WPF：无法将值从目标保存回源

Question

Thats the full error message I get:

System.Windows.Data Error: 8 : Cannot save value from target back to source. BindingExpression:Path=SelectedPupil; DataItem='AdministrationViewModel' (HashCode=52357250); target element is 'DataGrid' (Name=''); target property is 'SelectedItem' (type 'Object') NullReferenceException:'System.NullReferenceException

This error always occurs when I click/select from my selected pupil in the pupilListDataGrid into the ListBox with the schoolclasses.

This started to occur when I changed my Repository loading technique from Eager loading to Lazy loading.

My SelectedSchoolclass changes then I load the according pupils My SelectedPupil changes then I load the according documents

PupilListView.xaml:

<DataGrid  
                Grid.Row="1"                   
                IsReadOnly="True"
                HeadersVisibility="Column"
                SelectedItem="{Binding SelectedPupil}"                
                ItemsSource="{Binding Path=SelectedSchoolclass.PupilListViewModel}"
                AutoGenerateColumns="False"
                >
                <DataGrid.Columns>
                    <DataGridTextColumn Binding="{Binding FirstName}" Width="*" Header="Firstname" />
                    <DataGridTextColumn Binding="{Binding LastName}" Width="*" Header="Last name" />
                </DataGrid.Columns>
        </DataGrid

Here the ItemSource is set to the all the pupils from the SELECTED schoolclass. I do not need IsSynchronizedItem stuff because I do not really have aggregated data due to the new lazy loading entities just via id of the parent entity.

AdminViewMOdel:

public PupilViewModel SelectedPupil
        {
            get { return _selectedPupil; }
            set
            {
                _selectedPupil = value;
                this.RaisePropertyChanged("SelectedPupil");
                GetDocumentsForPupil();
            }
        }

private void GetDocumentsForPupil()
        {
            var documentsOC = new ObservableCollection<Document>(_docRepo.GetDocumentsByPupilId(_selectedPupil.Id));
            SelectedPupil.Documents.DocumentList = documentsOC;
        }

I guess the problem is the following:

When I jump from a selected Document or Pupil and select a schoolclass without any pupils it binds to NULL because my ObservableCollection is lazily created means only when I get data from database else the PupilListViewModel_Collection is NULL.

Well I would like to stick with Lazy loading and do not need binding hierarchy like DataGrid_ItemsSource=SchoolclassList/PupilList what I used for Eager loading.

How could I get rid of that exception?

Answer 1

It seems like you should be able to return an empty list of the appropriate type instead of a NULL. But, your post doesn't contain enough information for us to know that for sure.