StackOverflow user Samantha J T Star asked if there is a string method in C# to check if a string contains a specific character, upper or lower case. They would like to do something like this:

if (def.SomeFunction("s")) { }

There are multiple ways of doing this in C#. One is the Contains extension method from the System.Linq namespace:

using System.Linq;
...
    if (def.ToLower().Contains('s')) { }

Another way is to use one of the Contains overloads of the string class:

// Check by passing a char:
if (def.ToLower().Contains('s')) { }

// Check by passing a string:
if (def.Contains("s", StringComparison.CurrentCultureIgnoreCase) != -1) { }

Create your own extension methods

It is also possible to write your own extension methods for easier reuse:

public static class MyStringExtensions
{
    public static bool ContainsAnyCaseInvariant(this string haystack, char needle)
    {
        return haystack.IndexOf(needle, StringComparison.InvariantCultureIgnoreCase) != -1;
    }
    
    public static bool ContainsAnyCase(this string haystack, char needle)
    {
        return haystack.IndexOf(needle, StringComparison.CurrentCultureIgnoreCase) != -1;
    }
}

Then calling them would look like this:

if (def.ContainsAnyCase('s')) { }
// Or:
if (def.ContainsAnyCaseInvariant('s')) { }

In most cases, when dealing with user data, you probably want to use CurrentCultureIgnoreCase instead of InvariantCultureIgnoreCase, because that way you let the system handle upper/lowercase issues, which depend on the language. When dealing with computational issues, like names of HTML tags and so on, you want to use the invariant culture.

For example: In Turkish, the uppercase letter I in lowercase is actually ı without a dot, and not i with a dot.