Are there Weak References in .NET?

Question

I would like to keep a list of a certain class of objects in my application. But I still want the object to be garbage collected. Can you create weak references in .NET?

For reference:

• Garbage Collecting objects which keep track of their own instances in an internal Map
• Create a weak reference to an object

Answer From MSDN:

To establish a weak reference with an object, you create a WeakReference using the instance of the object to be tracked. You then set the Target property to that object and set the object to null. For a code example, see WeakReference in the class library.

Answer 1

Yes, there's a generic weak reference class.

MSDN > Weak Reference

Answer 2

Can you create weak references in .NET?

Yes:

WeakReference r = new WeakReference(obj);

Uses System.WeakReference.

Answer 3

Yes...

There is a pretty good example to be found here:

http://web.archive.org/web/20080212232542/http://www.robherbst.com/blog/2006/08/21/c-weakreference-example/

In your class you created two member variables:

WeakReference _weakRef = null;

Person _strongRef = null;

You created two new Person objects (which are simple objects I just created for this example, consisting of a Name property and some reference tracking code). Next you set the member variables to the newly created instances of the Person objects.

_strongRef = p;

_weakRef = new WeakReference(p1);

The difference here you’ll notice that _strongRef is just a regular normal reference, whereas _weakRef is set to a WeakReference object with the person object (p1) passed in as a parameter in the constructor.

If a garbage collection were to occur, or just for testing purposes you called it yourself with:

GC.Collect();

Then the p1 target object that is held by the _weakRef member variable should be garbage collected. You can write code to check:

if (_weakRef.IsAlive)

If the WeakReference is still alive you can convert the WeakReference to a strong or normal reference by using code like this:

Person p = _weakRef.Target as Person;

Now the p reference is treated as a strong reference and won’t be collected until it is no longer used. If you wanted to keep the reference around after the scope you could set that to a member variable.

Answer 4

Here is the full (non thread safe) implementation sample of WeakReference

ClassA objA = new ClassA();
WeakReference wr = new WeakReference(objA);
// do stuff 
GC.Collect();
ClassA objA2;
if (wr.IsAlive)
    objA2 = wr.Target as ClassA; 
else
    objA2 = new ClassA(); // create it directly if required

WeakReference is in the System namespace hence no need to include any special assembly for it.