Reading a file line by line in C#

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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.

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