How to compare strings in C#

מאמר
03/19/2024

You compare strings to answer one of two questions: "Are these two strings equal?" or "In what order should these strings be placed when sorting them?"

Those two questions are complicated by factors that affect string comparisons:

You can choose an ordinal or linguistic comparison.
You can choose if case matters.
You can choose culture-specific comparisons.
Linguistic comparisons are culture and platform-dependent.

The System.StringComparison enumeration fields represent these choices:

CurrentCulture: Compare strings using culture-sensitive sort rules and the current culture.
CurrentCultureIgnoreCase: Compare strings using culture-sensitive sort rules, the current culture, and ignoring the case of the strings being compared.
InvariantCulture: Compare strings using culture-sensitive sort rules and the invariant culture.
InvariantCultureIgnoreCase: Compare strings using culture-sensitive sort rules, the invariant culture, and ignoring the case of the strings being compared.
Ordinal: Compare strings using ordinal (binary) sort rules.
OrdinalIgnoreCase: Compare strings using ordinal (binary) sort rules and ignoring the case of the strings being compared.

Note

The C# examples in this article run in the Try.NET inline code runner and playground. Select the Run button to run an example in an interactive window. Once you execute the code, you can modify it and run the modified code by selecting Run again. The modified code either runs in the interactive window or, if compilation fails, the interactive window displays all C# compiler error messages.

When you compare strings, you define an order among them. Comparisons are used to sort a sequence of strings. Once the sequence is in a known order, it's easier to search, both for software and for humans. Other comparisons might check if strings are the same. These sameness checks are similar to equality, but some differences, such as case differences, might be ignored.

Default ordinal comparisons

By default, the most common operations:

String.Equals
String.Equality and String.Inequality, that is, equality operators == and !=, respectively perform a case-sensitive, ordinal comparison. String.Equals has an overload where a StringComparison argument can be provided to alter its sorting rules. The following example demonstrates that:

string root = @"C:\users";
string root2 = @"C:\Users";

bool result = root.Equals(root2);
Console.WriteLine($"Ordinal comparison: <{root}> and <{root2}> are {(result ? "equal." : "not equal.")}");

result = root.Equals(root2, StringComparison.Ordinal);
Console.WriteLine($"Ordinal comparison: <{root}> and <{root2}> are {(result ? "equal." : "not equal.")}");

Console.WriteLine($"Using == says that <{root}> and <{root2}> are {(root == root2 ? "equal" : "not equal")}");

The default ordinal comparison doesn't take linguistic rules into account when comparing strings. It compares the binary value of each Char object in two strings. As a result, the default ordinal comparison is also case-sensitive.

The test for equality with String.Equals and the == and != operators differs from string comparison using the String.CompareTo and Compare(String, String) methods. They all perform a case-sensitive comparison. However, while the tests for equality perform an ordinal comparison, the CompareTo and Compare methods perform a culture-aware linguistic comparison using the current culture. Make the intent of your code clear by calling an overload that explicitly specifies the type of comparison to perform.

Case-insensitive ordinal comparisons

The String.Equals(String, StringComparison) method enables you to specify a StringComparison value of StringComparison.OrdinalIgnoreCase for a case-insensitive ordinal comparison. There's also a static String.Compare(String, String, StringComparison) method that performs a case-insensitive ordinal comparison if you specify a value of StringComparison.OrdinalIgnoreCase for the StringComparison argument. These comparisons are shown in the following code:

string root = @"C:\users";
string root2 = @"C:\Users";

bool result = root.Equals(root2, StringComparison.OrdinalIgnoreCase);
bool areEqual = String.Equals(root, root2, StringComparison.OrdinalIgnoreCase);
int comparison = String.Compare(root, root2, comparisonType: StringComparison.OrdinalIgnoreCase);

Console.WriteLine($"Ordinal ignore case: <{root}> and <{root2}> are {(result ? "equal." : "not equal.")}");
Console.WriteLine($"Ordinal static ignore case: <{root}> and <{root2}> are {(areEqual ? "equal." : "not equal.")}");
if (comparison < 0)
    Console.WriteLine($"<{root}> is less than <{root2}>");
else if (comparison > 0)
    Console.WriteLine($"<{root}> is greater than <{root2}>");
else
    Console.WriteLine($"<{root}> and <{root2}> are equivalent in order");

These methods use the casing conventions of the invariant culture when performing a case-insensitive ordinal comparison.

