How can I check if a string contains a character in C#?

You can use the IndexOf method, which has a suitable overload for string comparison types:

if (def.IndexOf("s", StringComparison.OrdinalIgnoreCase) >= 0) ...

Also, you would not need the == true, since an if statement only expects an expression that evaluates to a bool.

You can use the extension method .Contains() from the namespace System.Linq:

using System.Linq;

    ...

    if (abc.ToLower().Contains('s')) { }

And also, to check if a boolean expression is true, you don't need == true

Since the Contains method is an extension method, my solution might be confusing. Here are two versions that don't require you to add using System.Linq;:

if (abc.ToLower().IndexOf('s') != -1) { }

// or:

if (abc.IndexOf("s", StringComparison.CurrentCultureIgnoreCase) != -1) { }

Update

If you want to, you can write your own extensions method 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 you can call them like this:

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

In most cases when dealing with user data, you actually want to use CurrentCultureIgnoreCase (or the ContainsAnyCase extension method), 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 ı (without a dot), and not i (with a dot).