将“PDF”解压缩的GZipStream转换为“PNG”C＃

Question

我要求从亚马逊运输标签，他们发回一个字符串是一个压缩的“GZIP”文件，我必须将其保存为“.gz”扩展名然后我可以对其进行解压缩（或解压缩），有时是“PNG”格式的字符串，有些是“PDF”格式！ 但我希望每次都以“PNG”格式打印，但当他们发回“PDF”格式时，我无法将解压缩（GZIPSTREAM）文件转换为“PNG”！ 有没有安装第三方库的帮助？

这里是我的代码

public async Task PrintLabel(string imageLabelString) 
    { 
     var byteStream = Convert.FromBase64String(imageLabelString); 
     GZipStream gzip = null; 
     Image image = null; 
     using (MemoryStream memoryStream = new MemoryStream(byteStream)) 
     { 
      image = null; 
      //File.WriteAllBytes("temp.gz", byteStream); 
      using (gzip = new GZipStream(memoryStream, CompressionMode.Decompress)) 
      { 
       image = Image.FromStream(gzip); 
      }; 

      memoryStream.Position = 0; 


      image.RotateFlip(RotateFlipType.Rotate90FlipNone); 

      PrintDocument pd = new PrintDocument(); 
      pd.PrintPage += (object o, PrintPageEventArgs e) => 
      { 
       Rectangle rectangle = new Rectangle(0, 0, 500, 750); 
       e.Graphics.DrawImage(image, rectangle); 
      }; 

      pd.PrinterSettings.PrinterName = "PrinterName"; 
      pd.Print(); 
     } 
     this.Close(); 
    } 

Answer 1

您可以通过查看前4个字节流的检测PNG。 PNG有一个可以很容易检测到的标记。

bool IsStreamPng(Stream s) 
{ 
    s.Seek(0, SeekOrigin.Begin); //Go to the start of the stream 
    var headerBytes = new byte[4]; //Create a buffer to hold 4 bytes of data 
    s.Read(headerBytes, 0, 4); //Copy 4 bytes of data to the buffer 

    var headerString = Encoding.ASCII.GetString(headerBytes); //convert the buffer to a string 
    return headerString.ToUpper().EndsWith("PNG"); //do the first 4 characters of the header end with "PNG"? 
} 

https://www.w3.org/TR/PNG/#5PNG-file-signature

Answer 2

...是改进@布拉德利的解决方案，以避免不必要的分配：

从http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html

第8个字节的PNG文件始终包含以下（十进制）值：

137 80 78 71 13 10 26 10 

public static Boolean IsPng(Stream stream) { 

    // assuming stream is at position 0 

    return 
     stream.ReadByte() == 137 && 
     stream.ReadByte() == 80 && 
     stream.ReadByte() == 78 && 
     stream.ReadByte() == 71 && 
     stream.ReadByte() == 13 && 
     stream.ReadByte() == 10 && 
     stream.ReadByte() == 26 && 
     stream.ReadByte() == 10; 
}