Question

I’m using WPF to create an application to enable an organisation to enter different pieces of data into the application.I have a tab control to allow them to do this.

Then in a separate view, I have a series of different data grids showing the user what data they have inserted into the database. Containing buttons to either, add, update or delete the data they want.

Which leads me to my question. Currently, I am able to delete, and add data with ease and with no problem. But then comes my issue with trying to get the selected item to update, which it doesn’t, resulting in a null reference exception.

If i set my property attributes programmatically though, it updates it fine. like so;public int _OrganisationTypeDetailID = 17; public int _OrganisationTypeID = 1;But I do not want this, as I want the ability for the user to select for themselves and update the data they need to.

Here’s some of the code that may help in resolving my issue;

View Model;

    public void UpdateOrganisationTypeDetail(OrganisationTypeDetail orgTypeDetail)
    {
        using (DBEntities context = new DBEntities())
        {
            var orgTD = context.OrganisationTypeDetails.Where(otd => otd.OrganisationTypeDetailID == SelectedType.OrganisationTypeDetailID).FirstOrDefault();

            if (orgTD != null)
            {
                orgTD.Title = Title;
                orgTD.FirstName = FirstName;
                orgTD.Surname = Surname;
                orgTD.Position = Position;
                orgTD.DateOfBirth = DateOfBirth;
                orgTD.Address = Address;
                orgTD.Country = Country;
                orgTD.Postcode = Postcode;
                orgTD.PhoneNumber = PhoneNumber;
                orgTD.MobileNumber = MobileNumber;
                orgTD.FaxNumber = FaxNumber;
                orgTD.Email = Email;
                orgTD.NINumber = NINumber;

                //context.OrganisationTypeDetails.Attach(orgTD);
                context.OrganisationTypeDetails.ApplyCurrentValues(orgTD);
                context.SaveChanges();

                MessageBox.Show("Updated Organisation Type Details");
            }
            else
            {
                MessageBox.Show("Unable to update selected 'Type'.");
            }
        }

    private OrganisationTypeDetail _SelectedType;
    public OrganisationTypeDetail SelectedType
    {
        get
        {
            return _SelectedType;
        }
        set
        {
            if (_SelectedType == value)
                return;

            _SelectedType = value;
            OnPropertyChanged("SelectedType");
        }
    }

    public List<OrganisationTypeDetail> GetOrganisationTypeDetail //Loads data 
    {
        get
        {
            using (DBEntities context = new DBEntities())
            {
                var query = from e in context.OrganisationTypeDetails
                            select e;
                return query.ToList<OrganisationTypeDetail>();
            }
        }
    }

    private ICommand showUpdateCommand;
    public ICommand ShowUpdateCommand //Update command
    {
        get
        {
            if (showUpdateCommand == null)
            {
                showUpdateCommand = new RelayCommand(this.UpdateFormExecute, this.UpdateFormCanExecute); //i => this.UpdateOrganisationTypeDetail()
            }
            return showUpdateCommand;
        }
    }

Code behind;

    private void btnUpdateOrgTypeDetail_Click(object sender, RoutedEventArgs e)
    {
        OrganisationTypeDetail selected = dgOrgTypeDetail.SelectedItem as OrganisationTypeDetail;
        OrganisationTypeDetailViewModel org = new OrganisationTypeDetailViewModel();

        if (selected == null)
            MessageBox.Show("You must select a 'Type' before updating.");
        else
        {
            OrganisationTypeDetailUpdateView update = new OrganisationTypeDetailUpdateView();

            update.ShowDialog();
            org.UpdateOrganisationTypeDetail(selected);
            Page_Loaded(null, null);
        }
    }

xaml;

            <DataGrid Name="dgOrgTypeDetail" Height="145" Width="555" 
            IsSynchronizedWithCurrentItem="True"
            ItemsSource="{Binding GetOrganisationTypeDetail}"
            SelectedItem="{Binding SelectedType, Mode=TwoWay}">

Hope this issue can be resolved.

Answer 1

I would say that your best bet for this is to use commanding in the MVVM pattern to achieve this..
It looks like you’re using a combination of MVVM and code behind and actually creating a new instance of the view model when your click event fires. Try binding the view model to your view once in the code behind of the view as the datacontext and then try updating the selected type..

Also when you’re trying to do the update on SelectedType – look at your View using Snoop – see if the SelectedType property is still bound to the view.

ICommand UpdateOrgTypeDetail { get;} 

Then in the view model constructor declare new instance

UpdateOrgTypeDetail = new DelegateCommand<object>(ExecuteUpdateOrgTypeDetail,    CanExecuteUpdateOrgTypeDetail);

These two delegates will then allow you to click your button (which needs to bind to UpdateOrgTypeDetail)

<Button Command="{Binding UpdateOrgTypeDetail}" />

You should find that the update on the property is done correctly from here.