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// Create POP3 object
Pop3 pop = new Pop3();

// Enable logging to file
pop.Log.Enabled = true;
pop.Log.Filename = @"C:\log.txt";
pop.Log.Clear();

// Connect to POP3 server
pop.Connect("mail.domain.com");
pop.Login("bounce", "secret");

// Download headers and bodies of all messages.
MailMessageCollection msgs = pop.DownloadMessageHeaders(1, -1, -1);
                        
// Loop through all messages in the mailbox
foreach (MailMessage msg in msgs)
{
        string strLine = msg.BodyPlainText;
        Console.WriteLine("From: " + msg.From.Email);
                                
        // Get failed email address
        string str_invalid_email =  GetInvalidEmailAddressME(msg);

        // If str_invalid_email is non-empty then failed email 
        // address was found
        if (str_invalid_email.Length > 0)
        {
                // Remove failed email from database
                RemoveEmailFromDatabase(str_invalid_email);

                // Display invalid adress
                Console.WriteLine("Invalid email: " + str_invalid_email);                

                // Delete bounced email from server to avoid
                // processing it next time
                pop.DeleteMessage(msg.IndexOnServer);
        }
}

// Disconnect from POP3 server
pop.Disconnect();

// The function checks whether the message is bounced and extracts 
// failed address 
// from bounced message. Valid only for MailEnable servers
static string  GetInvalidEmailAddressME(MailMessage msg)
{
        string str_invalid_email = msg.BodyPlainText;

        // Check if this is a bounced message report
        if (msg.Subject.IndexOf("Delivery Failure") == -1)
{ 
                return "";
        }
        if (msg.From.ToString().IndexOf("POSTMASTER") == -1)
        {
                return "";
        }

        // Now we're sure this is a bounced message report
        int i_start;
        i_start = str_invalid_email.IndexOf("SMTP:");

        // Check if bounced message report contains "Recipient:" field
        if (i_start == -1)
        {
                return "";
        }
                        
        // Get failed address
        i_start += 5;
        i_end = str_invalid_email.IndexOf("]",i_start);
        str_invalid_email.Substring(i_start, i_end);
                        
        return str_invalid_email;
}

// The function checks whether the message is bounced and extracts 
// failed address 
// from bounced message. Valid only for Communigate Pro servers
static string  GetInvalidEmailAddressCP(MailMessage msg)
{
string str_invalid_email = msg.BodyPlainText;

        // Check if this is a bounced message report
        if (msg.Subject.IndexOf("Undeliverable mail") == -1) 
        {
                return "";
        }
        if (msg.From.ToString().IndexOf("MAILER-DAEMON") == -1) 
        {
                return "";
        }

        // Now we're sure this is a bounced message report
        int i_start;
        i_start = str_invalid_email.IndexOf("to '<");

        // Check if bounced message report contains 
        // "Failed to deliver to " field
        if (i_start == -1)
        { 
                return "";
        }
                        
        // Get failed address
        i_start += 5;
        i_end = str_invalid_email.IndexOf("]",i_start);
        str_invalid_email.Substring(i_start, i_end);
                        
        return str_invalid_email;
}

// This function must remove (or disable) specified
// email address from mailing list
static void RemoveEmailFromDatabase(string str_invalid_email)
{
// TODO: Add your code here                        
}

Dim pop As New Pop3

' Enable logging to file
pop.Log.Enabled = True
pop.Log.Filename = "C:\log.txt"
pop.Log.Clear()

' Connect to POP3 server
pop.Connect("mail.domain.com")
pop.Login("jdoe", "secret")

' Download headers and bodies for all messages.
Dim msgs As MailMessageCollection = pop.DownloadMessageHeaders(1, -1, -1)

' Loop through all messages in the mailbox
Dim msg As MailMessage
For Each msg In msgs
        Dim strLine As String = msg.BodyPlainText
        Console.WriteLine("From: " + msg.From.Email)

        ' Get failed email address
        Dim str_invalid_email As String = GetInvalidEmailAddressME(msg)

        ' If str_invalid_email is non-empty then failed email 
        ' address was found
        If str_invalid_email.Length > 0 Then
                'Remove failed email from database
                RemoveEmailFromDatabase(str_invalid_email)

                ' Display invalid address
                Console.WriteLine("Invalid email: " & str_invalid_email)

                ' Delete bounced email from server to avoid
                ' processing it next time
                pop.DeleteMessage(msg.IndexOnServer)
        End If
Next
Console.ReadLine()

' Disconnect from POP3 server
pop.Disconnect()

' The function checks whether the message is bounced and extracts 
' failed address 
' from bounced message. Valid only for MailEnable servers
Function GetInvalidEmailAddressME(ByVal msg As MailMessage) As String
        Dim str_invalid_email As String = msg.BodyPlainText

