What's the fastest way to read a text file line-by-line?

Question

I want to read a text file line by line. I wanted to know if I'm doing it as efficiently as possible within the .NET C# scope of things.

This is what I'm trying so far:

var filestream = new System.IO.FileStream(textFilePath,
                                          System.IO.FileMode.Open,
                                          System.IO.FileAccess.Read,
                                          System.IO.FileShare.ReadWrite);
var file = new System.IO.StreamReader(filestream, System.Text.Encoding.UTF8, true, 128);

while ((lineOfText = file.ReadLine()) != null)
{
    //Do something with the lineOfText
}

Answer 1

To find the fastest way to read a file line by line you will have to do some benchmarking. I have done some small tests on my computer but you cannot expect that my results apply to your environment.

Using StreamReader.ReadLine

This is basically your method. For some reason you set the buffer size to the smallest possible value (128). Increasing this will in general increase performance. The default size is 1,024 and other good choices are 512 (the sector size in Windows) or 4,096 (the cluster size in NTFS). You will have to run a benchmark to determine an optimal buffer size. A bigger buffer is - if not faster - at least not slower than a smaller buffer.

const Int32 BufferSize = 128;
using (var fileStream = File.OpenRead(fileName))
  using (var streamReader = new StreamReader(fileStream, Encoding.UTF8, true, BufferSize)) {
    String line;
    while ((line = streamReader.ReadLine()) != null)
    {
      // Process line
    }
  }

The FileStream constructor allows you to specify FileOptions. For example, if you are reading a large file sequentially from beginning to end, you may benefit from FileOptions.SequentialScan. Again, benchmarking is the best thing you can do.

Using File.ReadLines

This is very much like your own solution except that it is implemented using a StreamReader with a fixed buffer size of 1,024. On my computer this results in slightly better performance compared to your code with the buffer size of 128. However, you can get the same performance increase by using a larger buffer size. This method is implemented using an iterator block and does not consume memory for all lines.

var lines = File.ReadLines(fileName);
foreach (var line in lines)
  // Process line

Using File.ReadAllLines

This is very much like the previous method except that this method grows a list of strings used to create the returned array of lines so the memory requirements are higher. However, it returns String[] and not an IEnumerable<String> allowing you to randomly access the lines.

var lines = File.ReadAllLines(fileName);
for (var i = 0; i < lines.Length; i += 1) {
  var line = lines[i];
  // Process line
}

Using String.Split

This method is considerably slower, at least on big files (tested on a 511 KB file), probably due to how String.Split is implemented. It also allocates an array for all the lines increasing the memory required compared to your solution.

using (var streamReader = File.OpenText(fileName)) {
  var lines = streamReader.ReadToEnd().Split("\r\n".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
  foreach (var line in lines)
    // Process line
}

My suggestion is to use File.ReadLines because it is clean and efficient. If you require special sharing options (for example you use FileShare.ReadWrite), you can use your own code but you should increase the buffer size.

Answer 2

If you're using .NET 4, simply use File.ReadLines which does it all for you. I suspect it's much the same as yours, except it may also use FileOptions.SequentialScan and a larger buffer (128 seems very small).

Answer 3

While File.ReadAllLines() is one of the simplest ways to read a file, it is also one of the slowest.

If you're just wanting to read lines in a file without doing much, according to these benchmarks, the fastest way to read a file is the age old method of:

using (StreamReader sr = File.OpenText(fileName))
{
        string s = String.Empty;
        while ((s = sr.ReadLine()) != null)
        {
               //do minimal amount of work here
        }
}

However, if you have to do a lot with each line, then this article concludes that the best way is the following (and it's faster to pre-allocate a string[] if you know how many lines you're going to read) :

AllLines = new string[MAX]; //only allocate memory here

using (StreamReader sr = File.OpenText(fileName))
{
        int x = 0;
        while (!sr.EndOfStream)
        {
               AllLines[x] = sr.ReadLine();
               x += 1;
        }
} //Finished. Close the file

//Now parallel process each line in the file
Parallel.For(0, AllLines.Length, x =>
{
    DoYourStuff(AllLines[x]); //do your work here
});

Answer 4

Use the following code:

foreach (string line in File.ReadAllLines(fileName))

This was a HUGE difference in reading performance.

