The problem with idatareader

why is my idatareader making a mistake .....?

 byteSize = _reader.GetBytes(_reader.GetOrdinal(sFieldName), 0, null, 0, 0);

      

        
        
        
      

    

I want to get an image from the database here sFieldName is the database column. Name ...... it shows me The order specified is invalid. message ...... what am I doing? working on c # window vs05 ....

sFieldName is a string ... this is the name of the database column and the type of the column image The name of the database column "BoardImage"

sFieldName = "BoardImage"

 oBoardDetail.BoardImage =oReader.GetImage("BoardImage");

      

        
        
        
      

    

here oBoardDetail is a class object and BoardImage is a property type of the Image class