able to handle situations in which it is called more than once.

In order to do this, you may need a way to detect that finalization has already occurred. It is always possible that other objects in the finalization queue might still have live references to your object, potentially even after your finalizer has run. You should be able to detect when your object is in an inconsistent state during the execution of any method.

If you define a finalizer that closes a resource used by your type, you may need to call GC. KeepAlive at the end of any instance method that doesn't use the this pointer Me in Visual Basic after doing some operation on that resource. This also implies that you need to be able to handle partially constructed instances, which can happen when the constructor never completes.

If the constructor throws an exception, Finalize will still be called. In this instance, it is possible that some of the fields have not been initialized. For example, in the following code, list may be null if the constructor throws an exception before list is assigned to. If list is null when the finalizer runs, it will throw an unhandled NullReferenceException , which will terminate the program.

To fix this error, you need to protect the finalizer like this:. Finalizers should not raise any unhandled exceptions, except in very system critical conditions, such as OutOfMemory.

As of. Finalizers should be simple enough so they don't fail. Since memory allocations may fail due to a lack of memory we are, after all, running in the context of the GC , you should never allocate memory from within a finalizer. This is especially important in a critical finalizer or from SafeHandle 's ReleaseHandle method.

Explicitly, it is called by user code and the class which is implementing dispose method, must has to implement IDisposable interface. There is performance costs associated with Finalize method since it doesn't clean the memory immediately and called by GC automatically. Take our free skill tests to evaluate your skill! In less than 5 minutes, with our skill test, you can identify your knowledge gaps and strengths.

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