From Decimal to Binary:
        int n = int.Parse(Console.ReadLine());
        var convert = Convert.ToString(n, 2);
        Console.WriteLine(convert);

From Binary to Decimal:
        string bin = Console.ReadLine();
        int result = Convert.ToInt32 (bin, 2 );
        Console.WriteLine(result);

From Decimal To Hexadecimal:
        int n = int.Parse(Console.ReadLine());
        var convert = Convert.ToString(n, 16);
        Console.WriteLine(convert);
2nd Version with placeholder:
       int n = int.Parse(Console.ReadLine());
        Console.WriteLine("{0:X}" , n);

From Hexadecimal to Decimal:
        string bin = Console.ReadLine();
        int result = Convert.ToInt32(bin, 16);
        Console.WriteLine(result);