Natural Sort Order in C#

Anyone have a good resource or provide a sample of a natural order sort in C# for an FileInfo array? I am implementing the IComparer interface in my sorts.

The easiest thing to do is just P/Invoke the built-in function in Windows, and use it as the comparison function in your IComparer:

[DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
private static extern int StrCmpLogicalW(string psz1, string psz2);

Michael Kaplan has some examples of how this function works here, and the changes that were made for Vista to make it work more intuitively. The plus side of this function is that it will have the same behaviour as the version of Windows it runs on, however this does mean that it differs between versions of Windows so you need to consider whether this is a problem for you.

So a complete implementation would be something like:

[SuppressUnmanagedCodeSecurity]
internal static class SafeNativeMethods
{
    [DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
    public static extern int StrCmpLogicalW(string psz1, string psz2);
}

public sealed class NaturalStringComparer : IComparer<string>
{
    public int Compare(string a, string b)
    {
        return SafeNativeMethods.StrCmpLogicalW(a, b);
    }
}

public sealed class NaturalFileInfoNameComparer : IComparer<FileInfo>
{
    public int Compare(FileInfo a, FileInfo b)
    {
        return SafeNativeMethods.StrCmpLogicalW(a.Name, b.Name);
    }
}

Just thought I'd add to this (with the most concise solution I could find):

public static IOrderedEnumerable<T> OrderByAlphaNumeric<T>(this IEnumerable<T> source, Func<T, string> selector)
{
    int max = source
        .SelectMany(i => Regex.Matches(selector(i), @"\d+").Cast<Match>().Select(m => (int?)m.Value.Length))
        .Max() ?? 0;

    return source.OrderBy(i => Regex.Replace(selector(i), @"\d+", m => m.Value.PadLeft(max, '0')));
}

The above pads any numbers in the string to the max length of all numbers in all strings and uses the resulting string to sort.

The cast to (int?) is to allow for collections of strings without any numbers (.Max() on an empty enumerable throws an InvalidOperationException).

None of the existing implementations looked great so I wrote my own. The results are almost identical to the sorting used by modern versions of Windows Explorer (Windows 7/8). The only differences I've seen are 1) although Windows used to (e.g. XP) handle numbers of any length, it's now limited to 19 digits - mine is unlimited, 2) Windows gives inconsistent results with certain sets of Unicode digits - mine works fine (although it doesn't numerically compare digits from surrogate pairs; nor does Windows), and 3) mine can't distinguish different types of non-primary sort weights if they occur in different sections (e.g. "e-1é" vs "é1e-" - the sections before and after the number have diacritic and punctuation weight differences).

public static int CompareNatural(string strA, string strB) {
	return CompareNatural(strA, strB, CultureInfo.CurrentCulture, CompareOptions.IgnoreCase);
}

public static int CompareNatural(string strA, string strB, CultureInfo culture, CompareOptions options) {
	CompareInfo cmp = culture.CompareInfo;
	int iA = 0;
	int iB = 0;
	int softResult = 0;
	int softResultWeight = 0;
	while (iA < strA.Length && iB < strB.Length) {
		bool isDigitA = Char.IsDigit(strA[iA]);
		bool isDigitB = Char.IsDigit(strB[iB]);
		if (isDigitA != isDigitB) {
			return cmp.Compare(strA, iA, strB, iB, options);
		}
		else if (!isDigitA && !isDigitB) {
			int jA = iA + 1;
			int jB = iB + 1;
			while (jA < strA.Length && !Char.IsDigit(strA[jA])) jA++;
			while (jB < strB.Length && !Char.IsDigit(strB[jB])) jB++;
			int cmpResult = cmp.Compare(strA, iA, jA - iA, strB, iB, jB - iB, options);
			if (cmpResult != 0) {
				// Certain strings may be considered different due to "soft" differences that are
				// ignored if more significant differences follow, e.g. a hyphen only affects the
				// comparison if no other differences follow
				string sectionA = strA.Substring(iA, jA - iA);
				string sectionB = strB.Substring(iB, jB - iB);
				if (cmp.Compare(sectionA + "1", sectionB + "2", options) ==
					cmp.Compare(sectionA + "2", sectionB + "1", options))
				{
					return cmp.Compare(strA, iA, strB, iB, options);
				}
				else if (softResultWeight < 1) {
					softResult = cmpResult;
					softResultWeight = 1;
				}
			}
			iA = jA;
			iB = jB;
		}
		else {
			char zeroA = (char)(strA[iA] - (int)Char.GetNumericValue(strA[iA]));
			char zeroB = (char)(strB[iB] - (int)Char.GetNumericValue(strB[iB]));
			int jA = iA;
			int jB = iB;
			while (jA < strA.Length && strA[jA] == zeroA) jA++;
			while (jB < strB.Length && strB[jB] == zeroB) jB++;
			int resultIfSameLength = 0;
			do {
				isDigitA = jA < strA.Length && Char.IsDigit(strA[jA]);
				isDigitB = jB < strB.Length && Char.IsDigit(strB[jB]);
				int numA = isDigitA ? (int)Char.GetNumericValue(strA[jA]) : 0;
				int numB = isDigitB ? (int)Char.GetNumericValue(strB[jB]) : 0;
				if (isDigitA && (char)(strA[jA] - numA) != zeroA) isDigitA = false;
				if (isDigitB && (char)(strB[jB] - numB) != zeroB) isDigitB = false;
				if (isDigitA && isDigitB) {
					if (numA != numB && resultIfSameLength == 0) {
						resultIfSameLength = numA < numB ? -1 : 1;
					}
					jA++;
					jB++;
				}
			}
			while (isDigitA && isDigitB);
			if (isDigitA != isDigitB) {
				// One number has more digits than the other (ignoring leading zeros) - the longer
				// number must be larger
				return isDigitA ? 1 : -1;
			}
			else if (resultIfSameLength != 0) {
				// Both numbers are the same length (ignoring leading zeros) and at least one of
				// the digits differed - the first difference determines the result
				return resultIfSameLength;
			}
			int lA = jA - iA;
			int lB = jB - iB;
			if (lA != lB) {
				// Both numbers are equivalent but one has more leading zeros
				return lA > lB ? -1 : 1;
			}
			else if (zeroA != zeroB && softResultWeight < 2) {
				softResult = cmp.Compare(strA, iA, 1, strB, iB, 1, options);
				softResultWeight = 2;
			}
			iA = jA;
			iB = jB;
		}
	}
	if (iA < strA.Length || iB < strB.Length) {
		return iA < strA.Length ? 1 : -1;
	}
	else if (softResult != 0) {
		return softResult;
	}
	return 0;
}

