Question: Is .NET behavior proper in the presence of the `\\?\` prefix? What does "\\?\." exactly mean?

[ericwj]

Description

I know \\?\ means 'do not parse', but might I halfway misuse this repo for a Win32 question about this syntax and the behavior of .NET path API's with them?

Is it even valid to use \\?\. unless there is something to talk to that is called exactly that? Like a device or file/directory that has such a weird name? Or should it in fact refer to the current directory and direct API's to maintain the \\?\ prefix to support long paths?

Also, I just give one example where path math isn't being done because the \\?\ prefix is present (presumably). Should this be and remain this way?

private const string PWD = "<PWD>";
[Theory]
[InlineData(@"\\?\.", @"\\?\" + PWD)]
[InlineData(@"\\?\C:\Users\..", @"\\?\C:\")]
public void GetFullPath(string path, string expected) {
    if (expected is object) expected = expected.Replace(PWD, Environment.CurrentDirectory);
    var actual = Path.GetFullPath(path);
    Assert.Equal(expected, actual);
}

First result

  Message: 
    Assert.Equal() Failure
                  ↓ (pos 4)
    Expected: \\?\C:\Users\(...)\b.Tests\b···
    Actual:   \\?\.
                  ↑ (pos 4)

Second result

  Message: 
    Assert.Equal() Failure
                     ↓ (pos 7)
    Expected: \\?\C:\
    Actual:   \\?\C:\Users\..
                     ↑ (pos 7)

Configuration

Same on netcoreapp3.1 as on net5.0

> dotnet --list-sdks
3.1.301 [C:\Program Files\dotnet\sdk]
3.1.400-preview-015178 [C:\Program Files\dotnet\sdk]
5.0.100-preview.5.20279.10 [C:\Program Files\dotnet\sdk]

Regression?

Might be intended behavior.

Opinion

It could be useful to be able to do path math with \\?\ with an increasing number of machines able to use long paths since recent versions of Windows.

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[ericwj]

The relevant documentations is here:

• Enable Long Paths in Windows 10, Version 1607, and Later

Scrolling slightly up, the section above that reads (emphasis mine):

The "\\?\" prefix (...) indicate that the path should be passed to the system with minimal modification, which means that you cannot use forward slashes to represent path separators, or a period to represent the current directory, or double dots to represent the parent directory.

But this does not mean that .NET API's might not provide support for doing path math and support . and .. before passing paths to the Win32 API if . and .. could not appear in valid paths with "\\?\" otherwise.

Besides long paths on systems not configured to accept them without \\?\, an oddity that might make such support by .NET handy is in rare, perhaps ill-adviced, but valid situations where applications might encounter GPT volume identifiers, e.g. \\?\Volume{bc850fae-cfcd-4783-9093-222ad1635ade}. These always requires the \\?\ prefix. Use of these paths may be required in the rare situation that the volume does not have any access path with a DOS device name.

Paths with \\?\ may be provided in several ways, amongst which as the working directory at application start, and they work with other API's such as Combine, GetPathRoot and GetDirectoryName.

[carlossanlop]

Thanks for the details.

We don't normalize paths prefixed with \\?\, and we are following the same behavior as Windows: a path with such prefix is considered as already normalized, and should not be modified.

Here is the relevant code in Path.GetFullPath:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Path.Windows.cs

Lines 54 to 61 in 6072e4d

	if (PathInternal.IsExtended(path.AsSpan()))
	{
	// \\?\ paths are considered normalized by definition. Windows doesn't normalize \\?\
	// paths and neither should we. Even if we wanted to GetFullPathName does not work
	// properly with device paths. If one wants to pass a \\?\ path through normalization
	// one can chop off the prefix, pass it to GetFullPath and add it again.
	return path;
	}

Here are a couple of articles that might interest you:

https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats
https://docs.microsoft.com/en-us/archive/blogs/jeremykuhne/path-normalization <- Written by a former owner of System.IO.

[ericwj]

Yes thanks for those links.

Stupid DOS Tricks

lol

Didn't know that \\?\C:\foo and \\?\C:\foo. would refer to different files. That is nasty.

What these don't say explicitly is that these paths with \\?\ are also always absolute paths, so, considering everything I know so far, \\?\. would mean the root of the DOS device namespace - referring to whatever 'current directory' would mean there, which can never exist unless by using an API that doesn't (necessarily) work with files only?

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