Linguistic comparisons

Many string comparison methods (such as String.StartsWith) use linguistic rules for the current culture by default to order their inputs. This linguistic comparison is sometimes referred to as "word sort order." When you perform a linguistic comparison, some nonalphanumeric Unicode characters might have special weights assigned. For example, the hyphen "-" might have a small weight assigned to it so that "co-op" and "coop" appear next to each other in sort order. Some nonprinting control characters might be ignored. In addition, some Unicode characters might be equivalent to a sequence of Char instances. The following example uses the phrase "They dance in the street." in German with the "ss" (U+0073 U+0073) in one string and 'ß' (U+00DF) in another. Linguistically (in Windows), "ss" is equal to the German Esszet: 'ß' character in both the "en-US" and "de-DE" cultures.

string first = "Sie tanzen auf der Straße.";
string second = "Sie tanzen auf der Strasse.";

Console.WriteLine($"First sentence is <{first}>");
Console.WriteLine($"Second sentence is <{second}>");

bool equal = String.Equals(first, second, StringComparison.InvariantCulture);
Console.WriteLine($"The two strings {(equal == true ? "are" : "are not")} equal.");
showComparison(first, second);

string word = "coop";
string words = "co-op";
string other = "cop";

showComparison(word, words);
showComparison(word, other);
showComparison(words, other);
void showComparison(string one, string two)
{
    int compareLinguistic = String.Compare(one, two, StringComparison.InvariantCulture);
    int compareOrdinal = String.Compare(one, two, StringComparison.Ordinal);
    if (compareLinguistic < 0)
        Console.WriteLine($"<{one}> is less than <{two}> using invariant culture");
    else if (compareLinguistic > 0)
        Console.WriteLine($"<{one}> is greater than <{two}> using invariant culture");
    else
        Console.WriteLine($"<{one}> and <{two}> are equivalent in order using invariant culture");
    if (compareOrdinal < 0)
        Console.WriteLine($"<{one}> is less than <{two}> using ordinal comparison");
    else if (compareOrdinal > 0)
        Console.WriteLine($"<{one}> is greater than <{two}> using ordinal comparison");
    else
        Console.WriteLine($"<{one}> and <{two}> are equivalent in order using ordinal comparison");
}

On Windows, prior to .NET 5, the sort order of "cop", "coop", and "co-op" changes when you change from a linguistic comparison to an ordinal comparison. The two German sentences also compare differently using the different comparison types. Prior to .NET 5, the .NET globalization APIs used National Language Support (NLS) libraries. In .NET 5 and later versions, the .NET globalization APIs use International Components for Unicode (ICU) libraries, which unifies .NET's globalization behavior across all supported operating systems.

Comparisons using specific cultures

The following example stores CultureInfo objects for the en-US and de-DE cultures. The comparisons are performed using a CultureInfo object to ensure a culture-specific comparison. The culture used affects linguistic comparisons. The following example shows the results of comparing the two German sentences using the "en-US" culture and the "de-DE" culture:

string first = "Sie tanzen auf der Straße.";
string second = "Sie tanzen auf der Strasse.";

Console.WriteLine($"First sentence is <{first}>");
Console.WriteLine($"Second sentence is <{second}>");

var en = new System.Globalization.CultureInfo("en-US");

// For culture-sensitive comparisons, use the String.Compare
// overload that takes a StringComparison value.
int i = String.Compare(first, second, en, System.Globalization.CompareOptions.None);
Console.WriteLine($"Comparing in {en.Name} returns {i}.");

var de = new System.Globalization.CultureInfo("de-DE");
i = String.Compare(first, second, de, System.Globalization.CompareOptions.None);
Console.WriteLine($"Comparing in {de.Name} returns {i}.");

bool b = String.Equals(first, second, StringComparison.CurrentCulture);
Console.WriteLine($"The two strings {(b ? "are" : "are not")} equal.");

string word = "coop";
string words = "co-op";
string other = "cop";

showComparison(word, words, en);
showComparison(word, other, en);
showComparison(words, other, en);
void showComparison(string one, string two, System.Globalization.CultureInfo culture)
{
    int compareLinguistic = String.Compare(one, two, en, System.Globalization.CompareOptions.None);
    int compareOrdinal = String.Compare(one, two, StringComparison.Ordinal);
    if (compareLinguistic < 0)
        Console.WriteLine($"<{one}> is less than <{two}> using en-US culture");
    else if (compareLinguistic > 0)
        Console.WriteLine($"<{one}> is greater than <{two}> using en-US culture");
    else
        Console.WriteLine($"<{one}> and <{two}> are equivalent in order using en-US culture");
    if (compareOrdinal < 0)
        Console.WriteLine($"<{one}> is less than <{two}> using ordinal comparison");
    else if (compareOrdinal > 0)
        Console.WriteLine($"<{one}> is greater than <{two}> using ordinal comparison");
    else
        Console.WriteLine($"<{one}> and <{two}> are equivalent in order using ordinal comparison");
}

Culture-sensitive comparisons are typically used to compare and sort strings input by users with other strings input by users. The characters and sorting conventions of these strings might vary depending on the locale of the user's computer. Even strings that contain identical characters might sort differently depending on the culture of the current thread.

