I have a list view where I bind multiple data templates to using a DataTemplateSelector. However, I cannot build my project, I'm getting an error of "Object reference not set to an instance of an object". I narrowed down the issue to the data binding ItemsSource of the combobox. If I comment out the combobox, my project will build. As I understand, the Binding keyword evaluates at runtime, but this is a compile time error. I tried to get around the error by swapping out to x:Bind, as that evaluates at compile-time but then that required the type of the DataTemplate to be defined.
x:DataType={x:Type templates:FilterDropdownDataTemplate}
However, x:Type isn't defined in a Windows 10 universal app. Any suggestions?
Here is my XAML:
<Page.Resources>
<DataTemplate x:Name="DateComboboxTemplate">
<TextBlock Text="{Binding}"/>
</DataTemplate>
<DataTemplate x:Name="FilterDropdownDataTemplate">
<StackPanel>
<TextBlock Text="{Binding Value}"/>
<ComboBox ItemsSource="{Binding DateRanges}"
SelectionChanged="{Binding DateSelection}"
SelectedItem="{Binding SelectedDateRange, Mode=TwoWay}"
ItemTemplate="{StaticResource DateComboboxTemplate}">
</ComboBox>
</StackPanel>
</DataTemplate>
<config:MainFilterTemplateSelector
x:Name="MainFilterSelector"
FilterDropdownTemplate="{StaticResource FilterDropdownDataTemplate}"
/>
</Page.Resources>
<ListView x:Name="FilterCategories"
Margin="0"
Padding="0"
BorderThickness="0"
ItemsSource="{Binding Filters}"
SelectedItem="{Binding SelectedFilterCategory, Mode=TwoWay}"
IsItemClickEnabled="True"
SelectionChanged="{x:Bind ViewModel.OpenSubFilters}"
ItemTemplateSelector="{StaticResource MainFilterSelector}">
</ListView>
And here is the relevant C# code (I'm using a MVVM pattern):
public class MainFilterViewDropdownListTemplate : Filter
{
private ObservableCollection<string> _dateRanges;
private string _selectedDateRange;
public MainFilterViewDropdownListTemplate() : base()
{
_dateRanges = new ObservableCollection<string>();
}
public ObservableCollection<string> DateRanges
{
get
{
return _dateRanges;
}
set
{
SetProperty(ref _dateRanges, value);
// hard coding these 4 lines for testing for now
_dateRanges.Add("Last 7 days");
_dateRanges.Add("Last 14 days");
_dateRanges.Add("Last 30 days");
_dateRanges.Add("Custom");
}
}
public string SelectedDateRange
{
get
{
return _selectedDateRange;
}
set
{
SetProperty(ref _selectedDateRange, value);
}
}
public IEnumerable<Filter> AllFilters { get; set; }
public void InitializeFields(Filter filter)
{
Column = filter.Column;
DisplayType = filter.DisplayType;
ID = filter.ID;
IsSelected = filter.IsSelected;
ParentId = filter.ParentId;
SelectedCount = filter.SelectedCount;
TotalCount = filter.TotalCount;
Value = filter.Value;
visibility = filter.visibility;
}
private void DateSelection()
{
}
}
And
public class MainFilterTemplateSelector : DataTemplateSelector
{
public DataTemplate FilterDropdownTemplate { get; set; }
protected override DataTemplate SelectTemplateCore(object selector)
{
if (selector == null)
{
throw new ArgumentNullException("Template is null");
}
if (!(selector is FilterBase))
{
throw new ArgumentException("This list can only be populated by a class that extends Filter.");
}
else
{
var filterType = selector as Filter;
switch (filterType.DisplayType)
{
case "date":
return FilterDropdownTemplate;
default:
return base.SelectTemplateCore(selector);
}
}
}
}
And
public class Filter : FilterBase
{
private int _selectedCount;
private int _totalCount;
private Visibility _visibility;
public IEnumerable<Filter> Children
{
get;
set;
}
public int TotalCount
{
get
{
return _totalCount;
}
set
{
SetProperty(ref _totalCount, value);
}
}
public int SelectedCount
{
get
{
return _selectedCount;
}
set
{
SetProperty(ref _selectedCount, value);
}
}
public Visibility visibility
{
get
{
return _visibility;
}
set
{
SetProperty(ref _visibility, value);
}
}
public void InitializeAllChildren(IEnumerable<Filter> allFilters)
{
foreach (Filter item in allFilters)
{
item.Children = allFilters.GetAllChildrenFiltersForParentID(item.ID) as IEnumerable<Filter>;
if (item.Children == null)
{
item.TotalCount = 0;
}
else
{
item.TotalCount = item.Children.Count();
item.SelectedCount = item.Children.Where(t => t.IsSelected.Value).Count();
}
}
Children = allFilters.GetAllChildrenFiltersForParentID(ID);
}
}
And
public class FilterBase : ViewModelBase
{
public int ID
{
get
{
return _id;
}
set
{
SetProperty(ref _id, value);
}
}
public string Value
{
get
{
return _value;
}
set
{
SetProperty(ref _value, value);
}
}
public string Column
{
get
{
return _column;
}
set
{
SetProperty(ref _column, value);
}
}
public int ParentId
{
get
{
return _parentId;
}
set
{
SetProperty(ref _parentId, value);
}
}
public string DisplayType
{
get
{
return _displayType;
}
set
{
SetProperty(ref _displayType, value);
}
}
public bool? IsSelected
{
get
{
return _isSelected;
}
set
{
SetProperty(ref _isSelected, value);
}
}
private int _id;
private string _value;
private string _column;
private int _parentId;
private string _displayType;
private bool? _isSelected;
}
And (BindableBase is from the Prism.Mvvm namespace)
public class ViewModelBase : BindableBase, IDisposable
{
PropertyChangeActionHelper _propertyChangeActionHelper;
protected void RegisterPropertyChangeAction(Action action, params string[] monitoredProperties)
{
if (_propertyChangeActionHelper == null)
{
_propertyChangeActionHelper = new PropertyChangeActionHelper(this);
}
_propertyChangeActionHelper.RegisterPropertyChangeAction(action, monitoredProperties);
}
#region IDisposable Support
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
if (_propertyChangeActionHelper != null)
{
_propertyChangeActionHelper.Dispose();
_propertyChangeActionHelper = null;
}
}
}
~ViewModelBase()
{
// Do not change this code. Put cleanup code in Dispose(bool disposing) above.
Dispose(false);
}
// This code added to correctly implement the disposable pattern.
public void Dispose()
{
// Do not change this code. Put cleanup code in Dispose(bool disposing) above.
Dispose(true);
GC.SuppressFinalize(this);
}
#endregion
}
x:Bindandx:DataType: Assuming that thetemplatesnamespace is declared andFilterDropdownDataTemplateis a type within that namespace, you should be able to define theDataTemplate's data type without usingx:Type:<DataTemplate x:DataType="templates:FilterDropdownDataTemplate">- andreask