using System;

public class Program

{

    public static void Main()

    {

        const string tmp = "17:23:24";

        //works

        var t1 = TimeSpan.Parse(tmp);

        Console.WriteLine(t1);

        //works

        var t2 = TimeSpan.ParseExact(tmp, "hh\\:mm\\:ss", System.Globalization.CultureInfo.InvariantCulture);

        Console.WriteLine(t2);

        //works

        var t3 = TimeSpan.Parse(tmp, System.Globalization.CultureInfo.InvariantCulture);

        Console.WriteLine(t3);

        //fails

        var t4 = TimeSpan.ParseExact(tmp, "hhmm", System.Globalization.CultureInfo.InvariantCulture);

        Console.WriteLine(t4);

        Console.WriteLine("Hello World");

    }

}