The signature matches the Comparison<string> delegate:

string[] files = Directory.GetFiles(@"C:\");
Array.Sort(files, CompareNatural);

Here's a wrapper class for use as IComparer<string>:

public class CustomComparer<T> : IComparer<T> {
	private Comparison<T> _comparison;

	public CustomComparer(Comparison<T> comparison) {
		_comparison = comparison;
	}

	public int Compare(T x, T y) {
		return _comparison(x, y);
	}
}

Example:

string[] files = Directory.EnumerateFiles(@"C:\")
    .OrderBy(f => f, new CustomComparer<string>(CompareNatural))
    .ToArray();

Here's a good set of filenames I use for testing:

Func<string, string> expand = (s) => { int o; while ((o = s.IndexOf('\\')) != -1) { int p = o + 1;
	int z = 1; while (s[p] == '0') { z++; p++; } int c = Int32.Parse(s.Substring(p, z));
	s = s.Substring(0, o) + new string(s[o - 1], c) + s.Substring(p + z); } return s; };
string encodedFileNames =
	"KDEqLW4xMiotbjEzKjAwMDFcMDY2KjAwMlwwMTcqMDA5XDAxNyowMlwwMTcqMDlcMDE3KjEhKjEtISox" +
	"LWEqMS4yNT8xLjI1KjEuNT8xLjUqMSoxXDAxNyoxXDAxOCoxXDAxOSoxXDA2NioxXDA2NyoxYSoyXDAx" +
	"NyoyXDAxOCo5XDAxNyo5XDAxOCo5XDA2Nio9MSphMDAxdGVzdDAxKmEwMDF0ZXN0aW5nYTBcMzEqYTAw" +
	"Mj9hMDAyIGE/YTAwMiBhKmEwMDIqYTAwMmE/YTAwMmEqYTAxdGVzdGluZ2EwMDEqYTAxdnNmcyphMSph" +
	"MWEqYTF6KmEyKmIwMDAzcTYqYjAwM3E0KmIwM3E1KmMtZSpjZCpjZipmIDEqZipnP2cgMT9oLW4qaG8t" +
	"bipJKmljZS1jcmVhbT9pY2VjcmVhbT9pY2VjcmVhbS0/ajBcNDE/ajAwMWE/ajAxP2shKmsnKmstKmsx" +
	"KmthKmxpc3QqbTAwMDNhMDA1YSptMDAzYTAwMDVhKm0wMDNhMDA1Km0wMDNhMDA1YSpuMTIqbjEzKm8t" +
	"bjAxMypvLW4xMipvLW40P28tbjQhP28tbjR6P28tbjlhLWI1Km8tbjlhYjUqb24wMTMqb24xMipvbjQ/" +
	"b240IT9vbjR6P29uOWEtYjUqb245YWI1Km/CrW4wMTMqb8KtbjEyKnAwMCpwMDEqcDAxwr0hKnAwMcK9" +
	"KnAwMcK9YSpwMDHCvcK+KnAwMipwMMK9KnEtbjAxMypxLW4xMipxbjAxMypxbjEyKnItMDAhKnItMDAh" +
	"NSpyLTAwIe+8lSpyLTAwYSpyLe+8kFwxIS01KnIt77yQXDEhLe+8lSpyLe+8kFwxISpyLe+8kFwxITUq" +
	"ci3vvJBcMSHvvJUqci3vvJBcMWEqci3vvJBcMyE1KnIwMCEqcjAwLTUqcjAwLjUqcjAwNSpyMDBhKnIw" +
