Chào anh Long, hiện tại em có CSDL gồm có 2 bảng DIENVIEN và NGUOIQUANLY:

CREATE DATABASE QUANLYDIENVIEN
GO
USE QUANLYDIENVIEN
GO
CREATE TABLE DIENVIEN
(
	ID INT IDENTITY PRIMARY KEY,
	NAME NVARCHAR(100)
)
GO
CREATE TABLE NGUOIQUANLY
(
	ID INT IDENTITY PRIMARY KEY,
	IDDIENVIEN INT NOT NULL,
	NAME NVARCHAR(100)
	FOREIGN KEY(IDDIENVIEN) REFERENCES dbo.DIENVIEN(ID)
)
GO
INSERT INTO dbo.DIENVIEN
(
    NAME
)
VALUES
(N'Vương Vũ Tiệp' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.DIENVIEN
(
    NAME
)
VALUES
(N'Tống Đạt Dân' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.DIENVIEN
(
    NAME
)
VALUES
(N'Tinh Hủy' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.DIENVIEN
(
    NAME
)
VALUES
(N'Phương Sổ Chân' -- NAME - nvarchar(100)
    )
GO
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   1,   -- IDDIENVIEN - int
    N'Nguyễn Văn A' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   1,   -- IDDIENVIEN - int
    N'Nguyễn Văn A' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   2,   -- IDDIENVIEN - int
    N'Nguyễn Văn B' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   3,   -- IDDIENVIEN - int
    N'Nguyễn Văn C' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   4,   -- IDDIENVIEN - int
    N'Nguyễn Văn C' -- NAME - nvarchar(100)
    )
INSERT INTO dbo.NGUOIQUANLY
(
    IDDIENVIEN,
    NAME
)
VALUES
(   4,   -- IDDIENVIEN - int
    N'Nguyễn Văn D' -- NAME - nvarchar(100)
    )
GO

và trong WPF em có 2 combobox:

MainWindow.xaml:

<Window x:Class="LoadTenDienVienTheoNguoiQuanLy.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:LoadTenDienVienTheoNguoiQuanLy"
        mc:Ignorable="d"
		Loaded="Window_Loaded"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
		<ComboBox x:Name="cbTenDienVien"></ComboBox>
		<ComboBox x:Name="cbNguoiQuanLy"></ComboBox>
    </StackPanel>
</Window>

MainWindow.xaml.cs:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace LoadTenDienVienTheoNguoiQuanLy
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        QUANLYDIENVIENEntities data = new QUANLYDIENVIENEntities();
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            cbTenDienVien.ItemsSource = data.DIENVIENs.Select(p => p.NAME).ToList();
            var DienVienTheoNguoiQuanLy= from m in data.NGUOIQUANLies
                                         from d in data.DIENVIENs
                                         where m.IDDIENVIEN==d.ID
                                         select m.NAME;
            cbNguoiQuanLy.ItemsSource = DienVienTheoNguoiQuanLy.ToList();
        }
    }
}

Giờ em muốn khi chọn 1 tên trong cbTenDienVien thì nó hiện lên tên tương ứng trong cbNguoiQuanLy thì em phải làm sao ạ? Em cảm ơn anh