C# 9.0 Windows 10の拡張子の分類情報等を取得する

Windows 10で拡張子の分類情報など（Perceived Type等）を取得するにはWin32APIのAssocGetPerceivedType関数を使用します。以下はそのサンプルコードです。recordやnew (...)を使用しているのでC# 9.0以降対応です。

using System;
using System.Runtime.InteropServices;

static class Program
{
    private record PerceivedInfo(
        PERCEIVED Perceived,
        PERCEIVEDFLAG PerceivedFlag,
        string TypeString);

    static void Main()
    {
        var exePerceivedTypeInfo = GetPerceivedType(".exe");
        // {PerceivedInfo { Perceived = PERCEIVED_TYPE_APPLICATION, PerceivedFlag = PERCEIVEDFLAG_HARDCODED, TypeString = application }}
        var bmpPerceivedTypeInfo = GetPerceivedType(".bmp");
        // {PerceivedInfo { Perceived = PERCEIVED_TYPE_IMAGE, PerceivedFlag = PERCEIVEDFLAG_HARDCODED, PERCEIVEDFLAG_NATIVESUPPORT, PERCEIVEDFLAG_GDIPLUS, TypeString = image }}
        var txtPerceivedTypeInfo = GetPerceivedType(".txt");
        // {PerceivedInfo { Perceived = PERCEIVED_TYPE_TEXT, PerceivedFlag = PERCEIVEDFLAG_SOFTCODED, PERCEIVEDFLAG_NATIVESUPPORT, TypeString = text }}
    }

    static PerceivedInfo GetPerceivedType(string extenionNameWithDot)
    {
        var str = NativeMethods.AssocGetPerceivedType(
            extenionNameWithDot, out var type, out var flag);
        return new (type, flag, str);
    }

    static class NativeMethods
    {
        // https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-assocgetperceivedtype
        [DllImport("shlwapi.dll", ExactSpelling = true, PreserveSig = false, CharSet = CharSet.Unicode)]
        [return: MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(CoTaskMemToStringUniMarshaler))]
        public static extern string AssocGetPerceivedType(
            [In] string pszExt,
            out PERCEIVED ptype,
            out PERCEIVEDFLAG pflag);
    }

    enum PERCEIVED : int
    {
        PERCEIVED_TYPE_FIRST = -3,
        PERCEIVED_TYPE_CUSTOM = -3,
        PERCEIVED_TYPE_UNSPECIFIED = -2,
        PERCEIVED_TYPE_FOLDER = -1,
        PERCEIVED_TYPE_UNKNOWN = 0,
        PERCEIVED_TYPE_TEXT = 1,
        PERCEIVED_TYPE_IMAGE = 2,
        PERCEIVED_TYPE_AUDIO = 3,
        PERCEIVED_TYPE_VIDEO = 4,
        PERCEIVED_TYPE_COMPRESSED = 5,
        PERCEIVED_TYPE_DOCUMENT = 6,
        PERCEIVED_TYPE_SYSTEM = 7,
        PERCEIVED_TYPE_APPLICATION = 8,
        PERCEIVED_TYPE_GAMEMEDIA = 9,
        PERCEIVED_TYPE_CONTACTS = 10,
        PERCEIVED_TYPE_LAST = 10,
    }


    [Flags]
    enum PERCEIVEDFLAG : ushort
    {
        PERCEIVEDFLAG_UNDEFINED = 0x0000,
        PERCEIVEDFLAG_SOFTCODED = 0x0001,
        PERCEIVEDFLAG_HARDCODED = 0x0002,
        PERCEIVEDFLAG_NATIVESUPPORT = 0x0004,
        PERCEIVEDFLAG_GDIPLUS = 0x0010,
        PERCEIVEDFLAG_WMSDK = 0x0020,
        PERCEIVEDFLAG_ZIPFOLDER = 0x0040,
    }
}

// https://potisan-programming-memo.hatenablog.jp/entry/2020/11/26/214836
internal sealed class CoTaskMemToStringUniMarshaler : ICustomMarshaler
{
    private static CoTaskMemToStringUniMarshaler instance;

    internal static ICustomMarshaler GetInstance(string cookie)
    {
        return instance ??= new CoTaskMemToStringUniMarshaler();
    }

    public void CleanUpManagedData(object ManagedObj) => throw new NotImplementedException();
    public IntPtr MarshalManagedToNative(object ManagedObj) => throw new NotImplementedException();
    public object MarshalNativeToManaged(IntPtr pNativeData) => Marshal.PtrToStringUni(pNativeData);
    public void CleanUpNativeData(IntPtr pNativeData) => Marshal.FreeCoTaskMem(pNativeData);
    public int GetNativeDataSize() => -1; // 値型ではない。
}

蛇足

• AssocGetPerceivedType関数は複数の出力引数を返すため、ラッパー関数でrecordに変換しています。C# 8.0以前では他の複合型やタプル（Tuple、ValueTuple）で代替します。
• AssocGetPerceivedType関数は戻り値HRESULT型でサフィックス（A/W）なくユニコード文字列を受け取る関数です。これらを示すためにDllImport属性にExactSpelling = true（文字列型を扱うがサフィックスは自動付加しない）、PreserveSig = false（HRESULT型戻り値を隠して失敗時は例外を発生させ、最後の引数を戻り値にする）、CharSet = CharSet.Unicode（文字列はユニコード）を指定しています。
• record PerceivedInfo(...);は...で与えられた引数をフィールドに持ち、作成時にそれらを指定する必要のあるレコードの実装です。;で終わらず{...}を続けることで引数以外のメンバー変数や関数なども追加できます。