RelayCommand implementation : Object reference not set to an instance of an object

Question

I have a WPF Project and I am using a RelayCommand for button click event. Here is the constractor of my MainViewModel

    private readonly DataService _dataService;
      public MainWindowModel(DataService dataService)
    {
        _dataService = dataService;
    }

// RelayCommandClass

    public class RelayCommand : ICommand
{
    #region Fields

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

    #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;
    }

    //public RelayCommand(Action<object> action)
    //{

    //    this.action = action;
    //}
    #endregion // Constructors

    #region ICommand Members

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

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

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

    #endregion // ICommand Members
}

//Region for Command Implementation

    private RelayCommand _validateResponse;

    public RelayCommand ValidateResponse
    {
        get
        {
            return _validateResponse ?? (_validateResponse = new RelayCommand(
                                                                 parm => _dataService.Validate("string1","string2"))
                     );


        }

    }

But When I run the project I keep getting a null reference exception. Did I miss something? Thanks

at which line you are getting null reference exception.And this exception comes when your object is not initialized so put break point check which object is null.hope it help :) — loop, May 16 '14 at 15:53
When does then exception is being thrown? on command execute? on app start? add it's stack trace — Jossef Harush Kadouri, May 16 '14 at 16:00
@loop the exception is at _dataService.Validate("string1","string2")) The _dataService shows null but Inject the DataService Class to the MainWindowModel() contractor. — HXD, May 16 '14 at 16:01
@HXD check when you creating the instance of 'MainWindowModel'. you might be passing a null value to 'dataService' — Jossef Harush Kadouri, May 16 '14 at 16:05
The first thing you're missing is the exception. And I don't mean a picture of the exception dialog. On the dialog you'll see a link for copying details to the clipboard. Click that, then [edit] and add them. — , May 16 '14 at 17:11

score 1 · Accepted Answer · answered May 16 '14 at 18:44

Initializing the _dataService object at the constructor(constructor two) used by xaml solved the problem

contractor one:

  public MainWindowModel(DataService dataService)
    {
        _dataService = dataService;
    }

contractor two:

 public MainWindowModel()
    {
         _dataService = new DataService()
    }

RelayCommand implementation : Object reference not set to an instance of an object

1 Answers1