What are weak references in .NET?

Weak references reduce the memory pressure placed on the managed heap by large objects.
When a root points to an object, the object cannot be collected because the application's code can reach the object. When a root points to an object, it's called a strong reference to the object. However, the garbage collector also supports weak references. Weak references allow the garbage collector to collect the object, but they also allow the application to access the object.

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Void Method() {
Object o = new Object(); // Creates a strong reference to the
// object.

// Create a strong reference to a short WeakReference object.
// The WeakReference object tracks the Object.
WeakReference wr = new WeakReference(o);

o = null; // Remove the strong reference to the object

o = wr.Target;
if (o == null) {
// A GC occurred and Object was reclaimed.
} else {
// a GC did not occur and we can successfully access the Object
// using o
}
}

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Once you've created a weak reference to an object, you usually set the strong reference to the object to null. If any strong reference remains, the garbage collector will be unable to collect the object.
To use the object again, you must turn the weak reference into a strong reference. You accomplish this simply by calling the WeakReference object's Target property and assigning the result to one of your application's roots. If the Target property returns null, then the object was collected. If the property does not return null, then the root is a strong reference to the object and the code may manipulate the object. As long as the strong reference exists, the object cannot be collected.

Note: It seems that "weak" references in .NET are equivalent to "soft" references in Java.