	"NSpyMDYqcjQqcjUqctmg2aYqctmkKnLZpSpy27Dbtipy27Qqctu1KnLfgN+GKnLfhCpy34UqcuClpuCl" +
	"rCpy4KWqKnLgpasqcuCnpuCnrCpy4KeqKnLgp6sqcuCppuCprCpy4KmqKnLgqasqcuCrpuCrrCpy4Kuq" +
	"KnLgq6sqcuCtpuCtrCpy4K2qKnLgrasqcuCvpuCvrCpy4K+qKnLgr6sqcuCxpuCxrCpy4LGqKnLgsasq" +
	"cuCzpuCzrCpy4LOqKnLgs6sqcuC1puC1rCpy4LWqKnLgtasqcuC5kOC5lipy4LmUKnLguZUqcuC7kOC7" +
	"lipy4LuUKnLgu5UqcuC8oOC8pipy4LykKnLgvKUqcuGBgOGBhipy4YGEKnLhgYUqcuGCkOGClipy4YKU" +
	"KnLhgpUqcuGfoOGfpipy4Z+kKnLhn6UqcuGgkOGglipy4aCUKnLhoJUqcuGlhuGljCpy4aWKKnLhpYsq" +
	"cuGnkOGnlipy4aeUKnLhp5UqcuGtkOGtlipy4a2UKnLhrZUqcuGusOGutipy4a60KnLhrrUqcuGxgOGx" +
	"hipy4bGEKnLhsYUqcuGxkOGxlipy4bGUKnLhsZUqcuqYoFwx6pilKnLqmKDqmKUqcuqYoOqYpipy6pik" +
	"KnLqmKUqcuqjkOqjlipy6qOUKnLqo5UqcuqkgOqkhipy6qSEKnLqpIUqcuqpkOqplipy6qmUKnLqqZUq" +
	"cvCQkqAqcvCQkqUqcvCdn5gqcvCdn50qcu+8kFwxISpy77yQXDEt77yVKnLvvJBcMS7vvJUqcu+8kFwx" +
	"YSpy77yQXDHqmKUqcu+8kFwx77yO77yVKnLvvJBcMe+8lSpy77yQ77yVKnLvvJDvvJYqcu+8lCpy77yV" +
	"KnNpKnPEsSp0ZXN02aIqdGVzdNmi2aAqdGVzdNmjKnVBZS0qdWFlKnViZS0qdUJlKnVjZS0xw6kqdWNl" +
	"McOpLSp1Y2Uxw6kqdWPDqS0xZSp1Y8OpMWUtKnVjw6kxZSp3ZWlhMSp3ZWlhMip3ZWlzczEqd2Vpc3My" +
	"KndlaXoxKndlaXoyKndlacOfMSp3ZWnDnzIqeSBhMyp5IGE0KnknYTMqeSdhNCp5K2EzKnkrYTQqeS1h" +
	"Myp5LWE0KnlhMyp5YTQqej96IDA1MD96IDIxP3ohMjE/ejIwP3oyMj96YTIxP3rCqTIxP1sxKl8xKsKt" +
	"bjEyKsKtbjEzKsSwKg==";
string[] fileNames = Encoding.UTF8.GetString(Convert.FromBase64String(encodedFileNames))
	.Replace("*", ".txt?").Split(new[] { "?" }, StringSplitOptions.RemoveEmptyEntries)
	.Select(n => expand(n)).ToArray();

Pure C# solution for linq orderby:

http://zootfroot.blogspot.com/2009/09/natural-sort-compare-with-linq-orderby.html

public class NaturalSortComparer<T> : IComparer<string>, IDisposable
{
    private bool isAscending;
 
    public NaturalSortComparer(bool inAscendingOrder = true)
    {
        this.isAscending = inAscendingOrder;
    }
 
    #region IComparer<string> Members
 
    public int Compare(string x, string y)
    {
        throw new NotImplementedException();
    }
 
