C# WEB/APP: Joining threads that execute Socket.Connect

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I write ip/port scanner so users using search on our LAN know if the particular ftp is online or not.

The class I write has the following structure:

Expand|Select|Wrap|Line Numbers

 
public class IPScanner

{

    public IPScanner() { }
 
    private void connect(object data)

    {

         Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
 
        try

        {

            socket.Connect((string)data, 21);

        }

        catch { }

        finally

        {

            socket.Close();

        }

    }
 
    public void Scan()

    {

        Thread[] threads = new Thread[255];

        for (int i = 0; i < 255; i++)

        {

            threads[i] = new Thread(new ParameterizedThreadStart(connect));

            threads[i].Start( "xxx.xxx.xxx." + ((int)(i+1)).ToString() );

        }
 
        for (int i = 0; i < 255; i++)

        {

            threads[i].Join();

        }

}

I use that class as

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        IPScanner ip_scanner = new IPScanner();

        ip_scanner.Scan();

But Joins work synchronously in that sense that the next Join in the for-loop executes only after previous one returns. So the scan of one address range consumes very much time. I test it using only 4 adresses I sure are offline and trying to Join to 5 threads. I receive the null reference error only after 1-2 minutes of executing of program.

But the following code works fine and returns in 10 seconds (this code is met often in google but applied not to sockets):

Expand|Select|Wrap|Line Numbers

 
public class IPScanner

{

    public IPScanner() { }
 
    private void connect(object data)

    {

         Sleep(10000);

         // Or do some math.

    }
 
    public void Scan()

    {

        Thread[] threads = new Thread[255];

        for (int i = 0; i < 255; i++)

        {

            threads[i] = new Thread(new ParameterizedThreadStart(connect));

            threads[i].Start( "xxx.xxx.xxx." + ((int)(i+1)).ToString() );

        }
 
        for (int i = 0; i < 255; i++)

        {

            threads[i].Join();

        }

}

...

        IPScanner ip_scanner = new IPScanner();

        ip_scanner.Scan();

What is the problem with sockets? I use asp.net2 and IIS 7 on Vista 64. (I tried the same in winform application with the same results.)
Thank you.

Dec 22 '07 #1

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