<Button Content="login" Height="23" HorizontalAlignment="Left" Margin="271,94,0,0" Name="btnLogin" VerticalAlignment="Top" Width="75"
                Command="{Binding LoginCommand}"
                />

public ICommand LoginCommand
        {
            get
            {
                if(_loginCommand!=null)
                {
                    _loginCommand = new RelayCommand(p => DoLogin(),
                                                        p => CanDoLogin());
                }
                return _loginCommand;
            }
        }

      
        public void DoLogin()
        {
            if(!CanDoLogin()) return;
            
        }

       
        public bool CanDoLogin()
        {
            return true;
            return !string.IsNullOrEmpty(_login)
                   && !string.IsNullOrEmpty(_password)
                   && !string.IsNullOrEmpty(_companyName);
        }

public class RelayCommand : ICommand
{
    #region Fields

    readonly Action<object> _execute;
    readonly Predicate<object> _canExecute;        

    #endregion // Fields

    #region Constructors

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

    public RelayCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;           
    }
    #endregion // Constructors

    #region ICommand Members

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }

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

    #endregion // ICommand Members
}

if(_loginCommand!=null)
                {
                    _loginCommand = new RelayCommand(p => DoLogin(),
                                                        p => CanDoLogin());
                }