    #endregion
 
    #region IComparer<string> Members
 
    int IComparer<string>.Compare(string x, string y)
    {
        if (x == y)
            return 0;
 
        string[] x1, y1;
 
        if (!table.TryGetValue(x, out x1))
        {
            x1 = Regex.Split(x.Replace(" ", ""), "([0-9]+)");
            table.Add(x, x1);
        }
 
        if (!table.TryGetValue(y, out y1))
        {
            y1 = Regex.Split(y.Replace(" ", ""), "([0-9]+)");
            table.Add(y, y1);
        }
 
        int returnVal;
 
        for (int i = 0; i < x1.Length && i < y1.Length; i++)
        {
            if (x1[i] != y1[i])
            {
                returnVal = PartCompare(x1[i], y1[i]);
                return isAscending ? returnVal : -returnVal;
            }
        }
 
        if (y1.Length > x1.Length)
        {
            returnVal = 1;
        }
        else if (x1.Length > y1.Length)
        { 
            returnVal = -1; 
        }
        else
        {
            returnVal = 0;
        }
 
        return isAscending ? returnVal : -returnVal;
    }
 
    private static int PartCompare(string left, string right)
    {
        int x, y;
        if (!int.TryParse(left, out x))
            return left.CompareTo(right);
 
        if (!int.TryParse(right, out y))
            return left.CompareTo(right);
 
        return x.CompareTo(y);
    }
 
    #endregion
 
    private Dictionary<string, string[]> table = new Dictionary<string, string[]>();
 
    public void Dispose()
    {
        table.Clear();
        table = null;
    }
}

Matthews Horsleys answer is the fastest method which doesn't change behaviour depending on which version of windows your program is running on. However, it can be even faster by creating the regex once, and using RegexOptions.Compiled. I also added the option of inserting a string comparer so you can ignore case if needed, and improved readability a bit.

    public static IEnumerable<T> OrderByNatural<T>(this IEnumerable<T> items, Func<T, string> selector, StringComparer stringComparer = null)
    {
        var regex = new Regex(@"\d+", RegexOptions.Compiled);

        int maxDigits = items
                      .SelectMany(i => regex.Matches(selector(i)).Cast<Match>().Select(digitChunk => (int?)digitChunk.Value.Length))
                      .Max() ?? 0;

        return items.OrderBy(i => regex.Replace(selector(i), match => match.Value.PadLeft(maxDigits, '0')), stringComparer ?? StringComparer.CurrentCulture);
    }

Use by

var sortedEmployees = employees.OrderByNatural(emp => emp.Name);

This takes 450ms to sort 100,000 strings compared to 300ms for the default .net string comparison - pretty fast!

My solution:

void Main()
{
	new[] {"a4","a3","a2","a10","b5","b4","b400","1","C1d","c1d2"}.OrderBy(x => x, new NaturalStringComparer()).Dump();
}

public class NaturalStringComparer : IComparer<string>
{
    private static readonly Regex _re = new Regex(@"(?<=\D)(?=\d)|(?<=\d)(?=\D)", RegexOptions.Compiled);

    public int Compare(string x, string y)
    {
        x = x.ToLower();
        y = y.ToLower();
        if(string.Compare(x, 0, y, 0, Math.Min(x.Length, y.Length)) == 0)
        {
            if(x.Length == y.Length) return 0;
            return x.Length < y.Length ? -1 : 1;
        }
        var a = _re.Split(x);
        var b = _re.Split(y);
        int i = 0;
        while(true)
        {
            int r = PartCompare(a[i], b[i]);
            if(r != 0) return r;
            ++i;
        }
    }

    private static int PartCompare(string x, string y)
    {
        int a, b;
        if(int.TryParse(x, out a) && int.TryParse(y, out b))
            return a.CompareTo(b);
        return x.CompareTo(y);
    }
}

Results:

1
a2
a3
a4
a10
b4
b5
b400
C1d
c1d2

You do need to be careful -- I vaguely recall reading that StrCmpLogicalW, or something like it, was not strictly transitive, and I have observed .NET's sort methods to sometimes get stuck in infinite loops if the comparison function breaks that rule.

A transitive comparison will always report that a < c if a < b and b < c. There exists a function that does a natural sort order comparison that does not always meet that criterion, but I can't recall whether it is StrCmpLogicalW or something else.

This is my code to sort a string having both alpha and numeric characters.

First, this extension method:

public static IEnumerable<string> AlphanumericSort(this IEnumerable<string> me)
{
    return me.OrderBy(x => Regex.Replace(x, @"\d+", m => m.Value.PadLeft(50, '0')));
}

Then, simply use it anywhere in your code like this:

List<string> test = new List<string>() { "The 1st", "The 12th", "The 2nd" };
test = test.AlphanumericSort();

How does it works ? By replaceing with zeros:

  Original  | Regex Replace |      The      |   Returned
    List    | Apply PadLeft |    Sorting    |     List
            |               |               |
 "The 1st"  |  "The 001st"  |  "The 001st"  |  "The 1st"
 "The 12th" |  "The 012th"  |  "The 002nd"  |  "The 2nd"
 "The 2nd"  |  "The 002nd"  |  "The 012th"  |  "The 12th"

Works with multiples numbers:

 Alphabetical Sorting | Alphanumeric Sorting
                      |
 "Page 21, Line 42"   | "Page 3, Line 7"
 "Page 21, Line 5"    | "Page 3, Line 32"
 "Page 3, Line 32"    | "Page 21, Line 5"
 "Page 3, Line 7"     | "Page 21, Line 42"

Hope that's will help.

