Best way to split string into lines with maximum length, without breaking words

Even when this post is 3 years old I wanted to give a better solution using Regex to accomplish the same:

If you want the string to be splitted and then use the text to be displayed you can use this:

public string SplitToLines(string stringToSplit, int maximumLineLength)
{
    return Regex.Replace(stringToSplit, @"(.{1," + maximumLineLength +@"})(?:\s|$)", "$1\n");
}

If on the other hand you need a collection you can use this:

public MatchCollection SplitToLines(string stringToSplit, int maximumLineLength)
{
    return Regex.Matches(stringToSplit, @"(.{1," + maximumLineLength +@"})(?:\s|$)");
}

NOTES

Remember to import regex (using System.Text.RegularExpressions;)

You can use string interpolation on the match:
$@"(.{{1,{maximumLineLength}}})(?:\s|$)"

The MatchCollection works almost like an Array

Matching example with explanation here

How about this as a solution:

IEnumerable<string> SplitToLines(string stringToSplit, int maximumLineLength)
{
    var words = stringToSplit.Split(' ').Concat(new [] { "" });
    return
        words
            .Skip(1)
            .Aggregate(
                words.Take(1).ToList(),
                (a, w) =>
                {
                    var last = a.Last();
                    while (last.Length > maximumLineLength)
                    {
                        a[a.Count() - 1] = last.Substring(0, maximumLineLength);
                        last = last.Substring(maximumLineLength);
                        a.Add(last);
                    }
                    var test = last + " " + w;
                    if (test.Length > maximumLineLength)
                    {
                        a.Add(w);
                    }
                    else
                    {
                        a[a.Count() - 1] = test;
                    }
                    return a;
                });
}

---

I reworked this as prefer this:

IEnumerable<string> SplitToLines(string stringToSplit, int maximumLineLength)
{
    var words = stringToSplit.Split(' ');
    var line = words.First();
    foreach (var word in words.Skip(1))
    {
        var test = $"{line} {word}";
        if (test.Length > maximumLineLength)
        {
            yield return line;
            line = word;
        }
        else
        {
            line = test;
        }
    }
    yield return line;
}

I don't think your solution is too bad. I do, however, think you should break up your ternary into an if else because you are testing the same condition twice. Your code might also have a bug. Based on your description, it seems you want lines <= maxLineLength, but your code counts the space after the last word and uses it in the <= comparison resulting in effectively < behavior for the trimmed string.

Here is my solution.

private static IEnumerable<string> SplitToLines(string stringToSplit, int maxLineLength)
    {
        string[] words = stringToSplit.Split(' ');
        StringBuilder line = new StringBuilder();
        foreach (string word in words)
        {
            if (word.Length + line.Length <= maxLineLength)
            {
                line.Append(word + " ");
            }
            else
            {
                if (line.Length > 0)
                {
                    yield return line.ToString().Trim();
                    line.Clear();
                }
                string overflow = word;
                while (overflow.Length > maxLineLength)
                {
                    yield return overflow.Substring(0, maxLineLength);
                    overflow = overflow.Substring(maxLineLength);
                }
                line.Append(overflow + " ");
            }
        }
        yield return line.ToString().Trim();
    }

It is a bit longer than your solution, but it should be more straightforward. It also uses a StringBuilder so it is much faster for large strings. I performed a benchmarking test for 20,000 words ranging from 1 to 11 characters each split into lines of 10 character width. My method completed in 14ms compared to 1373ms for your method.