这样做。更改了组合框中的值后，检查viewmodel中Companies.CurrentItem的值。如果您有任何疑问，请告诉我。

XAML代码 -

Do Like this. After changes the value in combobox then check the value for "Companies.CurrentItem" in viewmodel. If you have any doubt please let me know.
XAML Code-

<ComboBox x:Name="comboBox" HorizontalAlignment="Left" Margin="56,79,0,0"

         ItemsSource="{Binding Companies, Mode=OneWay}"

                                      DisplayMemberPath="Name"

                                      SelectedValue = "{Binding SelectedItem.CompanyId, ValidatesOnDataErrors=True, ValidatesOnExceptions=True}"

                                      SelectedValuePath="CompanyId"

                                      IsSynchronizedWithCurrentItem="True"

                                      IsEditable="True" VerticalAlignment="Top" Width="222"/> 

ViewModel Code-

ViewModel Code-

public class CompanyEntity
    {

        public int? CompanyId { get; set; }
        public string Name { get; set; }
    }
   
    class MainWindowVM
    {
        ObservableCollection<CompanyEntity> companies = new ObservableCollection<CompanyEntity>();
        ICommand saveCommand = null;
        public MainWindowVM()
        {
            companies.Add(new CompanyEntity { CompanyId = 1,Name= "Microsoft" });
            companies.Add(new CompanyEntity { CompanyId = 2,Name= "Apple" });
        }
        public ICommand SaveCommand
        {
            get
            {
                return saveCommand = new RelayCommand(Save);
            }
        }

        private void Save()
        {
            
        }

        public CollectionView Companies
        {
            get
            { 
                return CollectionViewSource.GetDefaultView(companies) as CollectionView;
            }
        }
    }