C#&Win API レジストリキーハンドルからキー名を取得する

動作確認環境：C# 8.0（.NET Core 3.1）、9.0（.NET 5.0）

C#ではWin32 APIで取得したレジストリキーハンドルHKEYからMicrosoft.Win32.RegistryKeyを作成するとキーの名前（Nameプロパティ）が空の文字列""になります。ZwQueryKey関数に準ずるNtQueryKey関数を使用すればキーの名前を取得できます。

ただし、NtQueryKeyは非公開APIであり仕様変更の可能性があります。また取得される名前はリダイレクト前の名前です。

トップレベルステートメント（C# 9）

using System;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.InteropServices;
using System.Text;
using Microsoft.Win32.SafeHandles;

const uint KEY_READ = 0x20019;

using var keyHandle = NativeMethods.RegOpenKeyExW(
    NativeMethods.HKEY_CLASSES_ROOT, ".txt", 0, KEY_READ);
var name = QueryRegistryKeyName(keyHandle);
// \REGISTRY\USER\<ログオンユーザーのSID>_Classes\.txt

[return: MaybeNull]
static string QueryRegistryKeyName(SafeRegistryHandle handle)
{
    const int STATUS_BUFFER_TOO_SMALL = unchecked((int)0xC0000023);

    if (handle == null)
        return null;
    var lstatus = NativeMethods.NtQueryKey(handle,
        KEY_INFORMATION_CLASS.KeyNameInformation,
        null, 0, out var bufferSize);
    if (lstatus != STATUS_BUFFER_TOO_SMALL)
        return null;

    var buffer = new byte[bufferSize];
    lstatus = NativeMethods.NtQueryKey(handle,
        KEY_INFORMATION_CLASS.KeyNameInformation,
        buffer, bufferSize, out bufferSize);
    if (lstatus != 0)
        return null;

    var nameLength = BitConverter.ToUInt32(buffer, 0);
    return Encoding.Unicode.GetString(buffer,
        sizeof(uint), buffer.Length - sizeof(uint));
}

static class NativeMethods
{
    [DllImport("advapi32.dll", CharSet = CharSet.Unicode, ExactSpelling = true)]
    public static extern int RegOpenKeyExW(
        IntPtr hKey,
        [In] string lpSubKey,
        uint ulOptions,
        uint samDesired,
        out SafeRegistryHandle phkResult);

    [return: MaybeNull]
    public static SafeRegistryHandle RegOpenKeyExW(
        IntPtr hKey,
        [In] string lpSubKey,
        uint ulOptions,
        uint samDesired)
    {
        // エラー時はhandleがIntPtr.Zeroなのでnullを返す。
        RegOpenKeyExW(hKey, lpSubKey, ulOptions, samDesired, out var handle);
        return handle;
    }

    public static IntPtr HKEY_CLASSES_ROOT => new IntPtr((uint)0x80000000);

    [DllImport("ntdll.dll")]
    public static extern int NtQueryKey(
        SafeRegistryHandle KeyHandle,
        KEY_INFORMATION_CLASS KeyInformationClass,
        byte[] KeyInformation,
        uint Length,
        out uint ResultLength);
}

enum KEY_INFORMATION_CLASS : int
{
    KeyNameInformation = 3,
}

コンソールアプリケーションの場合はプロジェクトファイルに<TargetFramework>net5-windows</TargetFramework>、<UseWindowsForms>true</UseWindowsForms>を追加します。

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net5-windows</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>
</Project>

Main関数（C# 8）

using System;
using System.Runtime.InteropServices;
using System.Text;
using Microsoft.Win32.SafeHandles;

class Program
{
    static void Main()
    {
        using var keyHandle = NativeMethods.RegOpenKeyExW(
            HKEY_CLASSES_ROOT, ".txt", 0, KEY_READ);
        var name = QueryRegistryKeyName(keyHandle);
        // \REGISTRY\USER\<ログオンユーザーのSID>_Classes\.txt
    }

    private static string QueryRegistryKeyName(SafeRegistryHandle handle)
    {
        if (handle == null)
            return null;
        var lstatus = NativeMethods.NtQueryKey(handle,
            KEY_INFORMATION_CLASS.KeyNameInformation,
            null, 0, out var bufferSize);
        if (lstatus != STATUS_BUFFER_TOO_SMALL)
            return null;

        var buffer = new byte[bufferSize];
        lstatus = NativeMethods.NtQueryKey(handle,
            KEY_INFORMATION_CLASS.KeyNameInformation,
            buffer, bufferSize, out bufferSize);
        if (lstatus != 0)
            return null;

        var nameLength = BitConverter.ToUInt32(buffer, 0);
        return Encoding.Unicode.GetString(buffer, 4, buffer.Length - 4);
    }

    private static class NativeMethods
    {
        [DllImport("advapi32.dll", CharSet = CharSet.Unicode, ExactSpelling = true)]
        public static extern int RegOpenKeyExW(
            IntPtr hKey,
            [In] string lpSubKey,
            uint ulOptions,
            uint samDesired,
            out SafeRegistryHandle phkResult);

        public static SafeRegistryHandle RegOpenKeyExW(
            IntPtr hKey,
            [In] string lpSubKey,
            uint ulOptions,
            uint samDesired)
        {
            // エラー時はhandleがIntPtr.Zeroなのでnullを返す。
            RegOpenKeyExW(hKey, lpSubKey, ulOptions, samDesired, out var handle);
            return handle;
        }

        [DllImport("ntdll.dll")]
        public static extern int NtQueryKey(
            SafeRegistryHandle KeyHandle,
            KEY_INFORMATION_CLASS KeyInformationClass,
            byte[] KeyInformation,
            uint Length,
            out uint ResultLength);
    }

    private static IntPtr HKEY_CLASSES_ROOT => new IntPtr((uint)0x80000000);
    private const uint KEY_READ = 0x20019;
    private const int STATUS_BUFFER_TOO_SMALL = unchecked((int)0xC0000023);

    public enum KEY_INFORMATION_CLASS : int
    {
        KeyNameInformation = 3,
    }
}

コンソールアプリケーションの場合はプロジェクトファイルに<UseWindowsForms>true</UseWindowsForms>を追加します。

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>
</Project>