It comes at the cost of memory consumption, but totally worth it!

Answer 5

If the file size is not big, then it is faster to read the entire file and split it afterwards

var filestreams = sr.ReadToEnd().Split(Environment.NewLine, 
                              StringSplitOptions.RemoveEmptyEntries);

Answer 6

There's a good topic about this in Stack Overflow question Is 'yield return' slower than "old school" return?.

It says:

ReadAllLines loads all of the lines into memory and returns a string[]. All well and good if the file is small. If the file is larger than will fit in memory, you'll run out of memory.

ReadLines, on the other hand, uses yield return to return one line at a time. With it, you can read any size file. It doesn't load the whole file into memory.

Say you wanted to find the first line that contains the word "foo", and then exit. Using ReadAllLines, you'd have to read the entire file into memory, even if "foo" occurs on the first line. With ReadLines, you only read one line. Which one would be faster?

Answer 7

If you have enough memory, I've found some performance gains by reading the entire file into a memory stream, and then opening a stream reader on that to read the lines. As long as you actually plan on reading the whole file anyway, this can yield some improvements.

Answer 8

You can't get any faster if you want to use an existing API to read the lines. But reading larger chunks and manually find each new line in the read buffer would probably be faster.

Answer 9

When you need to efficiently read and process a HUGE text file, ReadLines() and ReadAllLines() are likely to throw Out of Memory exception, this was my case. On the other hand, reading each line separately would take ages. The solution was to read the file in blocks, like below.

The class:

    //can return empty lines sometimes
    class LinePortionTextReader
    {
        private const int BUFFER_SIZE = 100000000; //100M characters
        StreamReader sr = null;
        string remainder = "";

        public LinePortionTextReader(string filePath)
        {
            if (File.Exists(filePath))
            {
                sr = new StreamReader(filePath);
                remainder = "";
            }
        }

        ~LinePortionTextReader()
        {
            if(null != sr) { sr.Close(); }
        }

        public string[] ReadBlock()
        {
            if(null==sr) { return new string[] { }; }
            char[] buffer = new char[BUFFER_SIZE];
            int charactersRead = sr.Read(buffer, 0, BUFFER_SIZE);
            if (charactersRead < 1) { return new string[] { }; }
            bool lastPart = (charactersRead < BUFFER_SIZE);
            if (lastPart)
            {
                char[] buffer2 = buffer.Take<char>(charactersRead).ToArray();
                buffer = buffer2;
            }
            string s = new string(buffer);
            string[] sresult = s.Split(new string[] { "\r\n" }, StringSplitOptions.None);
            sresult[0] = remainder + sresult[0];
            if (!lastPart)
            {
                remainder = sresult[sresult.Length - 1];
                sresult[sresult.Length - 1] = "";
            }
            return sresult;
        }

        public bool EOS
        {
            get
            {
                return (null == sr) ? true: sr.EndOfStream;
            }
        }
    }

Example of use:

    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 3)
            {
                Console.WriteLine("multifind.exe <where to search> <what to look for, one value per line> <where to put the result>");
                return;
            }

            if (!File.Exists(args[0]))
            {
                Console.WriteLine("source file not found");
                return;
            }
            if (!File.Exists(args[1]))
            {
                Console.WriteLine("reference file not found");
                return;
            }

            TextWriter tw = new StreamWriter(args[2], false);

            string[] refLines = File.ReadAllLines(args[1]);

            LinePortionTextReader lptr = new LinePortionTextReader(args[0]);
            int blockCounter = 0;
            while (!lptr.EOS)
            {
                string[] srcLines = lptr.ReadBlock();
                for (int i = 0; i < srcLines.Length; i += 1)
                {
                    string theLine = srcLines[i];
                    if (!string.IsNullOrEmpty(theLine)) //can return empty lines sometimes
                    {
                        for (int j = 0; j < refLines.Length; j += 1)
                        {
                            if (theLine.Contains(refLines[j]))
                            {
                                tw.WriteLine(theLine);
                                break;
                            }
                        }
                    }
                }

                blockCounter += 1;
                Console.WriteLine(String.Format("100 Mb blocks processed: {0}", blockCounter));
            }
            tw.Close();
        }
    }

I believe splitting strings and array handling can be significantly improved, yet the goal here was to minimize number of disk reads.