Using lazy ＜ t ＞ in C ＃ to realize singleton mode in WPF

Lazy Is a deferred object , It allows you to delay the creation of T object , in other words , Although the object has been declared , But actually only in use T When the object , To create this object .

stay WPF in , Public methods Can be written in a separate class , each viewmodel Use it when you need it Lazy call .

for example
Create public classes appData

using GalaSoft.MvvmLight;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using DAL;

namespace ShenhuaSQLite
{
    
    public class AppData: ObservableObject
    {
    
        public AppData()
        {
    

        }

        public static AppData Instance = new Lazy<AppData>(() => new AppData()).Value;

        private string systemName = "Zg Upper computer test ";

        public string SystemName
        {
    
            get {
     return systemName; }
            set {
     systemName = value; }
        }

        private SysAdmins sysAdmins = new SysAdmins();

        public SysAdmins CurrentUser
        {
    
            get {
     return sysAdmins; }
            set {
     sysAdmins = value;  RaisePropertyChanged(); }
        }

        /// <summary>
        ///  Main form 
        /// </summary>
        public MainWindow MainWindow {
     get; set; } = null;

        /// <summary>
        ///  Show or hide mask layers 
        /// </summary>
        /// <param name="isMark"></param>
        public void ShowMarkLayer(bool isMark)
        {
    
            if (MainWindow == null) return;
            //MainWindow.markLayer.Visibility = isMark ? System.Windows.Visibility.Visible : System.Windows.Visibility.Hidden;
        }

    }
}

In the login form , You can instantiate in the constructor lazy, Implementation calls appData All properties and methods in . This is somewhat similar to the multiple background data blocks on the way to the blog .

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using DAL;

namespace ShenhuaSQLite.ViewModel
{
    
    public class LoginViewModel : ViewModelBase
    {
    
        public AppData appData {
     get; private set; } = AppData.Instance;

        public LoginViewModel()
        {
    
            this.appData.CurrentUser.LoginName = "ZG";
            this.appData.CurrentUser.LoginPwd = "123";
        }

        public RelayCommand<Window> LoginCommand2 // Just write get Return
        {
    
            get
            {
    
                return new RelayCommand<Window>((arg) =>
                {
    
                    SysAdminsProvider sysAdminsProvider = new SysAdminsProvider();
                    //MemberProvider memberProvider = new MemberProvider();
                    var list = sysAdminsProvider.Select();
                    var user = list.FirstOrDefault(
                        u => u.LoginName == appData.CurrentUser.LoginName
                        && u.LoginPwd == appData.CurrentUser.LoginPwd) ; //u For database objects 

                    if (user == null)
                    {
    
                        MessageBox.Show(" Wrong user name or password ！");
                    }
                    else
                    {
    
                        MainWindow mainWindow = new MainWindow();
                        mainWindow.Show();
                        arg.Close();
                    }
                });
            }
        }


    }
}