WPF MVVM command canexecute enable/disable button

implement this for canexecute:

public bool CanExecute(object parameter)
        {if(thistext available)
            return true; 
         else
            return false;  
        }

Since, CanExecuteChanged is raised when the CanExecute method of an ICommand gets changed. it gets invoked when some command that could change canexecute. and can execute changed should be changed to this:

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

EDIT

in your view model constructor:

m_ButtonCommand= new RelayCommand(Submit, CanSubmit);

now method for this submit:
 private bool CanSubmit(object obj)
        {
            if(thistext available)
                return true; 
             else
                return false;  

        }
 public void Submit(object _)
  {//... code}

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

do it like this.

You need to modify your RelayCommand class like this

  class RelayCommand : ICommand
{
    private Action<object> _action;
    private Func<bool> _func;  

    public RelayCommand(Action<object> action,Func<bool> func)
    {
        _action = action;
        _func = func;
    }

    public void RaiseCanExecuteChanged()
    {
        if(CanExecuteChanged!=null)
            CanExecuteChanged(this,new EventArgs());
    }

    #region ICommand Members

    public bool CanExecute(object parameter)
    {
        if (_func != null)
           return _func();
        return true;
    }



    public event EventHandler CanExecuteChanged;

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

    #endregion
}

Initialize ButtonCommand as

ButtonCommand = new RelayCommand((s) => ShowMessage(s),()=>!string.IsNullOrEmpty(TextKomentar));

RaiseCanExcuteChanged from the setter of Text property

        public string TextKomentar
    {
        get
        {
            return this.textKomentar;
        }
        set
        {
            // Implement with property changed handling for INotifyPropertyChanged
            if (!string.Equals(this.textKomentar, value))
            {
                textKomentar = value;
                OnPropertyChanged("TextKomentar");
            }
            ButtonCommand.RaiseCanExecuteChanged();
        }
    }