Allow Incoming Connection from Specific IP Address or Subnet in UFW

Allow Incoming SSH from Specific IP Address or Subnet

sudo ufw allow from 15.15.15.0/24  to any port 22

sudo ufw allow from 15.15.15.65 to any port 22

Allow Incoming Rsync from Specific IP Address or Subnet

sudo ufw allow from 15.15.15.0/24 to any port 873

Allow All Incoming HTTP and HTTPS

sudo ufw allow proto tcp from any to any port 80,443

Allow MySQL from Specific IP Address or Subnet

sudo ufw allow from 15.15.15.0/24 to any port 3306

Allow MySQL to Specific Network Interface

sudo ufw allow in on eth1 to any port 3306

References
https://www.digitalocean.com/community/tutorials/ufw-essentials-common-firewall-rules-and-commands

Use await in Class Constructor in C#

use a static async method that returns a class instance created by a private constructor

public class ViewModel       
{       
    public ObservableCollection<TData> Data { get; set; }       

    //static async method that behave like a constructor       
    async public static Task<ViewModel> BuildViewModelAsync()  
    {       
        ObservableCollection<TData> tmpData = await GetDataTask();  
        return new ViewModel(tmpData);
    }       

    // private constructor called by the async method
    private ViewModel(ObservableCollection<TData> Data)
    {
        this.Data = Data;   
    }
}

References
https://stackoverflow.com/questions/8145479/can-constructors-be-async

Implementing the Singleton Pattern in C#

    public sealed class EventBus
    {
        private static readonly EventBus instance = new EventBus();

        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static EventBus()
        {
        }

        private EventBus()
        {
        }

        public static EventBus Instance
        {
            get { return instance; }
        }
    }

Using .NET 4’s Lazy<T> type

public sealed class Singleton5    
{    
    private Singleton5()    
    {    
    }    
    private static readonly Lazy<Singleton5> lazy = new Lazy<Singleton5>(() => new Singleton5());    
    public static Singleton5 Instance    
    {    
        get    
        {    
            return lazy.Value;    
        }    
    }    
}

 

References
https://csharpindepth.com/articles/singleton
https://www.c-sharpcorner.com/UploadFile/8911c4/singleton-design-pattern-in-C-Sharp/
https://riptutorial.com/csharp/example/6795/lazy–thread-safe-singleton–using-lazy-t–

Value conversion with IValueConverter in WPF

<Window x:Class="WpfTutorialSamples.DataBinding.ConverterSample"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfTutorialSamples.DataBinding"
        Title="ConverterSample" Height="140" Width="250">
    <Window.Resources>
        <local:YesNoToBooleanConverter x:Key="YesNoToBooleanConverter" />
    </Window.Resources>
    <StackPanel Margin="10">
        <TextBox Name="txtValue" />
        <WrapPanel Margin="0,10">
            <TextBlock Text="Current value is: " />
            <TextBlock Text="{Binding ElementName=txtValue, Path=Text, Converter={StaticResource YesNoToBooleanConverter}}"></TextBlock>
        </WrapPanel>
        <CheckBox IsChecked="{Binding ElementName=txtValue, Path=Text, Converter={StaticResource YesNoToBooleanConverter}}" Content="Yes" />
    </StackPanel>
</Window>
using System;
using System.Windows;
using System.Windows.Data;

namespace WpfTutorialSamples.DataBinding
{
    public partial class ConverterSample : Window
    {
        public ConverterSample()
        {
            InitializeComponent();
        }
    }

    public class YesNoToBooleanConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            switch(value.ToString().ToLower())
            {
                case "yes":
                case "oui":
                    return true;
                case "no":
                case "non":
                    return false;
            }
            return false;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if(value is bool)
            {
                if((bool)value == true)
                    return "yes";
                else
                    return "no";
            }
            return "no";
        }
    }
}

References
https://wpf-tutorial.com/data-binding/value-conversion-with-ivalueconverter/