typeof(TSecret) - the secret magic behind .NET generics
In last post we’ve talked about how .NET does code sharing for reference types. This time let’s take a look at how
typeof(T) does its black magic.
In particular, how does the code knows what
typeof(T) is, in the presence of code sharing?
Obviously if there is no code sharing at all, each method instantiation are different and the code would be instantiated with the correct
typeof(T) code where T is a real type, it obviously would “just work”.
Before we dive into the implementation, let’s take a first crack at this problem and see what are the challenges.
Recently I’ve been spending a bit of time playing with Go language. Coming from a C++ and C# background, I’ve been wondering what does C# async await (C++ coroutines, or node.js await/promise - pick your choice), would look like in Go.
It turns out go does not need async/await - every thing can be synchronous by default.
Let me explain why I arrived at this conclusion.
But before that, let’s take a look at why we need the async/await in the first place.
In last post I’ve talked how one would start writing a emulator. Now it’s time to dive a bit deeper to see how we write the main loop to drive the emulation.
Last time we talked about .NET dependent handle. It is a handle that promotes secondary if primary is promoted - as if there is a imaginary reference between them. This time let’s take a look at another secret handle - ref-counted handle. A ref-counted handle is a special handle that will become either strong or weak depending on the ref count. It’s only used in COM interop today internally in the CLR.
You can find its definition in gcinterface.h
* REFCOUNTED HANDLES
* Refcounted handles are handles that behave as strong handles while the
* refcount on them is greater than 0 and behave as weak handles otherwise.
* N.B. These are currently NOT general purpose.
* The implementation is tied to COM Interop.
HNDTYPE_REFCOUNTED = 5,
.NET publicly has documented 4 kind of handles:
Weak (also called Short Weak) - Don’t keep target object alive and will return null when object is gone. The target will become null when the object enters for finalization.
WeakTrackResurrection (also called Long Weak) - Don’t keep target object alive and will return null when object is gone. It’ll return the object even when the object is being finalized or resurrected.
Normal (also called strong) - keeps target object alive. If you are not careful, you may leak the object.
Pinned - Keeps the target object alive and prevents GC from moving this object around. Useful when you are passing this object to native code, as native code won’t know if GC moved it. Note that using a lot of pinning handles may degrade GC performance. The most common offender is pinned strings/arrays in interop calls.
You can also find them described in GCHandle enumeration.
However, besides these 4 types, there are actually more secret internal handle types that are not exposed. In this post I’ll be talking about dependent handle, and why it is totally awesome.