For .NET you use https://github.com/mgp25/Chat-API-NET

download installer for generate password https://github.com/mgp25/WART from this link

string nickname = "Nickname";
/* Your number in the format CCAANNNNNNNNN
    *   C - Country Code
    *   A - Area Code
    *   N - Phone number */
string sender = "***************"; //phone number
string password = "*****************"; // Obtain it with WART or Yowsup
WhatsApp wa = new WhatsApp(sender, password, nickname, true);
wa.OnConnectSuccess += () =>
{
    Console.WriteLine("Connected");
    wa.OnLoginSuccess += (phoneNumber, data) =>
    {
        Console.WriteLine("Connection success!");
        wa.SendMessage("**************"/* Number */, "Hello World!");
        Console.WriteLine("Message sent!");
    };
    wa.OnLoginFailed += (data) => {
        Console.WriteLine("Login failed: {0}", data);
    };
    wa.Login();
};
wa.OnConnectFailed += (ex) =>
{
    Console.WriteLine("Connect failed: {0}", ex.StackTrace);
};
wa.Connect();

Console.WriteLine("END");
Console.ReadLine();