Add support for IBuffer.AsMemory() and Memory.AsBuffer().

[jlaanstra]

Contributes to #824

WindowsRuntimeBuffer is now internally represented by a Memory<byte> instead of a byte[]. Also simplified a few codepaths to use the CopyTo functionality from Span.

AsMemory() follows the approach taken by ToArray() in that the capacity of the IBuffer is ignored and the Memory represents the length of the buffer.

[Sergio0694]

I don't think we should expose these APIs as is, because they are effectively unsafe, but hide that. Having a custom memory manager wrapping an IBuffer is equivalent to having a memory manager with a finalizer (because IBuffer will have a finalizer), which is explicitly not recommended specifically because it's unsafe.

We should definitely expose a way to convert between IBuffer and Memory<T> types, but I'm wondering whether it wouldn't be better to do so via an API that makes the fact they're unsafe explicit, and with proper documentation, so developers can know what to do. For instance, something like this:

public static class WindowsRuntimeMarshal
{
    public static Memory<byte> AsMemoryUnsafe(IBuffer buffer);
}

Just for context, pasting it here again, this is the scenario I'm worried about:

IBuffer buffer = GetSomeBuffer();
Span<byte> span = buffer.AsMemory().Span;
// <-- GC runs here
DoStuff(span); // Boom 💥

Also cc. @tannergooding, @GrabYourPitchforks, thoughts? 🙂

[jlaanstra]

I don't think we should expose these APIs as is, because they are effectively unsafe, but hide that. Having a custom memory manager wrapping an IBuffer is equivalent to having a memory manager with a finalizer (because IBuffer will have a finalizer), which is explicitly not recommended specifically because it's unsafe.

We should definitely expose a way to convert between IBuffer and Memory<T> types, but I'm wondering whether it wouldn't be better to do so via an API that makes the fact they're unsafe explicit, and with proper documentation, so developers can know what to do. For instance, something like this:

public static class WindowsRuntimeMarshal
{
    public static Memory<byte> AsMemoryUnsafe(IBuffer buffer);
}

Just for context, pasting it here again, this is the scenario I'm worried about:

IBuffer buffer = GetSomeBuffer();
Span<byte> span = buffer.AsMemory().Span;
// <-- GC runs here
DoStuff(span); // Boom 💥

Also cc. @tannergooding, @GrabYourPitchforks, thoughts? 🙂

Doesn't this apply equally to string?

string buffer = GetSomeString();
Span<char> span = buffer.AsMemory().Span;
// <-- GC runs here
DoStuff(span); // Boom 💥

[Sergio0694]

"Doesn't that equally apply to string?"

No, because a string is a managed object and the span will keep it alive (as it's an interior pointer to it).

[jlaanstra]

Superseded by #1556