Reading a file line by line in C#

Question

I am trying to read some text files, where each line needs to be processed. At the moment I am just using a StreamReader, and then reading each line individually.

I am wondering whether there is a more efficient way (in terms of LoC and readability) to do this using LINQ without compromising operational efficiency. The examples I have seen involve loading the whole file into memory, and then processing it. In this case however I don't believe that would be very efficient. In the first example the files can get up to about 50k, and in the second example, not all lines of the file need to be read (sizes are typically < 10k).

You could argue that nowadays it doesn't really matter for these small files, however I believe that sort of the approach leads to inefficient code.

First example:

// Open file
using(var file = System.IO.File.OpenText(_LstFilename))
{
    // Read file
    while (!file.EndOfStream)
    {
        String line = file.ReadLine();

        // Ignore empty lines
        if (line.Length > 0)
        {
            // Create addon
            T addon = new T();
            addon.Load(line, _BaseDir);

            // Add to collection
            collection.Add(addon);
        }
    }
}

Second example:

// Open file
using (var file = System.IO.File.OpenText(datFile))
{
    // Compile regexs
    Regex nameRegex = new Regex("IDENTIFY (.*)");

    while (!file.EndOfStream)
    {
        String line = file.ReadLine();

        // Check name
        Match m = nameRegex.Match(line);
        if (m.Success)
        {
            _Name = m.Groups[1].Value;

            // Remove me when other values are read
            break;
        }
    }
}

Answer 1

You can write a LINQ-based line reader pretty easily using an iterator block:

static IEnumerable<SomeType> ReadFrom(string file) {
    string line;
    using(var reader = File.OpenText(file)) {
        while((line = reader.ReadLine()) != null) {
            SomeType newRecord = /* parse line */
            yield return newRecord;
        }
    }
}

or to make Jon happy:

static IEnumerable<string> ReadFrom(string file) {
    string line;
    using(var reader = File.OpenText(file)) {
        while((line = reader.ReadLine()) != null) {
            yield return line;
        }
    }
}
...
var typedSequence = from line in ReadFrom(path)
                    let record = ParseLine(line)
                    where record.Active // for example
                    select record.Key;

then you have ReadFrom(...) as a lazily evaluated sequence without buffering, perfect for Where etc.

Note that if you use OrderBy or the standard GroupBy, it will have to buffer the data in memory; ifyou need grouping and aggregation, "PushLINQ" has some fancy code to allow you to perform aggregations on the data but discard it (no buffering). Jon's explanation is here.

Answer 2

It's simpler to read a line and check whether or not it's null than to check for EndOfStream all the time.

However, I also have a LineReader class in MiscUtil which makes all of this a lot simpler - basically it exposes a file (or a Func<TextReader> as an IEnumerable<string> which lets you do LINQ stuff over it. So you can do things like:

var query = from file in Directory.GetFiles("*.log")
            from line in new LineReader(file)
            where line.Length > 0
            select new AddOn(line); // or whatever

The heart of LineReader is this implementation of IEnumerable<string>.GetEnumerator:

public IEnumerator<string> GetEnumerator()
{
    using (TextReader reader = dataSource())
    {
        string line;
        while ((line = reader.ReadLine()) != null)
        {
            yield return line;
        }
    }
}

Almost all the rest of the source is just giving flexible ways of setting up dataSource (which is a Func<TextReader>).

Answer 3

Since .NET 4.0, the File.ReadLines() method is available.

int count = File.ReadLines(filepath).Count(line => line.StartsWith(">"));

Answer 4

NOTE: You need to watch out for the IEnumerable<T> solution, as it will result in the file being open for the duration of processing.

For example, with Marc Gravell's response:

foreach(var record in ReadFrom("myfile.csv")) {
    DoLongProcessOn(record);
}

the file will remain open for the whole of the processing.

Answer 5

Thanks all for your answers! I decided to go with a mixture, mainly focusing on Marc's though as I will only need to read lines from a file. I guess you could argue seperation is needed everywhere, but heh, life is too short!

Regarding the keeping the file open, that isn't going to be an issue in this case, as the code is part of a desktop application.

Lastly I noticed you all used lowercase string. I know in Java there is a difference between capitalised and non capitalised string, but I thought in C# lowercase string was just a reference to capitalised String?

public void Load(AddonCollection<T> collection)
{
    // read from file
    var query =
        from line in LineReader(_LstFilename)
        where line.Length > 0
        select CreateAddon(line);

    // add results to collection
    collection.AddRange(query);
}

protected T CreateAddon(String line)
{
    // create addon
    T addon = new T();
    addon.Load(line, _BaseDir);

    return addon;
}

protected static IEnumerable<String> LineReader(String fileName)
{
    String line;
    using (var file = System.IO.File.OpenText(fileName))
    {
        // read each line, ensuring not null (EOF)
        while ((line = file.ReadLine()) != null)
        {
            // return trimmed line
            yield return line.Trim();
        }
    }
}