What does "late-bound access to the destination object" mean?

Question

The docs for Interlocked.Exchange<T> contain the following remark:

This method overload is preferable to the Exchange(Object, Object) method overload, because the latter requires late-bound access to the destination object.

I am quite bewildered by this note. To me "late binding" refers to runtime method dispatch and doesn't seem to have anything to do with the technical specifics of atomically swapping two memory locations. What is the note talking about? What does "late-bound access" mean in this context?

Answer 1

canton7's answer is correct, and thanks for the shout-out. I'd like to add a few additional points.

This sentence, as is too often the case in the .NET documentation, both chooses to enstructure bizarre word usements, and thoroughly misses the point. For me, the poor word choice that stood out was not "late bound", which merely misses the point. The really awful word choice is using "destination object" to mean variable. A variable is not an object, any more than your sock drawer is a pair of socks. A variable contains a reference to an object, just as a sock drawer contains socks, and those two things should not be confused.

As you note, the reason to prefer the T version has nothing to do with late binding. The reason to prefer the T version is C# does not allow variant conversions on ref arguments. If you have a variable shelly of type Turtle, you cannot pass ref shelly to a method that takes ref object, because that method could write a Tiger into a ref object.

What then are the logical consequences of using the Object-taking overload on shelly? There are only two possibilities:

• We copy the value of shelly to a second variable of type Object, do the exchange, and then copy the new value back, and now our operation is no longer atomic, which was the whole point of calling the interlocked exchange.
• We change shelly to be of type Object, and now we are in a non-statically-typed and therefore bug-prone world, where we cannot ever be sure that shelly still contains a reference to Turtle.

Since both of those alternatives are terrible, you should use the generic version because it allows the aliased variable to be of the correct type throughout the operation.

Answer 2

The equivalent remark for Interlocked.Exchange(object, object) is:

Beginning with .NET Framework 2.0, the Exchange<T>(T, T) method overload provides a type-safe alternative for reference types. We recommend that you call it instead of this overload.

Although I haven't heard it used in this way before, I think by "late-bound" it simply means "non type-safe", as you need to cast the object to your concrete type (at runtime) before using it.

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As well as virtual method dispatch, "Late Binding" also commonly refers to reflection, as the exact method to be called similarly isn't known until runtime.

To quote Eric Lippert:

Basically by "early binding" we mean "the binding analysis is performed by the compiler and baked in to the generated program"; if the binding fails then the program does not run because the compiler did not get to the code generation phase. By "late binding" we mean "some aspect of the binding will be performed by the runtime" and therefore a binding failure will manifest as a runtime failure

(emphasis mine). Under this rather loose definition, casting object to a concrete type and then calling a method on it could be seen as "late bound", as there's an element of the binding which is performed at, and could fail at, runtime.