public class DelegateCommand : ICommand
{
    private readonly Predicate<object> _canExecute;
    private readonly Action<object> _execute;

    public event EventHandler CanExecuteChanged;

    public DelegateCommand(Action<object> execute)
        : this(execute, null)
    {
    }

    public DelegateCommand(Action<object> execute, Predicate<object> canExecute)
    {
        _execute = execute;
        _canExecute = canExecute;
    }

    public bool CanExecute(object parameter)
    {
        return _canExecute == null || _canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

    public void RaiseCanExecuteChanged()
    {
        CanExecuteChanged?.Invoke(this, EventArgs.Empty);
    }
}

#region RegisterCommand

private DelegateCommand _registerCommand;
public ICommand RegisterCommand
{
    get
    {
        _registerCommand = new DelegateCommand(param => Register());
        return _registerCommand;
    }
}

private bool CanRegister()
{
    return true;
}

private void Register()
{
    var newUser = new User
    {
        FirstName = _firstname,
        LastName = _lastname,
        Username = _username,
        Password = "", // TODO: Hashing and storing of passwords
    };
    using (var context = new WorkstreamContext())
    {
        var users = context.Set<User>();
        users.Add(newUser);
        context.SaveChanges();
    }
}

#endregion