Why combobox not select any value?

Here is the code of XAML:<br/>

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ComboBox x:Name="CB" SelectedValue="{Binding Model,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Value" VerticalAlignment="Center">
            
        </ComboBox>
    </Grid>
</Window>

And here is the code of code-behind:

public partial class MainWindow : Window
    {
        public List<TestModel> Models { get; set; } = new List<TestModel>();
        TestModel _Model = new TestModel() { Key = "Joe", Value = "456" };        

        public TestModel Model
        {
            get => _Model; set
            {
                if (_Model != value)
                {
                    _Model = value;                    
                }
            }
        }
       
        public MainWindow()
        {
            InitializeComponent();
            Models.Add(new TestModel() { Key = "John", Value = "123" });
            Models.Add(new TestModel() { Key = "Joe", Value = "456" });
            Models.Add(new TestModel() { Key = "Kay", Value = "547" });
            Models.Add(new TestModel() { Key = "Rose", Value = "258" });
            CB.ItemsSource = Models;
            this.DataContext = this;
        }
        public class TestModel
        {
            public string Key { get; set; }
            public string Value { get; set; }
        }
    }

I bind the SelectedValue to the Model which is already existed in the List. but the selection is still blank.

What's wrong with my code? I need the combobox select the item correctly.

Model should return one of the items from Models.

Add this to the constructor before DataContext setter:

Model = Models[1];

The problematic line is
_Model = new TestModel() { Key = "Joe", Value = "456" } - you create an instance of TestModel that is not present in the list. Even though it has the same property values, it is not the same object.

ComboBoxes use the .Equals() method of the objects they are displaying to find the SelectedItem in the ItemsSource.

Since the Equals() method for clasess by default works by comparing object references, you would need to set the SelectedItem to an exact same instance as an object inside the ItemsSource.

Another approach you could take is overriding the Equals() method on your object and making it Equal by value comparison or use a record class which automatically makes your object comparable by value.