SqlDataReader while(reader.Read())

Question

private bool ReadMagic(BinaryReader reader) 
{ 
     try 
     { 
          ini: 

          byte b0 = reader.ReadByte(); 
          if (b0 != 0xF9) goto ini; 

          b0 = reader.ReadByte(); 
          if (b0 != 0xbe) goto ini; 

          b0 = reader.ReadByte(); 
          if (b0 != 0xb4) goto ini; 

          b0 = reader.ReadByte(); 
          if (b0 != 0xd9) goto ini; 

          return true; 
     } 
     catch (EndOfStreamException) 
     { 
          return false; 
     } 
}

I found this piece of code on github and was wondering why someone would use the ReadMagic function instead of the usual reader.Read() function? Also could someone explain how the ReadMagic function works?

using(var reader = command.ExecuteReader())
{
     while(ReadMagic(reader));
}

var seq = new byte[] {0xF9, 0xbe, 0xb4, 0xd9}; int currentIndex = 0; while(!reader.EndOfStream) { if(reader.ReadByte() == seq[currentIndex]) currentIndex++; else currentIndex = 0; if(currentIndex == seq.Length) return true;} return false; — juharr
– juharr, Commented Jul 1, 2016 at 16:52

James Thorpe · Accepted Answer · 2016-07-01 16:27:58Z

4

A SqlDataReader is very much not the same thing as a BinaryReader, and you wouldn't use it as such. The BinaryReader is for reading streams (eg reading from a file). It has methods such as ReadByte, used here, for reading actual bytes from the stream, whereas SqlDataReader uses Read to advance to the next returned row, if there is one.

It looks like the ReadMagic function is consuming bytes from the stream until it gets a series of 0xF9, 0xBE, 0xB4, 0xD9 - presumably some sort of marker in whatever stream it happens to be reading, then returning true upon finding it.

edited Jul 1, 2016 at 16:27

answered Jul 1, 2016 at 16:24

James Thorpe

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1 Comment

Adam1980 Over a year ago

That makes sense. Thanks for the explanation.

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