Covariant and Contravariant Types made simple

https://medium.com/@EdwhardT/covariant-and-contravariant-types-made-simple-4ab6effb031b

How to swap two numbers without third variable?

int first,second ;
first = 1;
second = 2;
first = first + second;
second = first - second;
first = first - second;

C# Algorithms source code

https://github.com/aalhour/C-Sharp-Algorithms

Static Code Analysis Tool in Visual Studio 2013

http://blogs.msdn.com/b/hkamel/archive/2013/10/24/visual-studio-2013-static-code-analysis-in-depth-what-when-and-how.aspx

C# Safe Navigation Operator ?.

Read code to understand:

var g1 = parent?.child?.child?.child;
if (g1 != null) // TODO


http://blogs.msdn.com/b/jerrynixon/archive/2014/02/26/at-last-c-is-getting-sometimes-called-the-safe-navigation-operator.aspx

How to solve problem "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at"

To my case:
The password is wrong.

var client = new SmtpClient("smtp.gmail.com", 587)
            {
                Credentials = new NetworkCredential("myusername@gmail.com", "mypwd"),
                EnableSsl = true
            };
            client.Send("myusername@gmail.com", "myusername@gmail.com", "test", "testbody");

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at

How good is your C#?

http://volatileread.com/Wiki?id=1073

How to create a SFTP server on the fly for unit test?\

Usage:
NuaneSFTPServer server = new NuaneSFTPServer("username", "Passw0rd!", SftpRootFolder, 990);
server.Start();

server.Stop();

    public class NuaneSFTPServer
    {
        volatile SftpServer sFTPserver = null;
        public string UserName;
        public string Password;
        public string FtpPath;
        public int PortNumber;
        private static Thread workerThread;

        public NuaneSFTPServer(string userName, string password, string ftpPath, int portNumber)
        {
            UserName = userName;
            Password = password;
            FtpPath = ftpPath;
            PortNumber = portNumber;
            SshKey rsaKey = SshKey.Generate(SshKeyAlgorithm.RSA, 1024);
            SshKey dssKey = SshKey.Generate(SshKeyAlgorithm.DSS, 1024);

            // add keys, bindings and users
            sFTPserver = new SftpServer();
            sFTPserver.Log = Console.Out;
            sFTPserver.Keys.Add(rsaKey);
            sFTPserver.Keys.Add(dssKey);
            sFTPserver.Bindings.Add(IPAddress.Any, portNumber);
            sFTPserver.Users.Add(new SshUser(userName, password, ftpPath));
        }
        public void Start()
        {
            workerThread = new Thread(sFTPserver.Start);
            workerThread.Start();
            while (!workerThread.IsAlive) ;
        }
        public void Stop()
        {
            sFTPserver.Stop();
            workerThread.Join();
        }
    }

How to fix The thread xxx has exited with code 259 (0x103)?

When try to wait for a thread to be alive, but not call thread Start method, will get a lot of errors like this::The thread xxx has exited with code 259 (0x103)

            workerThread = new Thread(StartMethod);
//            workerThread.Start();
            while (!workerThread.IsAlive) ;

:)

How to check TcpListener is listening?

In TcpListener class, there is a property called Active but it a proctected one.
https://msdn.microsoft.com/en-us/library/system.net.sockets.tcplistener.active(v=vs.110).aspx

Solution:
Create a sub class from TcpListener, and expose this property by yourself.

Code example:
    public class Listener : TcpListener
    {
        public Listener(int portNumber): base(IPAddress.Any, portNumber) {}
        public bool IsAcitve
        {
            get { return this.Active; }
        }
    }

Why readonly in C#?

Make summary form following post:
http://stackoverflow.com/questions/277010/what-are-the-benefits-to-marking-a-field-as-readonly-in-c

1. Improve readability: tell how to not use in other way.
2. Nothing to do with performance
3. Immutable data structures in threading

What is real purpose of Enable Visual Studio Hosting Process?

According to MSDN:
"The Visual Studio hosting process improves debugger performance and enables new debugger features, such as partial-trust debugging and design-time expression evaluation"

In real world:
Sometimes, vshost.exe always running, and you can't even kill it from Task Manager. (After killed it, it restores automatically, need to quit Visual Studio)

Need to disable this feature/bug by following steps:
Properties of project =>
"Debug" tab =>
uncheck "Enable the Visual Studio hosting process"

Readonly vs const in C#

A const field can only be initialized at the declaration of the field.
A readonly field can be initialized either at the declaration or in a constructor

http://msdn.microsoft.com/en-us/library/vstudio/acdd6hb7(v=vs.100).aspx

How to solve problem "Cannot find the requested object." while create an X509Certificate2 object?

It throws this exception for following line:
X509Certificate2 certificate = new X509Certificate2(@"c:\temp\server.pvk", "password");

It means: the certificate file is not valid.
The most common reason is:
Need pvkimprt tool to generate PFX files

Reference:
http://msdn.microsoft.com/en-us/library/ms148420(v=vs.110).aspx
This constructor creates a new X509Certificate2 object using a certificate file name and a password needed to access the certificate. It is used with PKCS12 (PFX) files that contain the certificate's private key. Calling this constructor with the correct password decrypts the private key and saves it to a key container.

Following are the steps to generate X509Certificate2

"C:\Program Files (x86)\Windows Kits\8.1\bin\x64\makecert" -r -pe -n "CN=Test" -sky exchange c:\temp\server.cer -sv c:\temp\server.pvk -sr currentuser
"C:\Program Files (x86)\Windows Kits\8.1\bin\x64\cert2spc"  c:\temp\server.cer server.spc
pvkimprt -pfx server.spc server.pfx

What is the simplest way to solve "Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on"

Code Example

Replace
list.Items.Add(status);
with:
                    this.Invoke((MethodInvoker)delegate
                    {
                        //list.Items.Add(status);
                    });

C# Naming Conventions with example

use PascalCasing for class names and method names.
use camelCasing for method arguments and local variables
use noun or noun phrases to name a class.
prefix interfaces with the letter I.  Interface names are noun (phrases) or adjectives.

References:
http://msdn.microsoft.com/en-us/library/ms229043.aspx
http://www.dofactory.com/reference/csharp-coding-standards

How to solve Invalid Xml syntax error after updated an older Visual Studio project

Error message:
This application has failed to start because the application configuration is incorrect. Review the manifest file for possible errors.
Reinstalling the application may fix this problem. For more details, please see the application event log.

In EventView, the error message is:
Activation context generation failed for XXXXX
XXXX on line 1. Invalid Xml syntax.

The first line in config file is
<?xml version="1.0" encoding="Windows-1252"?>

Solution:
Change to
<?xml version="1.0" encoding="utf-8"?>

A Full-Featured FREE IDE Visual Studio Community 2013

http://www.visualstudio.com/en-us/products/visual-studio-community-vs

C# Autocomplete Demo Using Bing Code Search Engine

http://codesnippet.research.microsoft.com/#example

What is difference between String and string in C#

Same! like int is equal to System.int32