Adding to Greg Beech's answer (because I've just been searching for that), if you want to use this from Linq you can use the OrderBy that takes an IComparer. E.g.:

var items = new List<MyItem>();

// fill items

var sorted = items.OrderBy(item => item.Name, new NaturalStringComparer());

Here's a relatively simple example that doesn't use P/Invoke and avoids any allocation during execution.

Feel free to use the code from here, or if it's easier there's a NuGet package:

https://www.nuget.org/packages/NaturalSort

https://github.com/drewnoakes/natural-sort

internal sealed class NaturalStringComparer : IComparer<string>
{
    public static NaturalStringComparer Instance { get; } = new NaturalStringComparer();

    public int Compare(string x, string y)
    {
        // sort nulls to the start
        if (x == null)
            return y == null ? 0 : -1;
        if (y == null)
            return 1;

        var ix = 0;
        var iy = 0;

        while (true)
        {
            // sort shorter strings to the start
            if (ix >= x.Length)
                return iy >= y.Length ? 0 : -1;
            if (iy >= y.Length)
                return 1;

            var cx = x[ix];
            var cy = y[iy];

            int result;
            if (char.IsDigit(cx) && char.IsDigit(cy))
                result = CompareInteger(x, y, ref ix, ref iy);
            else
                result = cx.CompareTo(y[iy]);

            if (result != 0)
                return result;

            ix++;
            iy++;
        }
    }

    private static int CompareInteger(string x, string y, ref int ix, ref int iy)
    {
        var lx = GetNumLength(x, ix);
        var ly = GetNumLength(y, iy);

        // shorter number first (note, doesn't handle leading zeroes)
        if (lx != ly)
            return lx.CompareTo(ly);

        for (var i = 0; i < lx; i++)
        {
            var result = x[ix++].CompareTo(y[iy++]);
            if (result != 0)
                return result;
        }

        return 0;
    }

    private static int GetNumLength(string s, int i)
    {
        var length = 0;
        while (i < s.Length && char.IsDigit(s[i++]))
            length++;
        return length;
    }
}

It doesn't ignore leading zeroes, so 01 comes after 2.

Corresponding unit test:

public class NumericStringComparerTests
{
    [Fact]
    public void OrdersCorrectly()
    {
        AssertEqual("", "");
        AssertEqual(null, null);
        AssertEqual("Hello", "Hello");
        AssertEqual("Hello123", "Hello123");
        AssertEqual("123", "123");
        AssertEqual("123Hello", "123Hello");

        AssertOrdered("", "Hello");
        AssertOrdered(null, "Hello");
        AssertOrdered("Hello", "Hello1");
        AssertOrdered("Hello123", "Hello124");
        AssertOrdered("Hello123", "Hello133");
        AssertOrdered("Hello123", "Hello223");
        AssertOrdered("123", "124");
        AssertOrdered("123", "133");
        AssertOrdered("123", "223");
        AssertOrdered("123", "1234");
        AssertOrdered("123", "2345");
        AssertOrdered("0", "1");
        AssertOrdered("123Hello", "124Hello");
        AssertOrdered("123Hello", "133Hello");
        AssertOrdered("123Hello", "223Hello");
        AssertOrdered("123Hello", "1234Hello");
    }

    private static void AssertEqual(string x, string y)
    {
        Assert.Equal(0, NaturalStringComparer.Instance.Compare(x, y));
        Assert.Equal(0, NaturalStringComparer.Instance.Compare(y, x));
    }

    private static void AssertOrdered(string x, string y)
    {
        Assert.Equal(-1, NaturalStringComparer.Instance.Compare(x, y));
        Assert.Equal( 1, NaturalStringComparer.Instance.Compare(y, x));
    }
}

