Reading a line from a streamreader without consuming?

Question

Is there a way to read ahead one line to test if the next line contains specific tag data?

I'm dealing with a format that has a start tag but no end tag.

I would like to read a line add it to a structure then test the line below to make sure it not a new "node" and if it isn't keep adding if it is close off that struct and make a new one

the only solution i can think of is to have two stream readers going at the same time kinda suffling there way along lock step but that seems wastefull (if it will even work)

i need something like peek but peekline

Answer 1

The problem is the underlying stream may not even be seekable. If you take a look at the stream reader implementation it uses a buffer so it can implement TextReader.Peek() even if the stream is not seekable.

You could write a simple adapter that reads the next line and buffers it internally, something like this:

 public class PeekableStreamReaderAdapter     {         private StreamReader Underlying;         private Queue<string> BufferedLines;          public PeekableStreamReaderAdapter(StreamReader underlying)         {             Underlying = underlying;             BufferedLines = new Queue<string>();         }          public string PeekLine()         {             string line = Underlying.ReadLine();             if (line == null)                 return null;             BufferedLines.Enqueue(line);             return line;         }           public string ReadLine()         {             if (BufferedLines.Count > 0)                 return BufferedLines.Dequeue();             return Underlying.ReadLine();         }     } 

Answer 2

You could store the position accessing StreamReader.BaseStream.Position, then read the line next line, do your test, then seek to the position before you read the line:

            // Peek at the next line             long peekPos = reader.BaseStream.Position;             string line = reader.ReadLine();              if (line.StartsWith("<tag start>"))             {                 // This is a new tag, so we reset the position                 reader.BaseStream.Seek(pos);                  }             else             {                 // This is part of the same node.             } 

This is a lot of seeking and re-reading the same lines. Using some logic, you may be able to avoid this altogether - for instance, when you see a new tag start, close out the existing structure and start a new one - here's a basic algorithm:

        SomeStructure myStructure = null;         while (!reader.EndOfStream)         {             string currentLine = reader.ReadLine();             if (currentLine.StartsWith("<tag start>"))             {                 // Close out existing structure.                 if (myStructure != null)                 {                     // Close out the existing structure.                 }                  // Create a new structure and add this line.                 myStructure = new Structure();                                    // Append to myStructure.             }             else             {                 // Add to the existing structure.                 if (myStructure != null)                 {                     // Append to existing myStructure                 }                 else                 {                     // This means the first line was not part of a structure.                     // Either handle this case, or throw an exception.                 }             }         }