CertificateRequired Event

public event EventHandler<TEventArgs> CertificateRequired

Public Event CertificateRequired EventHandler(Of TEventArgs)

public:
 event EventHandler<TEventArgs>^ CertificateRequired

Show how to handle the CertificateRequired event.

using System;
using System.Security.Cryptography.X509Certificates;
using ComponentPro.Net;
using ComponentPro.Net.Mail;

...

public void HandleCertificateRequiredEvent()
{
    // Create a new instance. 
    Pop3 client = new Pop3();

    client.CertificateRequired += client_CertificateRequired;

    // Connect to the POP3 server. 
    client.Connect("myserver", 110, SslSecurityMode.Explicit);

    // Authenticate. 
    client.Authenticate("userName", "password");

    // Do something here... 
    // ... 
 
    // Disconnect. 
    client.Disconnect();
}

void client_CertificateRequired(object sender, ComponentPro.Security.CertificateRequiredEventArgs e)
{
    // Load certificates from the local machine. 
    X509Store my = new X509Store(StoreName.My, StoreLocation.CurrentUser);
    my.Open(OpenFlags.ReadOnly);

    // Retrieve a list of available certificates. 
    X509Certificate2Collection certs = my.Certificates;

    // If no certificate found, return. 
    if (certs.Count == 0)
    {
        e.Certificates = null;
        return;
    }

    // Show all certificates. 
    Console.WriteLine("Select certificate:");
    for (int i = 0; i <= certs.Count; i++)
    {
        if (i == 0)
        {
            Console.WriteLine(string.Format("{0}. [Nothing, skip this step]", i));
            continue;
        }

        Console.WriteLine(string.Format("{0}. {1}", i, certs[i - 1].SubjectName.Name));
    }

    // And ask user to choose an appropriate certificate. 
    while (true)
    {
        Console.Write(string.Format("Select certificate [0 - {0}]: ", certs.Count));

        int certIndex;

        try 
        {
            certIndex = int.Parse(Console.ReadLine());
        }
        catch 
        {
            Console.WriteLine("ERROR: Wrong certificate index input!");
            continue;
        }

        if (certIndex > 0 && certIndex <= certs.Count)
        {
            e.Certificates = new X509Certificate2Collection(certs[certIndex]);
            return;
        }

        if (certIndex == 0)
            break;

        Console.WriteLine(string.Format("ERROR: You must enter number between 0 and {0}.", certs.Count));
    }
}

Imports System.Security.Cryptography.X509Certificates
Imports ComponentPro.Net
Imports ComponentPro.Net.Mail

...

Public Sub HandleCertificateRequiredEvent()
    ' Create a new instance. 
    Dim client As New Pop3()

    AddHandler client.CertificateRequired, AddressOf client_CertificateRequired

    ' Connect to the POP3 server. 
    client.Connect("myserver", 110, SslSecurityMode.Explicit)

    ' Authenticate. 
    client.Authenticate("userName", "password")

    ' Do something here... 
    ' ... 
 
    ' Disconnect. 
    client.Disconnect()
End Sub 
 
Private Sub client_CertificateRequired(ByVal sender As Object, ByVal e As ComponentPro.Security.CertificateRequiredEventArgs)
    ' Load certificates from the local machine. 
    Dim my As New X509Store(StoreName.My, StoreLocation.CurrentUser)
    my.Open(OpenFlags.ReadOnly)

    ' Retrieve a list of available certificates. 
    Dim certs As X509Certificate2Collection = my.Certificates

    ' If no certificate found, return. 
    If certs.Count = 0 Then 
        e.Certificates = Nothing 
        Return 
    End If 
 
    ' Show all certificates. 
    Console.WriteLine("Select certificate:")
    For i As Integer = 0 To certs.Count
        If i = 0 Then 
            Console.WriteLine(String.Format("{0}. [Nothing, skip this step]", i))
            Continue For 
        End If 
 
        Console.WriteLine(String.Format("{0}. {1}", i, certs(i - 1).SubjectName.Name))
    Next i

    ' And ask user to choose an appropriate certificate. 
    Do 
        Console.Write(String.Format("Select certificate [0 - {0}]: ", certs.Count))

        Dim certIndex As Integer 
 
        Try 
            certIndex = Integer.Parse(Console.ReadLine())
        Catch 
            Console.WriteLine("ERROR: Wrong certificate index input!")
            Continue Do 
        End Try 
 
        If certIndex > 0 AndAlso certIndex <= certs.Count Then 
            e.Certificates = New X509Certificate2Collection(certs(certIndex))
            Return 
        End If 
 
        If certIndex = 0 Then 
            Exit Do 
        End If 
 
        Console.WriteLine(String.Format("ERROR: You must enter number between 0 and {0}.", certs.Count))
    Loop 
End Sub

Reference