I found that the ComboBox WPF binding in WPF does not work correctly if the binding value is of an object type. Even when I set SelectedValuePath="Id", the binding fails to set the value. When a different value in the ComboBox is selected, the new selected value should be set in the ViewModel too. However, this does not happen this way.

I got a solution for this issue, which is to set the SelectedValue to be the CategoryId:

<ComboBox Grid.Row="3" Grid.Column="1" 
          ItemsSource="{Binding CategoryList}"
          DisplayMemberPath="Name" 
          SelectedItem="CategoryId"
          SelectedValue="{Binding CategoryId, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          SelectedValuePath="Id" 
          Margin="5"/>
    

Inside the ViewModel:

public int CategoryId
{
    get => _selectedCategoryId;
    set
    {
        if (_selectedCategoryId != value)
        {
            //_selectedCategory = value;
            _item.CategoryId = value;
            _item.Category = CategoryList.FirstOrDefault(c => c.Id == value);
            OnPropertyChanged();
        }
    }
}
    

Then it will work!