C# Case insensitive Contains string check

In this tutorial we learn different ways to do case insensitive string contains check in C#.

It looks like a simple problem, but the default C# string.Contains() method is case sensitive.

And if the string is not in English language i.e., for other languages, we can’t compare text case insensitive directly.

The both strings should be in same culture and We should know the language culture.

Most of the times we will compare strings in English language only.

Method 1: Using C# string.IndexOf() method. #

We can use C# string.IndexOf() Method to do case insensitive string contains check.

IndexOf() method accepts StringComparison.OrdinalIgnoreCase parameter, which specify the type of search to use for the characters.

string textToCheck = "STRING Contains";
bool contains = textToCheck.IndexOf("string", StringComparison.OrdinalIgnoreCase) >= 0;

Method 2: Using string.Contains() method in .Net 5+ & .NET Core 2.0+ #

In latest versions of dot net, i.e., in .Net Core 2.0+ and .Net 5+. The string.Contains() method has an overloaded method which accepts StringComparison parameter.

string textToCheck = "STRING Contains";
bool checkContains = textToCheck.Contains("string",StringComparison.OrdinalIgnoreCase);

Method 3: Using Regex.IsMatch() method #

We can use regular expressions to do case insensitive contains string check.

If you are familiar with Regex, Use Regex.IsMatch() method and to check for case insensitive pass RegexOptions.IgnoreCase parameter.

var stringToSearch = "hI hEllo";
var substring = "hello";
bool contains = Regex.IsMatch(stringToCheck, Regex.Escape(substring), RegexOptions.IgnoreCase);

//true

Method 4: Using .ToUpper() & .ToLower() #

If the strings are in English language and the performance is not an issue, we can convert both string to same case then do the string contains check.

var stringToSearch = "hI hEllo";
var substring = "hello";
bool contains = stringToSearch.ToLower().Contains(substring.ToLower());
or 
bool contains = stringToSearch.ToUpper().Contains(substring.ToUpper());

//true

C# Case insensitive Contains check for other languages #

Case insensitivity is language dependent.

For example, in the English language I is the upper case version of i.

Whereas in the Turkish language upper case version of i is the unfamiliar character İ.

To do the case insensitive string check we need to use CultureInfo object.

var text = "İ";

var check = "i";
            
CultureInfo trCulture = new CultureInfo("tr-TR",false);

bool englishContains = text.IndexOf(check, StringComparison.OrdinalIgnoreCase) >= 0;
//false

var turkishContains = trCulture.CompareInfo.IndexOf(check, text, CompareOptions.IgnoreCase) >= 0;
//true

I have created CultureInfo object for turkish language. And compared both strings using CompareInfo as shown below.

trCulture.CompareInfo.IndexOf(check, text, CompareOptions.IgnoreCase) >= 0;

Best way to do Case insensitive Contains string check #

If you are using latest version of .Net use string.Contains() method.

Otherwise stick with string.IndexOf() method.

Do not prefer .ToUpper() or To.Lower() method as they may lead to performance problems.

Use CultureInfo object for other languages strings.