图片文件幻数

关于JPEG格式

二进制形式打开文件，文件开始字节为FF D8，文件结束两字节为FF D9
JPEG 文件有两种不同的元数据格式：JFIF 和 EXIF。
JFIF 以 ff d8 ff e0 开头，EXIF 以 ff d8 ff e1 开头。

代码示例

private static readonly byte[] Bmp = { 66, 77 };
private static readonly byte[] Jpeg = { 255, 216, 255 }; //第四位可能是224，也可能是225
private static readonly byte[] Gif = { 71, 73, 70, 56 };
private static readonly byte[] Tiff1 = { 77, 77, 00, 42 }; //TIFF format (Motorola - big endian)
private static readonly byte[] Tiff2 = { 73, 73, 42, 00 }; //TIFF format (Intel - little endian)
private static readonly byte[] Png = { 137, 80, 78, 71 };
private static readonly byte[] WebPRiff = { 82, 73, 70, 70 }; // 'RIFF'
private static readonly byte[] WebPWebP = { 87, 69, 66, 80 }; // 'WEBP'
private const int BufferLength = 12; // webp 需要这么长

/// <summary>
/// 解析入口
/// </summary>
/// <param name="stream">图片文件数据流</param>
/// <returns></returns>
public static ImageFormat GetImageFormat(Stream stream)
{
    var bytes = new byte[BufferLength];
    var read = stream.Read(bytes, 0, bytes.Length);
    return read < BufferLength ? ImageFormat.Unknown : GetImageFormat(bytes);
}

/// <summary>
/// 获取文件格式类型
/// </summary>
/// <param name="bytes"></param>
/// <returns></returns>
public static ImageFormat GetImageFormat(byte[] bytes)
{
    if (bytes.Length < BufferLength)
        return ImageFormat.Unknown;

    if (bytes.Take(Bmp.Length).SequenceEqual(Bmp))
        return ImageFormat.Bmp;
    if (bytes.Take(Jpeg.Length).SequenceEqual(Jpeg))
        return ImageFormat.Jpeg;
    if (bytes.Take(Gif.Length).SequenceEqual(Gif))
        return ImageFormat.Gif;
    if (bytes.Take(Tiff1.Length).SequenceEqual(Tiff1)
        || bytes.Take(Tiff2.Length).SequenceEqual(Tiff2))
        return ImageFormat.Tiff;
    if (bytes.Take(Png.Length).SequenceEqual(Png))
        return ImageFormat.Png;
    if (bytes.Take(WebPRiff.Length).SequenceEqual(WebPRiff)
        && bytes.Skip(8).Take(WebPWebP.Length).SequenceEqual(WebPWebP))
        return ImageFormat.WebP;

    return ImageFormat.Unknown;
}

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