em có 1 from main như vậy.

<Window x:Class="kho.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="CoffeeShop" Height="350" Width="525" WindowStartupLocation="CenterScreen" WindowState="Maximized" xmlns:my="clr-namespace:kho" Closing="Window_Closing" Loaded="Window_Loaded">
    <Grid Background="#8b92c6" Loaded="Grid_Loaded" Name="G_main">
     
    </Grid>
</Window>

còn này usercontrol

<UserControl x:Class="kho.USlogin"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="250" d:DesignWidth="560" Loaded="UserControl_Loaded">
    <UserControl.Resources>
        <ResourceDictionary Source="Resource/RDlogin.xaml"></ResourceDictionary>
    </UserControl.Resources>
    <Grid>
        <Grid Style="{StaticResource G_login}">
            <Border Style="{StaticResource BD_Login}">
                <Grid Style="{StaticResource BG_login}">
                    <Label Style="{StaticResource LB_login_TD}"
               Content="Đăng Nhập Hệ Thống" 
               Margin="0,-1,0,0">
                    </Label>
                    <Label Style="{StaticResource LB_DNMK_login}" 
               Margin="152,77,278,143"
               Content="Tên Đăng nhập"></Label>
                    <Label Style="{StaticResource LB_DNMK_login}"
               Content="Mật Khẩu"  Margin="152,123,278,99"/>
                    <TextBox  Style="{StaticResource TXT_Login}"
                  Name="txt_user"
                 Margin="0,82,68,139">
                    </TextBox>
                    <PasswordBox Style="{StaticResource PWB_Login}" Name="txt_pass"
             Margin="288,123,68,98"/>

                    <Image Name="im_login" Style="{StaticResource IMG_Login}"
               Margin="23,77,437,73">
                    </Image>
                    <Button Name="bt_login" Style="{StaticResource bt_login}"
                Content="Đăng nhập"
         Margin="288,174,0,42" Click="bt_login_Click" HorizontalAlignment="Left" Width="80"></Button>
                    <Button Content="Thoát" Style="{StaticResource bt_login}"
                Margin="0,174,68,42" 
                Name="bt_cancel"  Click="bt_cancel_Click" HorizontalAlignment="Right" Width="80" />
                </Grid>
            </Border>
        </Grid>
    </Grid>
</UserControl>

này là code xử lý của usercontrol

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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 kho
{
    /// <summary>
    /// Interaction logic for USlogin.xaml
    /// </summary>
    public partial class USlogin : UserControl
    {
        public USlogin()
        {
            InitializeComponent();
           
        }

        private void bt_cancel_Click(object sender, RoutedEventArgs e)
        {
            App.Current.Shutdown();
        }

        private void bt_login_Click(object sender, RoutedEventArgs e)
        {
            MainWindow fmain = new MainWindow();
            USlogin lg = new USlogin();
            USmanage mg=new USmanage();
            fmain.G_main.Children.Remove(lg);
            fmain.G_main.Children.Add(mg);
        }

        private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            
        }
    }
}

cho em hỏi là cái phần code này nó không remove được cái usercontrol và không add thêm usercontrol khác được ak

private void bt_login_Click(object sender, RoutedEventArgs e)
        {
            MainWindow fmain = new MainWindow();
            USlogin lg = new USlogin();
            USmanage mg=new USmanage();
            fmain.G_main.Children.Remove(lg);
            fmain.G_main.Children.Add(mg);
        }