c# WPF Cant get Parent Window

Question

I have a wpf page hosted in a Window. But i get Null exception when i tries to get use this. It works then i use this code in another method but not in alla methods why is it that way? please advice.

 NewPage page = new NewPage ();
 Window w = Window.GetWindow(this.Parent);
 w.Content = page;

Edit:

heres the full code:

    public HandOverListPage() {
        InitializeComponent();

        _settings = new Settings();
    }


    public void ShowCurrentInUseAssignment() {

        _currentDoc = (App.Current as App).SelectedHandOverDoc;

        var r = from item in (App.Current as App).SelectedHandOverDoc.Items
                where item.Status != 20
                select item;

        if(r.Count() == 0) {
            //Report assignment to QP with status finished
            ReportAssignment();

            HandOverPage page = new HandOverPage();

            Window w = Window.GetWindow(this.Parent);
            w.Content = page;

            return;
        } else {
            ICollectionView view = CollectionViewSource.GetDefaultView((App.Current as App).SelectedHandOverDoc.Items);
            view.SortDescriptions.Add(new SortDescription("Status", ListSortDirection.Ascending));

            ListBoxAssignmentItems.ItemsSource = view;
        }

        TxtBlockCounter.Text = r.Count().ToString();
    }

The error :

{"Value cannot be null.\r\nParameter name: dependencyObject"}

I get this when using immediate window

    ?this.GetType()
{Name = "HandOverListPage" FullName = "QP_Truck.Pages.HandOverListPage"}
    [System.RuntimeType]: {Name = "HandOverListPage" FullName = "QP_Truck.Pages.HandOverListPage"}
    base {System.Reflection.MemberInfo}: {Name = "HandOverListPage" FullName = "QP_Truck.Pages.HandOverListPage"}
    Assembly: {QP Truck, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null}
    AssemblyQualifiedName: "QP_Truck.Pages.HandOverListPage, QP Truck, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
    Attributes: Public | BeforeFieldInit
    BaseType: {Name = "Page" FullName = "System.Windows.Controls.Page"}
    ContainsGenericParameters: false
    DeclaringMethod: 'this.GetType().DeclaringMethod' threw an exception of type 'System.InvalidOperationException'
    DeclaringType: null
    FullName: "QP_Truck.Pages.HandOverListPage"
    GenericParameterAttributes: 'this.GetType().GenericParameterAttributes' threw an exception of type 'System.InvalidOperationException'
    GenericParameterPosition: 'this.GetType().GenericParameterPosition' threw an exception of type 'System.InvalidOperationException'
    GUID: {93eb30b9-a64e-3c6b-9182-0f93582d188d}
    HasElementType: false
    IsAbstract: false
    IsAnsiClass: true
    IsArray: false
    IsAutoClass: false
    IsAutoLayout: true
    IsByRef: false
    IsClass: true
    IsCOMObject: false
    IsContextful: false
    IsEnum: false
    IsExplicitLayout: false
    IsGenericParameter: false
    IsGenericType: false
    IsGenericTypeDefinition: false
    IsImport: false
    IsInterface: false
    IsLayoutSequential: false
    IsMarshalByRef: false
    IsNested: false
    IsNestedAssembly: false
    IsNestedFamANDAssem: false
    IsNestedFamily: false
    IsNestedFamORAssem: false
    IsNestedPrivate: false
    IsNestedPublic: false
    IsNotPublic: false
    IsPointer: false
    IsPrimitive: false
    IsPublic: true
    IsSealed: false
    IsSerializable: false
    IsSpecialName: false
    IsUnicodeClass: false
    IsValueType: false
    IsVisible: true
    MemberType: TypeInfo
    Module: {QP Truck.exe}
    Namespace: "QP_Truck.Pages"
    ReflectedType: null
    StructLayoutAttribute: {System.Runtime.InteropServices.StructLayoutAttribute}
    TypeHandle: {System.RuntimeTypeHandle}
    TypeInitializer: null
    UnderlyingSystemType: {Name = "HandOverListPage" FullName = "QP_Truck.Pages.HandOverListPage"}

Answer 1

Is the code that you posted in your constructor method?

The parent of a UserControl is always null in its constructor, so this.Parent is returning a null reference. Thus, calling Window.GetWindow(this.Parent) raises an ArgumentNullException because the dependency object that you specified has not been created yet.

To fix this, you need to place the code in the Initialized event handler. When this event is raised, you can be sure that the UserControl has been created.

Answer 2

Try Owner property, you have to assign it.

Sample:

public Activity ShowLookUp(Window owner)
{
     ActivityLookUp lookup = new ActivityLookUp();
     lookup.Owner = owner;
     lookup.ShowDialog();
}