WPF

public EquipmentIdentity()
        {
            InitializeComponent();
            EquipmentIdentityViewModel eivm = new EquipmentIdentityViewModel();
            DataContext = eivm;
            dataPager.OnDemandLoading += dataPager_OnDemandLoading;
        }

private void dataPager_OnDemandLoading(object sender, Syncfusion.UI.Xaml.Controls.DataPager.OnDemandLoadingEventArgs args)
        {
            //some codes here
        }

Quote:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

<Style TargetType="{x:Type local:MyControl}">

    <Setter Property="Template">

        <Setter.Value>

            <ControlTemplate TargetType="{x:Type local:MyControl}">

                <Border Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}">

                    <TreeView ItemsSource="{Binding TreeDataItems, RelativeSource={RelativeSource TemplatedParent}}">

                        <TreeView.Resources>
                            <HierarchicalDataTemplate DataType="{x:Type local:TreeItem}" ItemsSource="{Binding Children}">
                                <TextBlock Text="{Binding Path=Caption}" />
                            </HierarchicalDataTemplate>
                        </TreeView.Resources>

                        <i:Interaction.Triggers>
                            <i:EventTrigger EventName="TreeViewItem.Expanded">
                                <i:InvokeCommandAction Command="{Binding TreeItemExpandedCommand, RelativeSource={RelativeSource TemplatedParent}}"
                                                    CommandParameter="{Binding}">

                                </i:InvokeCommandAction>
                            </i:EventTrigger>
                        </i:Interaction.Triggers>

                    </TreeView>
                </Border>

            </ControlTemplate>

        </Setter.Value>

    </Setter>

</Style>

private ICommand _TreeItemExpandedCommand;
public ICommand TreeItemExpandedCommand
{
    get
    {
        if (_TreeItemExpandedCommand == null)
            _TreeItemExpandedCommand = new RelayCommand<object>(p => TreeItemExpandedExecuted(p));
        return _TreeItemExpandedCommand;
    }
}

private void TreeItemExpandedExecuted(object args)
{
}

<TreeView TreeViewItem.Expanded="TreeViewItem_Expanded">
    ...
</TreeView/>

and

 private void TreeViewItem_Expanded(object sender, RoutedEventArgs e)
    {
        TreeViewItem tvi = e.OriginalSource as TreeViewItem;
        if (tvi != null)
        {
          // Do something
        }
    }

<Style x:Key="dataGridCurrencyCellStyle" 
        BasedOn="{StaticResource dataGridCellStyle}"
        TargetType="{x:Type DataGridCell}">

    <Setter Property="Width" Value="75"/>
    <Setter Property="TextBlock.TextAlignment" Value="Left"/>
    <Setter Property="TextBlock.Text" Value="{Binding StringFormat=C}"/>
    <Setter Property="IsEnabled" Value="{Binding DataContext.AreFieldsEnabled, ElementName=budgetControl}"/>

</Style>

<Style x:Key="dataGridCurrencyCellStyle" 
        BasedOn="{StaticResource dataGridCellStyle}"
        TargetType="{x:Type DataGridCell}">

    <Setter Property="Width" Value="75"/>
    <Setter Property="TextBlock.TextAlignment" Value="Left"/>
    <Setter Property="TextBlock.Text" Value="{Binding StringFormat=C}"/>
    <Setter Property="IsEnabled" Value="False"/>

    <Style.Triggers>
        <MultiTrigger>
            <MultiTrigger.Conditions>
                <Condition Property="AreFieldsEnabled" Value="True" />
                <Condition Property="IsCellEnabled " Value="True" />
            </MultiTrigger.Conditions>
            <MultiTrigger.Setters>
                <Setter Property="IsEditing" Value="True" />
            </MultiTrigger.Setters>
        </MultiTrigger>
    </Style.Triggers>

</Style>

foreach (string item in columnNames)
{
    // Create binding
    Binding binding = new Binding(item);
    string styleName = "";

    // Get binding mode and style name
    if (item == "Type")
    {
        binding.Mode = BindingMode.OneTime;
        styleName = "budgetGridCellStyle";
    }
    else
    {
        binding.Mode = BindingMode.TwoWay;
        binding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
        styleName = "dataGridCurrencyCellStyle";
    }

    // Get the style
    var style = Application.Current.FindResource(styleName) as Style;

    // Bind the column
    var col = new DataGridTextColumn()
    {
        Header = item,
        Binding = binding,
        Visibility = Visibility.Visible,
        CellStyle = style
    };

    DataGridColumns.Add(col);
}

<Style x:Key="dataGridCurrencyCellStyle" 
        BasedOn="{StaticResource dataGridCellStyle}"
        TargetType="{x:Type DataGridCell}">

    <Setter Property="Width" Value="75"/>
    <Setter Property="TextBlock.TextAlignment" Value="Right"/>
    <Setter Property="TextBlock.Text" Value="{Binding StringFormat={}{0:0.##}}"/>
    <Setter Property="IsEnabled" Value="{Binding DataContext.AreFieldsEnabled, ElementName=budgetControl}"/>

</Style>

public interface IViewModelLocator
{
    object Get<T>();

    void Register(Type viewType, object viewModel);
}

public class ViewModelLocator : IViewModelLocator
{
    // Internal list of View Models
    private IDictionary<Type, object> _storage;

    public ViewModelLocator()
    {
        _storage = new Dictionary<Type, object>();
    }

    public object Get<T>()
    {
        if (_storage.ContainsKey(typeof(T)))
        {
            return _storage.FirstOrDefault(x => x.Key == typeof(T)).Value;
        }
        else
        {
            throw new Exception($"The requested view '{typeof(T)}' is not registered");
        }
    }

    public void Register(Type viewType, object viewModel)
    {
        if (!_storage.ContainsKey(viewType))
        {
            _storage.Add(viewType, viewModel);
        }
        else
        {
            throw new Exception($"An instance of the view model '{viewModel}' is already registered for the view '{viewType}'");
        }
    }
}

public partial class App : Application
{
    public static IServiceLocator ServiceLocator { get; private set; }
    public static IViewModelLocator ViewModelLocator { get; private set; }

    public App()
    {
        ServiceLocator = new ServiceLocator();
        ServiceLocator.Register(typeof(IMyService), new MyService());

        ViewModelLocator = new ViewModelLocator();
        ViewModelLocator.Register(typeof(MainWindowView), new MainWindowViewModel(ServiceLocator.Get<IMyService>()));
        ViewModelLocator.Register(typeof(LoginView), new LoginViewModel());
    }
}

public partial class MainWindowView : Window
{
    public MainWindowView()
    {
        InitializeComponent();

        this.DataContext = App.ViewModelLocator.Get<MainWindowView>();
    }
}

x:Class="My.MainView"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   DataContext="{Binding Source={StaticResource MyViewModelLocator}, Path=Main}">

1. It required a resources entry in App.XAML:

   <Application.Resources>
   <core:ViewModelLocator x:Key="MyViewModelLocator" />
   </Application.Resources>

2. It requires a property on the VM Locator called "Main". Since these classes are going to be in my framework, there won't be a property for each ViewModel that other apps use on the VM Locator.

DataContext="{Binding Path="{x:static TestApp:App.ViewModelLocator.Get<???????>()}"