What is simpliest way to get Line number from char position in String?

Question

What is simpliest way to get Line number from char position in String in C#? (or get Position of line (first char in line) ) Is there any built-in function ? If there are n

Answer 1

If you are going to call the function many times on the same long string, this class can be usefull. It caches the new line positions, so that later it can perform O(log (line breaks in string)) lookup for GetLine and O(1) for GetOffset.

public class LineBreakCounter
{
    List lineBreaks_ = new List();
    int length_;

    public LineBreakCounter(string text)
    {
        if (text == null)
            throw new ArgumentNullException(nameof(text));

        length_ = text.Length;
        for (int i = 0; i < text.Length; i++)
        {
            if (text[i] == '\n')
                lineBreaks_.Add(i);

            else if (text[i] == '\r' && i < text.Length - 1 && text[i + 1] == '\n')
                lineBreaks_.Add(++i);
        }
    }

    public int GetLine(int offset)
    {
        if (offset < 0 || offset > length_)
            throw new ArgumentOutOfRangeException(nameof(offset));

        var result = lineBreaks_.BinarySearch(offset);
        if (result < 0)
            return ~result;
        else
            return result;
    }

    public int Lines => lineBreaks_.Count + 1;

    public int GetOffset(int line)
    {
        if (line < 0 || line >= Lines)
            throw new ArgumentOutOfRangeException(nameof(line));

        if (line == 0)
            return 0;

        return lineBreaks_[line - 1] + 1;
    }
}

Here is my test case:

[TestMethod]
public void LineBreakCounter_ShouldFindLineBreaks()
{
    var text = "Hello\nWorld!\r\n";
    var counter = new LineBreakCounter(text);

    Assert.AreEqual(0, counter.GetLine(0));
    Assert.AreEqual(0, counter.GetLine(3));
    Assert.AreEqual(0, counter.GetLine(5));
    Assert.AreEqual(1, counter.GetLine(6));
    Assert.AreEqual(1, counter.GetLine(8));
    Assert.AreEqual(1, counter.GetLine(12));
    Assert.AreEqual(1, counter.GetLine(13));
    Assert.AreEqual(2, counter.GetLine(14));

    Assert.AreEqual(3, counter.Lines);
    Assert.AreEqual(0, counter.GetOffset(0));
    Assert.AreEqual(6, counter.GetOffset(1));
    Assert.AreEqual(14, counter.GetOffset(2));
}