Listview SelectedItem

This is because you have used Element binding but did not give path of property

<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <ListView x:Name="MyListView" Grid.Column="0" ItemsSource="{Binding WebServiceDataCollection}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding UserName}"/>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
        <StackPanel Grid.Column="1">
            <TextBox Text="{Binding ElementName=MyListView, Path=SelectedItem.UserName  }"/>
        </StackPanel>
    </Grid>

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-- Vishal Kaushik --

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