WPF listbox sreach with data template

Question

Hello ,

   I am trying to filer the data in the wpf  listbox based on the input given .

 Currently I am able to do if I have only one field in the template , what if I have multiple fields.

I am writing a textchanged in the TextBox function

    private void ListSearch_TextChanged(object sender, TextChangedEventArgs e)
        {
            string txtOrig = txtNameToSearch.Text;
            string upper = txtOrig.ToUpper();
            string lower = txtOrig.ToLower();


            var filteration= from test in lstEmployee
                              let ename = test .firstname
                              where ename.StartsWith(upper) || ename.StartsWith(lower) || ename.Contains(txtOrig)
                              select test ;
         
            lstEmpData.ItemsSource = filteration;
        }

My Xaml file

<ListBox>

  <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding firstname}"></TextBlock>                                          
                    </StackPanel>                                     
                </DataTemplate>               
            </ListBox.ItemTemplate>

<ListBox>

 <TextBox Height="27" HorizontalAlignment="Left"
Name="ListSearch" TextChanged="ListSearch_TextChanged" />

SO I type in some name and the listbox shows me the filtered data with the name visible , [this is only for  one text block]

  Now the above one works fine .

============================

 If I want to add a new textblock

<TextBlock Text="{Binding Lastname}"></TextBlock>

How do I process this second content Lastname? So  now the list box contains a template containing firstname and Lastname,

IF I type the name the reulsts are fine but number , it doesnt show up. Even If I type the number , I need to get the list item  to be shown [which inturn contains firstname and Lastname].. I am assuming something needs to be done in the Linq query.[if  my assumption is correct]

 and when I just change the query to Last name  without adding extra textblock , I get a null referece exception unhandeled error  (Object reference not set to an instance of an object.)

Could you please provide me some light how do I go about this?

Thank You