What is the fastest (built-in) comparison for string-types in C#

Question

What is the fastest built-in comparison-method for string-types in C#? I don\'t mind about the typographical/semantical meaning: the aim is to use the comparator in sorted lists

Answer 1

I'm assuming you want a less-than/equal/greater-than comparison rather than just equality; equality is a slightly different topic, although the principles are basically the same. If you're actually only searching for presence in something like a SortedList, I'd consider using a Dictionary<string, XXX> instead - do you really need all that sorting?

String.CompareOrdinal, or using an overload of String.Compare which allows the comparison to be provided, and specifying an ordinal (case-sensitive) comparison, e.g. String.Compare(x, y, StringComparison.Ordinal) will be the fastest.

Basically an ordinal comparison just needs to walk the two strings, character by character, until it finds a difference. If it doesn't find any differences, and the lengths are the same, the result is 0. If it doesn't find any differences but the lengths aren't the same, the longer string is deemed "larger". If it does find a difference, it can immediately work out which is deemed "larger" based on which character is "larger" in ordinal terms.

To put is another way: it's like doing the obvious comparison between two char[] values.

Culture-sensitive comparisons have to perform all kinds of tortuous feats, depending on the precise culture you use. For an example of this, see this question. It's pretty clear that having more complex rules to follow can make this slower.

Answer 2

I just noticed a 50% performance increase in my own code by comparing string lengths first and if equal then using the string.compare methods. So in a loop I have:

VB:

If strA.length = strB.length then
   if string.compare(strA,strB,true) = 0 then
      TheyAreEqual
   End if
End if

C#:

if(strA.Length == strB.Length)
{
   if(string.Compare(strA,strB,true) == 0)
   {
       //they are equal
   }
}

This could be dependant on your own strings but its seems to have worked well for me.

Answer 3

This might be useful to someone, but changing one line of my code brought the unit testing of my method down from 140ms to 1ms!

Original

Unit test: 140ms

public bool StringsMatch(string string1, string string2)
{
    if (string1 == null && string2 == null) return true;
    return string1.Equals(string2, StringComparison.Ordinal);
}

New

Unit test: 1ms

public bool StringsMatch(string string1, string string2)
{
    if (string1 == null && string2 == null) return true;
    return string.CompareOrdinal(string1, string2) == 0 ? true : false;
}

Unit Test (NUnit)

[Test]
public void StringsMatch_OnlyString1NullOrEmpty_ReturnFalse()
{
    Authentication auth = new Authentication();
    Assert.IsFalse(auth.StringsMatch(null, "foo"));
    Assert.IsFalse(auth.StringsMatch("", "foo"));
}

Interestingly StringsMatch_OnlyString1NullOrEmpty_ReturnFalse() was the only unit test that took 140ms for the StringsMatch method. StringsMatch_AllParamsNullOrEmpty_ReturnTrue() was always 1ms and StringsMatch_OnlyString2NullOrEmpty_ReturnFalse() always <1ms.

Answer 4

I think there's a few ways most C# developers go about comparing strings, with the following being the most common:

• Compare - as you mentioned
• CompareOrdinal - as you mentioned
• ==
• String.Equals
• writing a custom algorithm to compare char by char

If you want to go to extremes, you can use other objects/methods that aren't so obvious:

• SequenceEqual example:

  c1 = str1.ToCharArray(); c2 = str2.ToCharArray(); if (c1.SequenceEqual(c2))

• IndexOf example: if (stringsWeAreComparingAgainst.IndexOf(stringsWeWantToSeeIfMatches, 0 , stringsWeWantToSeeIfMatches.Length) == 0)

• Or you can implement Dictionary and HashSets, using the strings as "keys" and testing to see if they exist already with the string you want to compare against. For instance: if (hs.Contains(stringsWeWantToSeeIfMatches))

So feel free to slice and dice to find your own ways of doing things. Remember though someone is going to have to maintain the code and probably won't want to spend time trying to figure out why you're using whatever method you've decided to use.