        ' Check if this is a bounced message report
        If msg.Subject.IndexOf("Delivery Failure") = -1 Then
                Return ""
        End If
        If msg.From.ToString().IndexOf("POSTMASTER") = -1 Then
                Return ""
        End If

        ' Now we're sure this is a bounced message report
        Dim i_start As Integer, i_end As Integer
        i_start = str_invalid_email.IndexOf("SMTP:")

        ' Check if bounced message report contains "Recipient:" field
        If i_start = -1 Then
                Return ""
        End If

        ' Get failed address
        i_start += 5
        i_end = str_invalid_email.IndexOf("]", i_start)
        str_invalid_email.Substring(i_start, i_end)

        Return str_invalid_email
End Function

' The function checks whether the message is bounced and extracts 
' failed address 
' from bounced message. Valid only for Communigate Pro servers
Function GetInvalidEmailAddressCP(ByVal msg As MailMessage) As String
        Dim str_invalid_email As String = msg.BodyPlainText

        ' Check if this is a bounced message report
        If msg.Subject.IndexOf("Undeliverable mail") = -1 Then
                Return ""
        End If
        If msg.From.ToString().IndexOf("MAILER-DAEMON") = -1 Then
                Return ""
        End If

        ' Now we're sure this is a bounced message report
        Dim i_start As Integer, i_end As Integer
        i_start = str_invalid_email.IndexOf("to '<")

        ' Check if bounced message report contains 
        ' "Failed to deliver to " field
        If i_start = -1 Then
                Return ""
        End If

        ' Get failed address
        i_start += 5
        i_end = str_invalid_email.IndexOf("]", i_start)
        str_invalid_email.Substring(i_start, i_end)

        Return str_invalid_email
End Function

' This function must remove (or disable) specified
' email address from mailing list
Sub RemoveEmailFromDatabase(ByVal str_invalid_email As String)
        ' TODO: Add your code here                        
End Sub

static void SimpleOCRCalculator(string filePath)
      {
         RasterCodecs codecs = new RasterCodecs();

         RasterImage image = codecs.Load(filePath);

         string[] calculations;
         using (IOcrEngine engine = OcrEngineManager.CreateEngine(OcrEngineType.Advantage, false))
         {
            engine.Startup(null, null, null, null);
            IOcrPage page = engine.CreatePage(image, OcrImageSharingMode.None);

            page.AutoZone(null);
            page.Recognize(null);

            calculations = new string[page.Zones.Count];

            for (int i = 0; i < page.Zones.Count; i++)
            {
               calculations[i] = page.GetText(i);
            }

            engine.Shutdown();
         }

         Dictionary<string, Action<double, double>> operands = new Dictionary<string, Action<double, double>>();
         operands.Add("+", new Action<double, double>(delegate(double a, double b) { double ans = a + b; Console.WriteLine("{0} + {1} = {2}", a, b, ans); }));
         operands.Add("-", new Action<double, double>(delegate(double a, double b) { double ans = a - b; Console.WriteLine("{0} - {1} = {2}", a, b, ans); }));
         operands.Add("x", new Action<double, double>(delegate(double a, double b) { double ans = a * b; Console.WriteLine("{0} * {1} = {2}", a, b, ans); }));
         operands.Add("/", new Action<double, double>(delegate(double a, double b) { double ans = a / b; Console.WriteLine("{0} / {1} = {2}", a, b, ans); }));

         for (int i = 0; i < calculations.Length; i++)
         {
            string equation = Regex.Replace(calculations[i], @"\n|\r| ", "");
            string[] ops = new string[] { "+", "-", "x", "/" };

            for (int j = 0; j < ops.Length; j++)
            {
               int index = equation.IndexOf(ops[j]);

               if (index > 0 && index < equation.Length)
               {
                  string op1 = equation.Substring(0, index);
                  string op2 = equation.Substring(index + 1);

                  double arg1 = double.Parse(op1);
                  double arg2 = double.Parse(op2);

                  operands[ops[j]](arg1, arg2);

                  break;
               }
            }
         }

         codecs.Dispose();
         image.Dispose();
      }

• .NET Standard是一项API规范，每一个特定的版本，都定义了必须实现的基类库。
• .NET Core是一个托管框架，针对构建控制台、云、ASP.NET Core和UWP应用程序进行了优化。每一种托管实现（如Xamarin、.NET Core或.NET Framework）都必须遵循.NET Standard实现BCL。

• .NET Framework用于构建桌面应用程序和运行在互联网信息服务器（IIS）上的ASP.NET应用程序。它是第一个托管框架。
• Xamarin则是一个用于构建iOS、Android、macOS和桌面应用程序的框架。