C# 单线程将8位Bitmap保存为bmp格式图片，怎样做速度最快，占用磁盘空间最小，EncoderValue.CompressionRLE没有效果，并且在重新加载图片后能得到压缩前的图片

如果要在重新加载图片后得到压缩前的图片，就需要使用无损压缩方式，比如EncoderValue.CompressionNone。这种压缩方式不会降低图片质量，但会增加文件大小。

以下是一个示例代码，可以将8位Bitmap保存为bmp格式图片，速度较快，同时占用磁盘空间较小，且使用无损压缩方式：

public static void SaveBitmap(Bitmap bitmap, string fileName)
{
    BitmapData bmpData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format8bppIndexed);
    int stride = bmpData.Stride;
    int height = bmpData.Height;
    byte[] data = new byte[stride * height];
    Marshal.Copy(bmpData.Scan0, data, 0, data.Length);
    bitmap.UnlockBits(bmpData);

    EncoderParameters encoderParams = new EncoderParameters(1);
    encoderParams.Param[0] = new EncoderParameter(Encoder.Compression, (long)EncoderValue.CompressionNone);

    using (FileStream fs = new FileStream(fileName, FileMode.Create))
    {
        BitmapInfoHeader bmiHeader = new BitmapInfoHeader();
        bmiHeader.biSize = 40;
        bmiHeader.biWidth = bitmap.Width;
        bmiHeader.biHeight = bitmap.Height;
        bmiHeader.biPlanes = 1;
        bmiHeader.biBitCount = 8;
        bmiHeader.biCompression = 0;
        bmiHeader.biSizeImage = data.Length;
        bmiHeader.biXPelsPerMeter = 0;
        bmiHeader.biYPelsPerMeter = 0;
        bmiHeader.biClrUsed = 256;
        bmiHeader.biClrImportant = 256;

        byte[] header = new byte[bmiHeader.biSize];
        BitConverter.GetBytes(bmiHeader.biSize).CopyTo(header, 0);
        BitConverter.GetBytes(bmiHeader.biWidth).CopyTo(header, 4);
        BitConverter.GetBytes(bmiHeader.biHeight).CopyTo(header, 8);
        BitConverter.GetBytes(bmiHeader.biPlanes).CopyTo(header, 12);
        BitConverter.GetBytes(bmiHeader.biBitCount).CopyTo(header, 14);
        BitConverter.GetBytes(bmiHeader.biCompression).CopyTo(header, 16);
        BitConverter.GetBytes(bmiHeader.biSizeImage).CopyTo(header, 20);
        BitConverter.GetBytes(bmiHeader.biXPelsPerMeter).CopyTo(header, 24);
        BitConverter.GetBytes(bmiHeader.biYPelsPerMeter).CopyTo(header, 28);
        BitConverter.GetBytes(bmiHeader.biClrUsed).CopyTo(header, 32);
        BitConverter.GetBytes(bmiHeader.biClrImportant).CopyTo(header, 36);

        fs.Write(header, 0, header.Length);
        fs.Write(data, 0, data.Length);

        ImageCodecInfo bmpCodecInfo = ImageCodecInfo.GetImageEncoders().FirstOrDefault(codec => codec.FormatID == ImageFormat.Bmp.Guid);
        if (bmpCodecInfo != null)
        {
            fs.Flush();
            fs.Position = header.Length;
            bmp.Save(fs, bmpCodecInfo, encoderParams);
        }
    }
}

注意：由于使用了无损压缩方式，文件大小可能会比原始图片大。