I'm using C# (.Net 2.0), and I have a fairly large text file (~1600 lines on average) that I need to check periodically to make sure a certain line of text is there.
What is the most efficient way of doing this? Do I really have to load the entire file into memory each time?
Is there a file-content-search api of some sort that I could use?
Thanks for any help/advice.
If the line of text is always going to be the same then using RegEx to match the text of the line is probably more efficient than looping through a file to match the text using String.Equals() or ==.
That said, I don't know of anyway in c# to find text in a file with out opening the file into memory and reading the lines.
This link is a nice tutorial on using RegEx to match lines in a file using c#.
Well, you can always use the FileSystemWatcher to give you an event when the file has changed, that way you only scan the file on demand.
You should be able to just loop over the lines like this:
String line;
while ((line = file.ReadLine()) != null)
{
if (line matches regex blah)
return true;
}
return false;
The ReadLine method only loads a single line of the file into memory, not the whole file. When the loop runs again, the only reference to that line is lost and so, the line will be garbage collected when needed.
It really depends on your definition of "efficient".
If you mean memory-efficient then you could use a stream reader so that you only have one line of text in memory at a time, unfortunately this is slower than loading the whole thing in at once and may lock the file.
If you mean in the shortest possible time, then this is a task that will gain great benefits from a parallel architecture. Split the file into chunks and pass each chunk off to a different thread to process. Of course that isn't especially CPU efficient, as it may put all your cores at a high level of usage.
If you are looking to just do the least amount of work is there anything you already know about the file? How often will it be updated? Are the first 10 characters of each line always the same? If you looked at 100 lines last time do you need to rescan those lines again? Any of these could create huge savings for both time and memory usage.
At the end of the day though there is no magic bullet, and to search a file is (at worst case) an O(n) operation.
Sorry, just re-read that, and it may come across as sarcastic, and I don't mean it to be. I just meant to emphasize that any gains you make in one area are likely to be loses elsewhere and "efficient" is a very ambiguous term in circumstances like these.
Unless they are very long lines, in modern computing terms 1600 lines is not a lot! The file IO will be handled by the runtime, and will be buffered, and will be astonishingly fast, and the memory footprint astonishingly unremarkable.
Simply read the file line by line, or use System.IO.File.ReadAllLines(), and then see if the line exists e.g. using a whole line comparision with a string.
This isn't going to be your bottleneck.
Your bottleneck might occur if you are polling frequently and/or using regular expressions unnecessarily. Its best to use a file system watcher to avoid parsing the file at all if it is unchanged.
List<String> lines = System.IO.File.ReadAllLines(file).ToList()
lines.Contains("foo");
I would combine a couple of techniques used here:
1). Set a FileSystemWatcher on the file. Set the necessary filters to prevent false positives. You don't want to check the file unecessarily.
2). When the FSW raises the event, grab the contents using string fileString = File.ReadAllLines().
3). Use a simple regex to find the match for your string.
4). If the match has an index greater than -1, then the file contains the string at whatever value is in the index.
You've successfully avoided having to parse the file line by line, you have potentially loaded a large amount of data (although 1600 lines of text is hardly that large) into memory. When the string literal goes out of scope it'll be reclaimed by the garbage collector.