Here's a version for .NET Core 2.1+ / .NET 5.0+, using spans to avoid allocations

public class NaturalSortStringComparer : IComparer<string>
{
    public static NaturalSortStringComparer Ordinal { get; } = new NaturalSortStringComparer(StringComparison.Ordinal);
    public static NaturalSortStringComparer OrdinalIgnoreCase { get; } = new NaturalSortStringComparer(StringComparison.OrdinalIgnoreCase);
    public static NaturalSortStringComparer CurrentCulture { get; } = new NaturalSortStringComparer(StringComparison.CurrentCulture);
    public static NaturalSortStringComparer CurrentCultureIgnoreCase { get; } = new NaturalSortStringComparer(StringComparison.CurrentCultureIgnoreCase);
    public static NaturalSortStringComparer InvariantCulture { get; } = new NaturalSortStringComparer(StringComparison.InvariantCulture);
    public static NaturalSortStringComparer InvariantCultureIgnoreCase { get; } = new NaturalSortStringComparer(StringComparison.InvariantCultureIgnoreCase);

    private readonly StringComparison _comparison;

    public NaturalSortStringComparer(StringComparison comparison)
    {
        _comparison = comparison;
    }

    public int Compare(string? x, string? y)
    {
        // Let string.Compare handle the case where x or y is null
        if (x is null || y is null)
            return string.Compare(x, y, _comparison);

        var xSegments = GetSegments(x);
        var ySegments = GetSegments(y);

        while (xSegments.MoveNext() && ySegments.MoveNext())
        {
            bool xIsNumber = int.TryParse(xSegments.Current, out var xValue);
            bool yIsNumber = int.TryParse(ySegments.Current, out var yValue);

            int cmp;

            // If they're both numbers, compare the value
            if (xIsNumber && yIsNumber)
            {
                cmp = xValue.CompareTo(yValue);
                if (cmp != 0)
                    return cmp;
            }
            // If x is a number and y is not, x is "lesser than" y
            else if (xIsNumber)
            {
                return -1;
            }
            // If y is a number and x is not, x is "greater than" y
            else if (yIsNumber)
            {
                return 1;
            }

            // OK, neither are number, compare the segments as text
            cmp = xSegments.Current.CompareTo(ySegments.Current, _comparison);
            if (cmp != 0)
                return cmp;
        }

        // At this point, either all segments are equal, or one string is shorter than the other

        // If x is shorter, it's "lesser than" y
        if (x.Length < y.Length)
            return -1;
        // If x is longer, it's "greater than" y
        if (x.Length > y.Length)
            return 1;

        // If they have the same length, they're equal
        return 0;
    }

    private static StringSegmentEnumerator GetSegments(string s) => new StringSegmentEnumerator(s);

    private struct StringSegmentEnumerator
    {
        private readonly string _s;
        private int _start;
        private int _length;
        private int _currentPosition;

        public StringSegmentEnumerator(string s)
        {
            _s = s;
            _start = -1;
            _length = 0;
            _currentPosition = 0;
        }

        public ReadOnlySpan<char> Current => _s.AsSpan(_start, _length);

        public bool MoveNext()
        {
            if (_currentPosition >= _s.Length)
                return false;

            int start = _currentPosition;
            bool isFirstCharDigit = Char.IsDigit(_s[_currentPosition]);

            while (++_currentPosition < _s.Length && Char.IsDigit(_s[_currentPosition]) == isFirstCharDigit)
            {
            }

            _start = start;
            _length = _currentPosition - start;
            return true;
        }
    }
}

I've actually implemented it as an extension method on the StringComparer so that you could do for example:

• StringComparer.CurrentCulture.WithNaturalSort() or
• StringComparer.OrdinalIgnoreCase.WithNaturalSort().

The resulting IComparer<string> can be used in all places like OrderBy, OrderByDescending, ThenBy, ThenByDescending, SortedSet<string>, etc. And you can still easily tweak case sensitivity, culture, etc.

The implementation is fairly trivial and it should perform quite well even on large sequences.

---

I've also published it as a tiny NuGet package, so you can just do:

Install-Package NaturalSort.Extension

The code including XML documentation comments and suite of tests is available in the NaturalSort.Extension GitHub repository.

---

The entire code is this (if you cannot use C# 7 yet, just install the NuGet package):

public static class StringComparerNaturalSortExtension
{
    public static IComparer<string> WithNaturalSort(this StringComparer stringComparer) => new NaturalSortComparer(stringComparer);

    private class NaturalSortComparer : IComparer<string>
    {
        public NaturalSortComparer(StringComparer stringComparer)
        {
            _stringComparer = stringComparer;
        }

        private readonly StringComparer _stringComparer;
        private static readonly Regex NumberSequenceRegex = new Regex(@"(\d+)", RegexOptions.Compiled | RegexOptions.CultureInvariant);
        private static string[] Tokenize(string s) => s == null ? new string[] { } : NumberSequenceRegex.Split(s);
        private static ulong ParseNumberOrZero(string s) => ulong.TryParse(s, NumberStyles.None, CultureInfo.InvariantCulture, out var result) ? result : 0;

        public int Compare(string s1, string s2)
        {
            var tokens1 = Tokenize(s1);
            var tokens2 = Tokenize(s2);

            var zipCompare = tokens1.Zip(tokens2, TokenCompare).FirstOrDefault(x => x != 0);
            if (zipCompare != 0)
                return zipCompare;

            var lengthCompare = tokens1.Length.CompareTo(tokens2.Length);
            return lengthCompare;
        }
        
        private int TokenCompare(string token1, string token2)
        {
            var number1 = ParseNumberOrZero(token1);
            var number2 = ParseNumberOrZero(token2);

            var numberCompare = number1.CompareTo(number2);
            if (numberCompare != 0)
                return numberCompare;

            var stringCompare = _stringComparer.Compare(token1, token2);
            return stringCompare;
        }
    }
}

