为什么可观察集合为 xaml 数据绑定提供空值 c# wpf

Question

I have a datagrid which need to represent two types of dataset at two different times of event.

1. On form loading - the datagrid show modelclasswithcombobox (This works fine)

2. on button click - the datagrid show modelforseperateusers (The problem is here it shows empty datagrid eventhough the data is populated in collection)

XAML:

<DataGrid  Height="150" x:Name="datagrid1" AutoGenerateColumns="True" CanUserAddRows="False" Width="467" Margin="299,-135,40,9" HorizontalAlignment="Left">
    <DataGrid.Style>
        <Style  TargetType="{x:Type DataGrid}">
            <Setter  Property="ItemsSource" Value="{Binding Populatedatagridwithobservablecollection.modelclasswithcombobox}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Isauditorsearch}" Value="True">
                    <Setter Property="ItemsSource" Value="{Binding 
     modelforseperateusers,
     diag:PresentationTraceSources.TraceLevel=High,Mode=TwoWay }"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </DataGrid.Style>
</DataGrid>

code:

  public interface INofificationActionfordatagrid {
ObservableCollection < datagridmodel > modelclasswithcombobox {
    get;
    set;
}
void getdatausinglinq();
}

public class PopulateDatagrid: INofificationActionfordatagrid {
public ObservableCollection < datagridmodel > modelclasswithcombobox {
    get;
    set;
}
   public void getdatausinglinq() {
    using(Operations_Productivity_ToolEntities context = new 
 Operations_Productivity_ToolEntities()) {
        var a1 = from t1 in context.Test_ImportedAuditdata
        where t1.status == "Pending"
        select t1;

        if (modelclasswithcombobox == null) modelclasswithcombobox = new 
  ObservableCollection < datagridmodel > ();

        foreach(var a in a1) {
            modelclasswithcombobox.Add(new datagridmodel {

                AuditId = a.AuditId.ToString(),
                Claimnumber = a.ClaimNumber.ToString(),
                UserName = a.username,
                CreateDate = a.Created_Date.ToString(),
                ID = a.id.ToString(),
                Status = a.status

            });
        }
        context.SaveChanges();

    }
    }
 }

public class ViewModel {
public ObservableCollection < datagridmodelforSeperateUsers > 
modelforseperateusers {
    get;
    set;
}

INofificationActionfordatagrid
_populatedatagridwithobservablecollection = new PopulateDatagrid();

public ViewModel() {
    Isauditorsearch = false;
    _populatedatagridwithobservablecollection.getdatausinglinq();
    ReassignusersSeperately = new RelayCommand(o = 
   >Filterbind(Currentcomboboxitem3));
}

public INofificationActionfordatagrid
Populatedatagridwithobservablecollection {
    get {
        return _populatedatagridwithobservablecollection;
    }
    set {
        if (value != _populatedatagridwithobservablecollection) {
            _populatedatagridwithobservablecollection = value;

            OnPropertyChanged("Populatedatagridwithobservablecollection");
        }
    }
   }

   public void Filterbind(string an) {
    Isauditorsearch = true;

    using(Operations_Productivity_ToolEntities context = new 
   Operations_Productivity_ToolEntities()) {
        var a1 = from t1 in context.Test_ImportedAuditdata
        where t1.username == an
        select t1;
        try {
            if (modelforseperateusers == null) modelforseperateusers = new 
     ObservableCollection < datagridmodelforSeperateUsers > ();

            foreach(var a in a1) {
                modelforseperateusers.Add(new 
        datagridmodelforSeperateUsers {
                    toUpdate = false,
                    AuditId = a.AuditId.ToString(),
                    Claimnumber = a.ClaimNumber.ToString(),
                    UserName = a.username,
                    CreateDate = a.Created_Date.ToString(),
                    ID = a.id.ToString(),
                    Status = a.status

                });
            }
            context.SaveChanges();
        } catch(Exception e) {
            MessageBox.Show(e.ToString());
        }
    }
}
}

Problem : The observable collection modelforseperateusers is populated with the data in the viewmodel but it is not binded with the xaml.

On debugging the xaml :

     Warning: 56 : Created BindingExpression (hash=36729282) for Binding (hash=25749409)
     System.Windows.Data Warning: 58 :   Path: 'modelforseperateusers'
     System.Windows.Data Warning: 61 : BindingExpression (hash=36729282): Default update trigger resolved to PropertyChanged
     System.Windows.Data Warning: 62 : BindingExpression (hash=36729282): Attach to System.Windows.Controls.DataGrid.ItemsSource (hash=55797203)
     System.Windows.Data Warning: 67 : BindingExpression (hash=36729282): Resolving source 
     System.Windows.Data Warning: 70 : BindingExpression (hash=36729282): Found data context element: DataGrid (hash=55797203) (OK)
     System.Windows.Data Warning: 78 : BindingExpression (hash=36729282): Activate with root item ViewModel (hash=38471091)
     System.Windows.Data Warning: 107 : BindingExpression (hash=36729282):   At level 0 using cached accessor for ViewModel.modelforseperateusers: RuntimePropertyInfo(modelforseperateusers)
     System.Windows.Data Warning: 104 : BindingExpression (hash=36729282): Replace item at level 0 with ViewModel (hash=38471091), using accessor RuntimePropertyInfo(modelforseperateusers)
     System.Windows.Data Warning: 101 : BindingExpression (hash=36729282): GetValue at level 0 from ViewModel (hash=38471091) using RuntimePropertyInfo(modelforseperateusers): <null>
     System.Windows.Data Warning: 80 : BindingExpression (hash=36729282): TransferValue - got raw value <null>
     System.Windows.Data Warning: 84 : BindingExpression (hash=36729282): TransferValue - implicit converter produced <null>
     System.Windows.Data Warning: 89 : BindingExpression (hash=36729282): TransferValue - using final value <null>

On seeing the debug information we can see that the collection is providing null value and I believe that is the reason behind the empty datagrid. Please provide inputs for the issue.

Answer 1

I had the same issue with my ObservableCollection and other binding. I found out that when the application loads you cannot bind to null object.

So I now initialize all my binding object and everything is working much better.

private ObservableCollection<RaceObject> _obsRaces = new ObservableCollection<RaceObject>();

        public ObservableCollection<RaceObject> ObsRaces
        {
            get => _obsRaces;
        }

Once the binding is done on an existing object instead of null object, you will be able to do anything you want and the binding will always work.

Besides this applies to DependencyObject kind of ViewModel. I'm not using INotifiChange.