Case insensitive ‘Contains(string)’ in C#

A common question – Is there a way to use the String.Contains method in C# to perform a case-insensitive match?

string title = "TEST";
title.Contains("test");

The Answer

The answer is to not use the String.Contains method, but instead use String.IndexOf method. In C#, you can test for a case-insensitive match on a partial string using the String.IndexOf method with the StringComparison.OrdinalIgnoreCase option. This allows you to search for a substring within a larger string without considering differences in case.

Here’s an example of how to test for a case-insensitive match on a partial string in C#:

string str = "The quick brown fox jumps over the lazy dog.";
string substr = "BROWN";

if (str.IndexOf(substr, StringComparison.OrdinalIgnoreCase) >= 0)
{
    Console.WriteLine("The substring was found (case-insensitive).");
}
else
{
    Console.WriteLine("The substring was not found (case-insensitive).");
}

In this example, we define a string variable str that contains a sentence, and a string variable substr that contains a partial string we want to search for within str. We then use the String.IndexOf method to search for substr within str using the StringComparison.OrdinalIgnoreCase option, which ignores differences in case.

If substr is found within str, the IndexOf method will return the starting position of the first occurrence of substr within str. In this case, we check if the return value is greater than or equal to 0 to confirm that the substring was found. If it was found, the program will print “The substring was found (case-insensitive).” to the console. Otherwise, it will print “The substring was not found (case-insensitive).”.

It’s important to note that when using String.IndexOf with StringComparison.OrdinalIgnoreCase, the search is faster but less precise than using the StringComparison.CurrentCultureIgnoreCase or StringComparison.InvariantCultureIgnoreCase options. The StringComparison.CurrentCultureIgnoreCase option uses the culture-specific rules for case-insensitive comparisons, while the StringComparison.InvariantCultureIgnoreCase option uses a culture-independent comparison that is more consistent across different cultures.