LibraryImportGenerator can generate non-compilable code with specific string marshalling (it forgets a global:: on CharSet DllImport properties)

[smourier]

Description

When using the LibraryImportAttribute attribute on any method, if the project redefines a sub System namespace in the same namespace of generated functions, then the LibraryImportGenerator generates incorrect code.

Reproduction Steps

Here is a .csproj:

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>net9.0</TargetFramework>
		<Nullable>enable</Nullable>
		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
	</PropertyGroup>

</Project>

And a Program.cs:

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

[assembly: DisableRuntimeMarshalling]

namespace ConsoleApp;

internal partial class Program
{
    static void Main()
    {
    }

    [LibraryImport("whatever", StringMarshalling = StringMarshalling.Utf16)]
    public static partial uint Whatever();
}

public class System // this is what causes the generated code to fail to compile
{
}

Expected behavior

The project should compile file.

Actual behavior

Here is the output of CSC:

Rebuild started at 20:06...
1>------ Rebuild All started: Project: ConsoleApp, Configuration: Debug Any CPU ------
Restored E:\smo\work\ConsoleApp\ConsoleApp\ConsoleApp.csproj (in 0,8 ms).
Restored E:\smo\work\ConsoleApp\ConsoleAppFail\ConsoleAppFail.csproj (in 10 ms).
Restored E:\smo\work\ConsoleApp\ClassLibrary1\ClassLibrary1.csproj (in 13 ms).
1>E:\smo\work\ConsoleApp\ConsoleApp\obj\Debug\net9.0\Microsoft.Interop.LibraryImportGenerator\Microsoft.Interop.LibraryImportGenerator\LibraryImports.g.cs(6,144,6,151): error CS0117: 'System' does not contain a definition for 'Runtime'
1>Done building project "ConsoleApp.csproj" -- FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========
========== Rebuild completed at 20:06 and took 00,409 seconds ==========

As we can see there's CS0117 error: 'System' does not contain a definition for 'Runtime' coming from generated file LibraryImports.g.cs.

Looking at generated source code, it looks something like this:

public static unsafe partial class Functions
{
    [global::System.Runtime.InteropServices.DllImportAttribute("whatever", EntryPoint = "whatever", ExactSpelling = true, CharSet = System.Runtime.InteropServices.CharSet.Unicode)]
    public static extern partial uint whatever();
}

Where CharSet is missing a global:: prefix.

Regression?

I've not tested under .NET 8 (but I think it's probably there)

Known Workarounds

I don't know any.

Configuration

.NET 9.0.1
Visual Studio 2022 17.12.4
Windows 11 x64

Other information

No response

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[AaronRobinsonMSFT]

@jtschuster or @jkoritzinsky Can one of you handle this?