Confused about PWSTR.From(string)

[jonpryor]

I'm confused about the safety (sanity?) of PWSTR.From(string?):

DirectNAot/DirectN/Partials/PWSTR.partial..cs

Lines 24 to 33 in 7e1bbe4

	public unsafe static PWSTR From(string? str)
	{
	if (str == null)
	return Null;
	fixed (char* chars = str)
	{
	return new PWSTR(chars);
	}
	}

In particular the fixed block:

DirectNAot/DirectN/Partials/PWSTR.partial..cs

Lines 29 to 32 in 7e1bbe4

	fixed (char* chars = str)
	{
	return new PWSTR(chars);
	}

The fixed statement:

prevents the garbage collector from relocating a moveable variable and declares a pointer to that variable.

but that variable is only prevented from relocating within the fixed statement.

Yet here you retain the pointer value and reuse outside the fixed statement.

How does this not "interact poorly" with the GC?

var s = PWSTR.From("foo");

// GC happens, causing `foo` to move in memory

// use `s`. "boom"?

(It's probably "fine" for string constants as those are often interned, but for the output of StringBuilder.ToString()…?)

Am I missing something, or is this as horrifying as I think it is?

[smourier]

Good remark, I was surprised myself the first times I used it, and indeed, that code is not actually "100% guaranteed by the book". I never saw it fail but I'm still curious, maybe we can cook some sample that make it fail.

Assumption, the idea is to use it just for API calls, not to keep it around as a PWSTR.

First reason it works is, as you say in .NET 6+ (DirectNAOT targets 9+), string literals baked into your assemblies are placed on the non-GC frozen object heap, never moved or collected, so PWSTR.From("foo") is a no risk.

Second, more fragile, reason for other strings is between the return and the native call there's typically no allocation, so no GC fire, so the string doesn't get a chance to be deallocated (maybe to move?).

But still, if you don't trust it, you're allowed to not use it :)

[jonpryor]

@smourier wrote:

But still, if you don't trust it, you're allowed to not use it :)

I don't see how you don't use it when it's the parameter type used in DirectNAot-related bindings. Consider https://github.com/smourier/WebView2Aot/blob/cbdcd2270768412e7da72ca353e7f31570601f49/WebView2/Generated/WebView2/Functions.cs#L16:

partial class Functions
{
    [LibraryImport("WebView2Loader")]
    [PreserveSig]
    public static partial HRESULT CreateCoreWebView2EnvironmentWithOptions(
        PWSTR browserExecutableFolder,
        PWSTR userDataFolder,
        [MarshalUsing(typeof(UniqueComInterfaceMarshaller<ICoreWebView2EnvironmentOptions>))] ICoreWebView2EnvironmentOptions environmentOptions, 
        [MarshalUsing(typeof(UniqueComInterfaceMarshaller<ICoreWebView2CreateCoreWebView2EnvironmentCompletedHandler>))] ICoreWebView2CreateCoreWebView2EnvironmentCompletedHandler environmentCreatedHandler);
}

(Code reformatted for legibility.)

I think, at least for my uses, I'll need to review PWSTR.From(string)-using code to separately wrap the string value in a fixed statement…

[smourier]

You don't have to use PWSTR.From, you can allocate and/or free PWSTR by yourself, and DirectN.Extensions is filled with a lot of utilities, there is

Pwstr: https://github.com/smourier/DirectNAot/blob/main/DirectN.Extensions/Utilities/Pwstr.cs
AllocPwstr: https://github.com/smourier/DirectNAot/blob/main/DirectN.Extensions/Utilities/AllocPwstr.cs