C# String comparisons: Difference between CurrentCultureIgnoreCase and InvariantCultureIgnoreCase

string test = "testvalue";
test.Equals("TESTVALUE", StringComparison.CurrentCultureIgnoreCase);

string test = "testvalue";
test.Equals("TESTVALUE", StringComparison.InvariantCultureIgnoreCase);

using System;
using System.Globalization;
using System.Threading;

class Test
{
    static void Main()
    {
        CultureInfo turkish = CultureInfo.CreateSpecificCulture("tr");
        Thread.CurrentThread.CurrentCulture = turkish;

        // In Turkey, "i" does odd things
        string lower = "i";
        string upper = "I";

        Console.WriteLine(lower.Equals(upper,
            StringComparison.CurrentCultureIgnoreCase));
        Console.WriteLine(lower.Equals(upper,
            StringComparison.InvariantCultureIgnoreCase));
    }
}