Here is a naive one-line regex-less LINQ way (borrowed from python):

var alphaStrings = new List<string>() { "10","2","3","4","50","11","100","a12","b12" };
var orderedString = alphaStrings.OrderBy(g => new Tuple<int, string>(g.ToCharArray().All(char.IsDigit)? int.Parse(g) : int.MaxValue, g));
// Order Now: ["2","3","4","10","11","50","100","a12","b12"]

Inspired by Michael Parker's solution, here is an IComparer implementation that you can drop in to any of the linq ordering methods:

private class NaturalStringComparer : IComparer<string>
{
    public int Compare(string left, string right)
    {
        int max = new[] { left, right }
            .SelectMany(x => Regex.Matches(x, @"\d+").Cast<Match>().Select(y => (int?)y.Value.Length))
            .Max() ?? 0;

        var leftPadded = Regex.Replace(left, @"\d+", m => m.Value.PadLeft(max, '0'));
        var rightPadded = Regex.Replace(right, @"\d+", m => m.Value.PadLeft(max, '0'));

        return string.Compare(leftPadded, rightPadded);
    }
}

Expanding on a couple of the previous answers and making use of extension methods, I came up with the following that doesn't have the caveats of potential multiple enumerable enumeration, or performance issues concerned with using multiple regex objects, or calling regex needlessly, that being said, it does use ToList(), which can negate the benefits in larger collections.

The selector supports generic typing to allow any delegate to be assigned, the elements in the source collection are mutated by the selector, then converted to strings with ToString().

	private static readonly Regex _NaturalOrderExpr = new Regex(@"\d+", RegexOptions.Compiled);

	public static IEnumerable<TSource> OrderByNatural<TSource, TKey>(
		this IEnumerable<TSource> source, Func<TSource, TKey> selector)
	{
		int max = 0;

		var selection = source.Select(
			o =>
			{
				var v = selector(o);
				var s = v != null ? v.ToString() : String.Empty;

				if (!String.IsNullOrWhiteSpace(s))
				{
					var mc = _NaturalOrderExpr.Matches(s);

					if (mc.Count > 0)
					{
						max = Math.Max(max, mc.Cast<Match>().Max(m => m.Value.Length));
					}
				}

				return new
				{
					Key = o,
					Value = s
				};
			}).ToList();

		return
			selection.OrderBy(
				o =>
				String.IsNullOrWhiteSpace(o.Value) ? o.Value : _NaturalOrderExpr.Replace(o.Value, m => m.Value.PadLeft(max, '0')))
					 .Select(o => o.Key);
	}

	public static IEnumerable<TSource> OrderByDescendingNatural<TSource, TKey>(
		this IEnumerable<TSource> source, Func<TSource, TKey> selector)
	{
		int max = 0;

		var selection = source.Select(
			o =>
			{
				var v = selector(o);
				var s = v != null ? v.ToString() : String.Empty;

				if (!String.IsNullOrWhiteSpace(s))
				{
					var mc = _NaturalOrderExpr.Matches(s);

					if (mc.Count > 0)
					{
						max = Math.Max(max, mc.Cast<Match>().Max(m => m.Value.Length));
					}
				}

				return new
				{
					Key = o,
					Value = s
				};
			}).ToList();

		return
			selection.OrderByDescending(
				o =>
				String.IsNullOrWhiteSpace(o.Value) ? o.Value : _NaturalOrderExpr.Replace(o.Value, m => m.Value.PadLeft(max, '0')))
					 .Select(o => o.Key);
	}

We had a need for a natural sort to deal with text with the following pattern:

"Test 1-1-1 something"
"Test 1-2-3 something"
...

For some reason when I first looked on SO, I didn't find this post and implemented our own. Compared to some of the solutions presented here, while similar in concept, it could have the benefit of maybe being simpler and easier to understand. However, while I did try to look at performance bottlenecks, It is still a much slower implementation than the default OrderBy().

