How to get selected item text through selectionchanged event in WPF combobox?

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C#

WPF

combobox

I want to realize a general and simple function: in a combox, get the selected item text through user selection in a WPF ComboBox, but I could not find a way to do it even after search internet. Unbelievable. Hope anyone can help me.

the XAML and c# code are as follows. I have given the comments and my question as well the code. Note that I have tried the similar functions in both MainWindow() and Selectionchanged event. The problem occurs only in the latter function. But I need get the selected item, e.g. "item1" when I select the combox item to be the first item. But it gives an error (See below) when starting the debugging.

strangly, the XAML code can not be fully copied to here. however, you should assume the xaml code has no any problem. it is not complete only due to the editor here.

<ComboBox x:Name="ComBoxTest" Height="50" SelectionChanged="ComBoxTest_SelectionChanged" SelectedValuePath="Content">

        <ComboBoxItem IsSelected="True">item1</ComboBoxItem>

        <ComboBoxItem>item2</ComboBoxItem>

</ComboBox>

namespace WPFComboBoxTest
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        
 MessageBox.Show(ComBoxTest.SelectedItem.ToString());//System.Windows.Controls.ComboxItem:item1
 MessageBox.Show(ComBoxTest.SelectedValue.ToString());//item1 (OK)
string text = ((ComboBoxItem)ComBoxTest.SelectedItem).Content.ToString();//item1 (Ok, too)
            MessageBox.Show(text);
           
        }

        private void ComBoxTest_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            MessageBox.Show(ComBoxTest.SelectedItem.ToString());//System.Windows.Controls.ComboxItem, only, after starting the programm, no item1, which is selected by default i XAML code. But after starting programm, it shows System.Windows.Controls.ComboxItem:item1 when I select the first item
            MessageBox.Show(ComBoxTest.SelectedValue.ToString());//problem, when start debugging:  an unhandled exception of teyp 'System.Refection.TragetInvocationException' occured. I think this will work if I really hit the ComboBox and select one item. But I have no chance since this error occurs once starting the programm.
            string text = ((ComboBoxItem)ComBoxTest.SelectedItem).Content.ToString();//same problem
            MessageBox.Show(text);
            //my question: how to deal with this problem?
        }
    }
}

What I have tried:

tried different ways, selecteditem, selectedvalue, items, none works.
search also internet, but it is not successful.