What's the purpose of StrongBox<T>?

[msedi]

While digging through WeakReferenceList<T> I came across the StrongBox<T> where I'm not really sure what this class does.
The docs are a bit vague about this, what's the difference between holding an object in a StrongBox<T> or holding the object itself in, for example, a ConcurrentQueue<T>?

The reason why I ask is also this article:

https://stackoverflow.com/questions/13612098/usage-of-concurrentqueuestrongboxt

that made things even worse. Here some author say that it improved performance of some code of a company he worked for when switching from a direct reference to a StrongBox<T>.

[huoyaoyuan]

Used by C++/CLI? It has syntax to define reference of value types, like DateTime^.

[stephentoub]

It let's you "box" a value type to the heap but in a strongly-typed manner, e.g. instead of doing:

object box = 42;
... // some time later
int i = (int)box;

and then being able to pass around the weakly-typed box, you can do:

var box = new StrongBox<int>(42);
... // some time later
int i = box.Value;

and then pass around box and access its Value, which is strongly typed as int (via the generic T). Hence the name "strong box", i.e. "strongly-typed box". (It's not limited for use with value types, though.)

You can see example uses of it here:
https://source.dot.net/#System.Private.CoreLib/StrongBox.cs,bdbb335cbbdee41b,references

Some typical (advanced) use cases include:

• Sharing an individual value type instance between multiple threads in a mutable fashion
• Enabling value types of arbitrary size to be atomically written as a reference
• Working around the lack of ref fields (by passing around the box with its mutable contents)

[msedi]

@stephentoub: Thanks a lot for your explanation.