Here is the extension method I implement:

public static class EnumerableExtensions
{
    // set up the regex parser once and for all
    private static readonly Regex Regex = new Regex(@"\d+|\D+", RegexOptions.Compiled | RegexOptions.Singleline);

    // stateless comparer can be built once
    private static readonly AggregateComparer Comparer = new AggregateComparer();

    public static IEnumerable<T> OrderByNatural<T>(this IEnumerable<T> source, Func<T, string> selector)
    {
        // first extract string from object using selector
        // then extract digit and non-digit groups
        Func<T, IEnumerable<IComparable>> splitter =
            s => Regex.Matches(selector(s))
                      .Cast<Match>()
                      .Select(m => Char.IsDigit(m.Value[0]) ? (IComparable) int.Parse(m.Value) : m.Value);
        return source.OrderBy(splitter, Comparer);
    }

    /// <summary>
    /// This comparer will compare two lists of objects against each other
    /// </summary>
    /// <remarks>Objects in each list are compare to their corresponding elements in the other
    /// list until a difference is found.</remarks>
    private class AggregateComparer : IComparer<IEnumerable<IComparable>>
    {
        public int Compare(IEnumerable<IComparable> x, IEnumerable<IComparable> y)
        {
            return
                x.Zip(y, (a, b) => new {a, b})              // walk both lists
                 .Select(pair => pair.a.CompareTo(pair.b))  // compare each object
                 .FirstOrDefault(result => result != 0);    // until a difference is found
        }
    }
}

The idea is to split the original strings into blocks of digits and non-digits ("\d+|\D+"). Since this is a potentially expensive task, it is done only once per entry. We then use a comparer of comparable objects (sorry, I can't find a more proper way to say it). It compares each block to its corresponding block in the other string.

I would like feedback on how this could be improved and what the major flaws are. Note that maintainability is important to us at this point and we are not currently using this in extremely large data sets.

A version that's easier to read/maintain.

public class NaturalStringComparer : IComparer<string>
{
    public static NaturalStringComparer Instance { get; } = new NaturalStringComparer();

    public int Compare(string x, string y) {
        const int LeftIsSmaller = -1;
        const int RightIsSmaller = 1;
        const int Equal = 0;

        var leftString = x;
        var rightString = y;

        var stringComparer = CultureInfo.CurrentCulture.CompareInfo;

        int rightIndex;
        int leftIndex;

        for (leftIndex = 0, rightIndex = 0;
             leftIndex < leftString.Length && rightIndex < rightString.Length;
             leftIndex++, rightIndex++) {
            var leftChar = leftString[leftIndex];
            var rightChar = rightString[leftIndex];

            var leftIsNumber = char.IsNumber(leftChar);
            var rightIsNumber = char.IsNumber(rightChar);

            if (!leftIsNumber && !rightIsNumber) {
                var result = stringComparer.Compare(leftString, leftIndex, 1, rightString, leftIndex, 1);
                if (result != 0) return result;
            } else if (leftIsNumber && !rightIsNumber) {
                return LeftIsSmaller;
            } else if (!leftIsNumber && rightIsNumber) {
                return RightIsSmaller;
            } else {
                var leftNumberLength = NumberLength(leftString, leftIndex, out var leftNumber);
                var rightNumberLength = NumberLength(rightString, rightIndex, out var rightNumber);

                if (leftNumberLength < rightNumberLength) {
                    return LeftIsSmaller;
                } else if (leftNumberLength > rightNumberLength) {
                    return RightIsSmaller;
                } else {
                    if(leftNumber < rightNumber) {
                        return LeftIsSmaller;
                    } else if(leftNumber > rightNumber) {
                        return RightIsSmaller;
                    }
                }
            }
        }

        if (leftString.Length < rightString.Length) {
            return LeftIsSmaller;
        } else if(leftString.Length > rightString.Length) {
            return RightIsSmaller;
        }

        return Equal;
    }

    public int NumberLength(string str, int offset, out int number) {
        if (string.IsNullOrWhiteSpace(str)) throw new ArgumentNullException(nameof(str));
        if (offset >= str.Length) throw new ArgumentOutOfRangeException(nameof(offset), offset, "Offset must be less than the length of the string.");

        var currentOffset = offset;

        var curChar = str[currentOffset];

        if (!char.IsNumber(curChar))
            throw new ArgumentException($"'{curChar}' is not a number.", nameof(offset));

        int length = 1;

        var numberString = string.Empty;

        for (currentOffset = offset + 1;
            currentOffset < str.Length;
            currentOffset++, length++) {

            curChar = str[currentOffset];
            numberString += curChar;

            if (!char.IsNumber(curChar)) {
                number = int.Parse(numberString);

                return length;
            }
        }

        number = int.Parse(numberString);

        return length;
    }
}

Let me explain my problem and how i was able to solve it.

Problem:- Sort files based on FileName from FileInfo objects which are retrieved from a Directory.

Solution:- I selected the file names from FileInfo and trimed the ".png" part of the file name. Now, just do List.Sort(), which sorts the filenames in Natural sorting order. Based on my testing i found that having .png messes up sorting order. Have a look at the below code

var imageNameList = new DirectoryInfo(@"C:\Temp\Images").GetFiles("*.png").Select(x =>x.Name.Substring(0, x.Name.Length - 4)).ToList();
imageNameList.Sort();