Create a web service that can answer to WhatsApp messages

For Java, you can try WhatsUp

For Python, see YowsUp.

Beware that WhatsApp threatens legal action against many of these library developers and does not officially support using the service this way.

I have also spoken directly with WhatsApp representatives who have said no commercial API use of WhatsApp is acceptable.

Also note that bulk messaging is against the WhatsApp terms of service.

There used to be a PHP implementation at WhatsAPI, and another Java implementation, WhatsApi4J. Both are no longer available due to legal threats.

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();