//use case 1 : https://github.com/aviferdman/Workshop-on-Software-Engineering-Project/issues/11
//using concurrent dictionary try add if usename already exist
//than fail and return error message otherwise return success
public async Task <string> SignUp(string username, string password, string phone)
{
if (username == null)
{
return("username cannot be null");
}
if (await usersDAL.AddDataUser(new DataUser(username, EncryptString(key, password), phone)))
{
await usersDAL.AddNewMemberState(username);
await usersDAL.AddNewShoppingCart(username);
return("success");
}
return("username: "******" is already taken please choose a different one");
}