Linguistic sorting and searching strings in arrays

The following examples show how to sort and search for strings in an array using a linguistic comparison dependent on the current culture. You use the static Array methods that take a System.StringComparer parameter.

The following example shows how to sort an array of strings using the current culture:

string[] lines = new string[]
{
    @"c:\public\textfile.txt",
    @"c:\public\textFile.TXT",
    @"c:\public\Text.txt",
    @"c:\public\testfile2.txt"
};

Console.WriteLine("Non-sorted order:");
foreach (string s in lines)
{
    Console.WriteLine($"   {s}");
}

Console.WriteLine("\n\rSorted order:");

// Specify Ordinal to demonstrate the different behavior.
Array.Sort(lines, StringComparer.CurrentCulture);

foreach (string s in lines)
{
    Console.WriteLine($"   {s}");
}

Once the array is sorted, you can search for entries using a binary search. A binary search starts in the middle of the collection to determine which half of the collection would contain the sought string. Each subsequent comparison subdivides the remaining part of the collection in half. The array is sorted using the StringComparer.CurrentCulture. The local function ShowWhere displays information about where the string was found. If the string wasn't found, the returned value indicates where it would be if it were found.

string[] lines = new string[]
{
    @"c:\public\textfile.txt",
    @"c:\public\textFile.TXT",
    @"c:\public\Text.txt",
    @"c:\public\testfile2.txt"
};
Array.Sort(lines, StringComparer.CurrentCulture);

string searchString = @"c:\public\TEXTFILE.TXT";
Console.WriteLine($"Binary search for <{searchString}>");
int result = Array.BinarySearch(lines, searchString, StringComparer.CurrentCulture);
ShowWhere<string>(lines, result);

Console.WriteLine($"{(result > 0 ? "Found" : "Did not find")} {searchString}");

void ShowWhere<T>(T[] array, int index)
{
    if (index < 0)
    {
        index = ~index;

        Console.Write("Not found. Sorts between: ");

        if (index == 0)
            Console.Write("beginning of sequence and ");
        else
            Console.Write($"{array[index - 1]} and ");

        if (index == array.Length)
            Console.WriteLine("end of sequence.");
        else
            Console.WriteLine($"{array[index]}.");
    }
    else
    {
        Console.WriteLine($"Found at index {index}.");
    }
}

Ordinal sorting and searching in collections

The following code uses the System.Collections.Generic.List<T> collection class to store strings. The strings are sorted using the List<T>.Sort method. This method needs a delegate that compares and orders two strings. The String.CompareTo method provides that comparison function. Run the sample and observe the order. This sort operation uses an ordinal case-sensitive sort. You would use the static String.Compare methods to specify different comparison rules.

List<string> lines = new List<string>
{
    @"c:\public\textfile.txt",
    @"c:\public\textFile.TXT",
    @"c:\public\Text.txt",
    @"c:\public\testfile2.txt"
};

Console.WriteLine("Non-sorted order:");
foreach (string s in lines)
{
    Console.WriteLine($"   {s}");
}

Console.WriteLine("\n\rSorted order:");

lines.Sort((left, right) => left.CompareTo(right));
foreach (string s in lines)
{
    Console.WriteLine($"   {s}");
}

Once sorted, the list of strings can be searched using a binary search. The following sample shows how to search the sorted list using the same comparison function. The local function ShowWhere shows where the sought text is or would be:

List<string> lines = new List<string>
{
    @"c:\public\textfile.txt",
    @"c:\public\textFile.TXT",
    @"c:\public\Text.txt",
    @"c:\public\testfile2.txt"
};
lines.Sort((left, right) => left.CompareTo(right));

string searchString = @"c:\public\TEXTFILE.TXT";
Console.WriteLine($"Binary search for <{searchString}>");
int result = lines.BinarySearch(searchString);
ShowWhere<string>(lines, result);

Console.WriteLine($"{(result > 0 ? "Found" : "Did not find")} {searchString}");

void ShowWhere<T>(IList<T> collection, int index)
{
    if (index < 0)
    {
        index = ~index;

        Console.Write("Not found. Sorts between: ");

        if (index == 0)
            Console.Write("beginning of sequence and ");
        else
            Console.Write($"{collection[index - 1]} and ");

        if (index == collection.Count)
            Console.WriteLine("end of sequence.");
        else
            Console.WriteLine($"{collection[index]}.");
    }
    else
    {
        Console.WriteLine($"Found at index {index}.");
    }
}

Always make sure to use the same type of comparison for sorting and searching. Using different comparison types for sorting and searching produces unexpected results.

Collection classes such as System.Collections.Hashtable, System.Collections.Generic.Dictionary<TKey,TValue>, and System.Collections.Generic.List<T> have constructors that take a System.StringComparer parameter when the type of the elements or keys is string. In general, you should use these constructors whenever possible, and specify either StringComparer.Ordinal or StringComparer.OrdinalIgnoreCase.

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