using static System.Console;

using System;

using System.Globalization;

public class Program {

    public static void Main() {

        var mainStr = "José João";

        WriteLine(mainStr.ContainsInsensitive("JOA"));

        WriteLine(mainStr.ContainsInsensitive("jose"));

        WriteLine(mainStr.ContainsInsensitive("josé"));

    }

}

​

namespace System {

    public static class StringExt {

        public static bool ContainsInsensitive(this string source, string search) => (new CultureInfo("pt-BR").CompareInfo).IndexOf(source, search, CompareOptions.IgnoreCase | CompareOptions.IgnoreNonSpace) >= 0;

    }

}