I'm trying to get the quality value from a Bitmap which was loaded from a jpeg image. This is what I have so far:

    int GetQuality(Bitmap bitmap) {
        if (!bitmap.RawFormat.Equals(ImageFormat.Jpeg)) return -1;

        ImageCodecInfo jpegEncoder = ImageCodecInfo.GetImageEncoders().First(c => c.FormatID == ImageFormat.Jpeg.Guid);
        var jpegGuid = jpegEncoder.Clsid;
        var jpegParams = bitmap.GetEncoderParameterList(jpegGuid);
        EncoderParameter qualityParam = jpegParams?.Param[1]!;

        BindingFlags bindFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;
        FieldInfo? field = typeof(EncoderParameter).GetField("_parameterValue", bindFlags);
        object obj = field!.GetValue(qualityParam)!;
        IntPtr qualityParamValue = (IntPtr) obj;
        unsafe {
            //byte* qpvPtr = (byte*) qualityParamValue;
            //var quality = *qpvPtr;    // val is type byte with a value of 0

            var quality = *(byte*) qualityParamValue; // val is type byte with a value of 0
            return quality;
        }
    }

But the quality value comes back zero for a jpeg bitmap which was saved at a quality compression of 50% (EncoderParameter(Encoder.Quality, 50L)). Either my reflection and unsafe code is wrong, or the quality value is not saved in the jpeg bitmap. I derived this code by looking at the EncoderParameter constructor, which looks looks like this:

public EncoderParameter(Encoder encoder, byte value)
{
    _parameterGuid = encoder.Guid;

    _parameterValueType = EncoderParameterValueType.ValueTypeByte;
    _numberOfValues = 1;
    _parameterValue = Marshal.AllocHGlobal(sizeof(byte));

    *(byte*)_parameterValue = value;
    GC.KeepAlive(this);
}

where byte value is the quality (from 0 - 100). Does my unsafe code correctly do the inverse of *(byte*)_parameterValue = value;? If not, what am I doing wrong?. If so, does that mean the quality is not actually stored in the compressed bitmap?