As always, optimize as your own risk. :-)

Answer 5

Fastest is interned strings with reference equality test, but you only get equality testing and it's at the heavy expense of memory - so expensive that it's almost never the recommended course.

Past that, a case-sensitive ordinal test will be the fastest, and this method is absolutely recommended for non-culture-specific strings. Case-sensitive is faster if it works for your use case.

When you specify either StringComparison.Ordinal or StringComparison.OrdinalIgnoreCase, the string comparison will be non-linguistic. That is, the features that are specific to the natural language are ignored when making comparison decisions. This means the decisions are based on simple byte comparisons and ignore casing or equivalence tables that are parameterized by culture. As a result, by explicitly setting the parameter to either the StringComparison.Ordinal or StringComparison.OrdinalIgnoreCase, your code often gains speed, increases correctness, and becomes more reliable.

Source

Answer 6

I designed a unit test to test string comparison speed using some of the methods mentioned in this post. This test was ran using .NET 4

In short, there isn't much much difference, and I had to go to 100,000,000 iterations to see a significant difference. Since it seems the characters are compared in turn until a difference is found, inevitably how similar the strings are plays a part.

These results actually seem to suggest that using str1.Equals(str2) is the fastest way to compare strings.

These are the results of the test, with the test class included:

######## SET 1 compared strings are the same: 0
#### Basic == compare: 413
#### Equals compare: 355
#### Equals(compare2, StringComparison.Ordinal) compare: 387
#### String.Compare(compare1, compare2, StringComparison.Ordinal) compare: 426
#### String.CompareOrdinal(compare1, compare2) compare: 412

######## SET 2 compared strings are NOT the same: 0
#### Basic == compare: 710
#### Equals compare: 733
#### Equals(compare2, StringComparison.Ordinal) compare: 840
#### String.Compare(compare1, compare2, StringComparison.Ordinal) compare: 987
#### String.CompareOrdinal(compare1, compare2) compare: 776

using System;
using System.Diagnostics;
using NUnit.Framework;

namespace Fwr.UnitTests
{
    [TestFixture]
    public class StringTests
    {
        [Test]
        public void Test_fast_string_compare()
        {
            int iterations = 100000000;
            bool result = false;
            var stopWatch = new Stopwatch();

            Debug.WriteLine("######## SET 1 compared strings are the same: " + stopWatch.ElapsedMilliseconds);

            string compare1 = "xxxxxxxxxxxxxxxxxx";
            string compare2 = "xxxxxxxxxxxxxxxxxx";

            // Test 1

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1 == compare2;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Basic == compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 2

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1.Equals(compare2);
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Equals compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 3

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1.Equals(compare2, StringComparison.Ordinal);
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Equals(compare2, StringComparison.Ordinal) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 4

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = String.Compare(compare1, compare2, StringComparison.Ordinal) != 0;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### String.Compare(compare1, compare2, StringComparison.Ordinal) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 5

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = String.CompareOrdinal(compare1, compare2) != 0;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### String.CompareOrdinal(compare1, compare2) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            Debug.WriteLine("######## SET 2 compared strings are NOT the same: " + stopWatch.ElapsedMilliseconds);

            compare1 = "ueoqwwnsdlkskjsowy";
            compare2 = "sakjdjsjahsdhsjdak";

            // Test 1

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1 == compare2;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Basic == compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 2

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1.Equals(compare2);
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Equals compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 3

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = compare1.Equals(compare2, StringComparison.Ordinal);
            }

            stopWatch.Stop();

            Debug.WriteLine("#### Equals(compare2, StringComparison.Ordinal) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 4

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = String.Compare(compare1, compare2, StringComparison.Ordinal) != 0;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### String.Compare(compare1, compare2, StringComparison.Ordinal) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();

            // Test 5

            stopWatch.Start();

            for (int i = 0; i < iterations; i++)
            {
                result = String.CompareOrdinal(compare1, compare2) != 0;
            }

            stopWatch.Stop();

            Debug.WriteLine("#### String.CompareOrdinal(compare1, compare2) compare: " + stopWatch.ElapsedMilliseconds);

            stopWatch.Reset();
        }
    }
}