C# JPEGファイルEXIFデータのWindows用評価とキーワードを取得・設定・削除する

追記2024/10/6：FormatterServices等のクラスはObsoluteになりました。FormatterServices.GetUninitializedObject(Type)よりもRuntimeHelpers.GetUninitializedObject(Type)が推奨されます（ソース）。

JPEGファイルEXIFデータのWindows用評価とキーワードを取得・設定・削除するサンプルコードです。

#nullable enable

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.Serialization;
using System.Text;

const string path = <対象とするJPEGのパス　※EXIF改変注意！>;

using (var stream = new MemoryStream(File.ReadAllBytes(path)))
{
    using var img = Image.FromStream(stream);

    var rating = ExifPropertyTagUtility.GetRating(img);
    var keywords = ExifPropertyTagUtility.GetXPKeywords(img);

    ExifPropertyTagUtility.SetRating(img, 5);
    ExifPropertyTagUtility.SetXPKeywords(img, new[] { "Hello", "EXIF" });

    img.Save(path);
}

Console.ReadKey();

static class ExifPropertyTagUtility
{
    // https://exiv2.org/tags.html
    public enum PropertyTags : int
    {
        Rating = 0x4746,
        XPKeywords = 0x9c9e,
    }

    public enum PropertyTypes : int
    {
        Byte = 1,
        ASCII = 2,
        Short = 3,
        Long = 4,
        Rational = 5,
        SByte = 6,
        Undefined = 7,
        SShort = 8,
        SLONG = 9,
        SRational = 10,
        Float = 11,
        Double = 12,
    }

    public static PropertyItem CreatePropertyItem()
    {
        return (PropertyItem)FormatterServices.GetSafeUninitializedObject(typeof(PropertyItem));
    }

    public static short? GetValueShort(in Image jpeg, PropertyTags tag)
    {
        var item = jpeg.GetPropertyItem((int)tag);
        if (item is null || item.Type != (int)PropertyTypes.Short || item.Len != 2)
            return null;
        return BitConverter.ToInt16(item.Value);
    }

    public static void SetValueShort(in Image jpeg, PropertyTags tag, short value)
    {
        var item = CreatePropertyItem();
        item.Id = (int)tag;
        item.Type = (int)PropertyTypes.Short;
        item.Len = 2;
        item.Value = BitConverter.GetBytes(value);
        jpeg.SetPropertyItem(item);
    }

    public static string? GetValueUni(in Image jpeg, PropertyTags tag)
    {
        var item = jpeg.GetPropertyItem((int)tag);
        if (item is null || item.Type != (int)PropertyTypes.Byte)
            return null;
        return Encoding.Unicode.GetString(item.Value!, 0, item.Len - 2);
    }

    public static void SetValueUni(in Image jpeg, PropertyTags tag, string value)
    {
        var bytes = Encoding.Unicode.GetBytes(value + '\0');
        var item = CreatePropertyItem();
        item.Id = (int)tag;
        item.Type = (int)PropertyTypes.Byte;
        item.Len = bytes.Length;
        item.Value = bytes;
        jpeg.SetPropertyItem(item);
    }

    public static short? GetRating(in Image jpeg)
    {
        return GetValueShort(jpeg, PropertyTags.Rating);
    }

    public static void SetRating(in Image jpeg, short value)
    {
        SetValueShort(jpeg, PropertyTags.Rating, value);
    }

    public static void RemoveRating(in Image jpeg)
    {
        jpeg.RemovePropertyItem((int)PropertyTags.Rating);
    }

    public static string[]? GetXPKeywords(in Image jpeg)
    {
        return GetValueUni(jpeg, PropertyTags.XPKeywords)?.Split(';') ?? null;
    }

    public static void SetXPKeywords(in Image jpeg, string[] value)
    {
        SetValueUni(jpeg, PropertyTags.XPKeywords, String.Join<string>(';', value));
    }

    public static void RemoveXPKeywords(in Image jpeg)
    {
        jpeg.RemovePropertyItem((int)PropertyTags.XPKeywords);
    }
}