C#

public void ReadFromStream(StreamReader stream) 
{
	// before attempting to read: save current stream position
	long safePos = stream.BaseStream.Position;

	// read first line
	string line = stream.ReadLine();

	// extract keyword and name
	string[] keyAndName = line.Split(new char[]{' '});

	// if keyword invalid
	if(keyAndName[0].Trim() != keyWord) 
	{
		// move stream back to original position
		stream.BaseStream.Position = safePos;

		// a debug check to see whether it actually moves back to the original position
		// NO IT DOESN'T: the message shown is the line following 'line' and is not 'line'
		System.Windows.Forms.MessageBox.Show(stream.ReadLine());

		// invalid keyword --> throw exception and stream should be left in same state as when this method was entered
		throw new Exception();
	}

// further reading of the object...
}