using System;

using System.Linq;  

​

public class Program

{

    public static void Main()

    {

        var hex = "49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d";

        var base64 = Convert.ToBase64String(hex.HexToBytes());

        Console.WriteLine(base64);

    }

}

public static class CryptoExtensions

{

    public static byte[] HexToBytes(this String hex)

    {

        return Enumerable.Range(0, hex.Length).Where(x => x % 2 == 0).Select(x => Convert.ToByte(hex.Substring(x, 2), 16)).ToArray();

    }

} 

​