Use DownloadMessageHeadersAsync methods to asynchronously download headers of a message. These methods asynchronously download headers of a message with execution occurring on a new thread, therefore they allow your next line of code to execute immediately.

In Task-based Asynchronous programs, the single asynchronous method represents the initiation and completion of an asynchronous operation. You may create the continuation code either explicitly, through methods on the Task class (for example, ContinueWith) or implicitly, by using language support built on top of continuations (for example, await in C#, Await in Visual Basic).

In Event-based Asynchronous programs, the event DownloadMessageHeadersCompleted is raised when the DownloadMessageHeadersAsync completes. In the event handler method of the DownloadMessageHeadersCompleted, you can check if there were any errors by using the Error  property of the event data object.

The following example demonstrates how to asynchronously download headers of a message:

Task-based approach

using System;
using System.IO;
using ComponentPro;
using ComponentPro.Net.Mail;

...

public async void DoDownloadMessageHeadersAsync()
{
    // Create a new instance of the Imap class. 
    Imap client = new Imap();

    // Connect to the server. 
    client.Connect("myserver");

    // Or you can specify the IMAP port with 
    // client.Connect("myserver", 143); 
 
    // Login to the server. 
    client.Authenticate("user", "password");

    // ... 
 
    // Select 'INBOX' mailbox. 
    client.Select("INBOX");

    MemoryStream stream = new MemoryStream();
    // Download a mail message's headers with sequence number 1 to the specified stream. 
    await client.DownloadMessageHeadersAsync(1, stream);

    // ... 
 
    // Load the message for reading. 
    stream.Seek(0, SeekOrigin.Begin);
    MailMessage msg = new MailMessage(stream);

    Console.WriteLine("Message downloaded successfully.");

    foreach (Header h in msg.Headers)
    {
        Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw);
    }

    // Disconnect. 
    client.Disconnect();
}

void client_DownloadMessageHeadersCompleted(object sender, ExtendedAsyncCompletedEventArgs<long> e)
{
    // Imap client = (Imap)sender; 
    if (e.Error != null)
    {
        Console.WriteLine("Error: " + e.Error.ToString());
    }
    else 
    {
        Stream stream = (Stream) e.UserState;

        // Load the message for reading. 
        MailMessage msg = new MailMessage(stream);

        Console.WriteLine("Message downloaded successfully.");

        foreach (Header h in msg.Headers)
        {
            Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw);
        }
    }
}

Imports System.IO
Imports ComponentPro
Imports ComponentPro.Net.Mail

...

Public Async Sub DoDownloadMessageHeadersAsync()
    ' Create a new instance of the Imap class. 
    Dim client As New Imap()

    ' Connect to the server. 
    client.Connect("myserver")

    ' Or you can specify the IMAP port with 
    ' client.Connect("myserver", 143); 
 
    ' Login to the server. 
    client.Authenticate("user", "password")

    ' ... 
 
    ' Select 'INBOX' mailbox. 
    client.Select("INBOX")

    Dim stream As New MemoryStream()
    ' Download a mail message's headers with sequence number 1 to the specified stream. 
    Await client.DownloadMessageHeadersAsync(1, stream)

    ' ... 
 
    ' Load the message for reading. 
    stream.Seek(0, SeekOrigin.Begin)
    Dim msg As New MailMessage(stream)

    Console.WriteLine("Message downloaded successfully.")

    For Each h As Header In msg.Headers
        Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw)
    Next h

    ' Disconnect. 
    client.Disconnect()
End Sub 
 
Private Sub client_DownloadMessageHeadersCompleted(ByVal sender As Object, ByVal e As ExtendedAsyncCompletedEventArgs(Of Long))
    ' Imap client = (Imap)sender; 
    If e.Error IsNot Nothing Then 
        Console.WriteLine("Error: " & e.Error.ToString())
    Else 
        Dim stream As Stream = CType(e.UserState, Stream)

        ' Load the message for reading. 
        Dim msg As New MailMessage(stream)

        Console.WriteLine("Message downloaded successfully.")

        For Each h As Header In msg.Headers
            Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw)
        Next h
    End If 
End Sub

Event-based approach

using System;
using System.IO;
using ComponentPro;
using ComponentPro.Net.Mail;

...

public void DoDownloadMessageHeadersAsync()
{
    // Create a new instance of the Imap class. 
    Imap client = new Imap();

    // Connect to the server. 
    client.Connect("myserver");

    // Or you can specify the IMAP port with 
    // client.Connect("myserver", 143); 
 
    // Login to the server. 
    client.Authenticate("user", "password");

    // ... 
 
    // Register an event handler. 
    client.DownloadMessageHeadersCompleted += client_DownloadMessageHeadersCompleted;

    // Select 'INBOX' mailbox. 
    client.Select("INBOX");

    MemoryStream stream = new MemoryStream();
    // Download a mail message's headers with sequence number 1 to the specified stream. 
    client.DownloadMessageHeadersAsync(1, stream, stream);

    // ... 
 
    // Disconnect. 
    client.Disconnect();
}

void client_DownloadMessageHeadersCompleted(object sender, ExtendedAsyncCompletedEventArgs<long> e)
{
    // Imap client = (Imap)sender; 
    if (e.Error != null)
    {
        Console.WriteLine("Error: " + e.Error.ToString());
    }
    else 
    {
        Stream stream = (Stream) e.UserState;

        stream.Seek(0, SeekOrigin.Begin);

        // Load the message for reading. 
        MailMessage msg = new MailMessage(stream);

        Console.WriteLine("Message downloaded successfully.");

        foreach (Header h in msg.Headers)
        {
            Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw);
        }
    }
}

Imports System.IO
Imports ComponentPro
Imports ComponentPro.Net.Mail

...

Public Sub DoDownloadMessageHeadersAsync()
    ' Create a new instance of the Imap class. 
    Dim client As New Imap()

    ' Connect to the server. 
    client.Connect("myserver")

    ' Or you can specify the IMAP port with 
    ' client.Connect("myserver", 143); 
 
    ' Login to the server. 
    client.Authenticate("user", "password")

    ' ... 
 
    ' Register an event handler. 
    AddHandler client.DownloadMessageHeadersCompleted, AddressOf client_DownloadMessageHeadersCompleted

    ' Select 'INBOX' mailbox. 
    client.Select("INBOX")

    Dim stream As New MemoryStream()
    ' Download a mail message's headers with sequence number 1 to the specified stream. 
    client.DownloadMessageHeadersAsync(1, stream, stream)

    ' ... 
 
    ' Disconnect. 
    client.Disconnect()
End Sub 
 
Private Sub client_DownloadMessageHeadersCompleted(ByVal sender As Object, ByVal e As ExtendedAsyncCompletedEventArgs(Of Long))
    ' Imap client = (Imap)sender; 
    If e.Error IsNot Nothing Then 
        Console.WriteLine("Error: " & e.Error.ToString())
    Else 
        Dim stream As Stream = CType(e.UserState, Stream)

        stream.Seek(0, SeekOrigin.Begin)

        ' Load the message for reading. 
        Dim msg As New MailMessage(stream)

        Console.WriteLine("Message downloaded successfully.")

        For Each h As Header In msg.Headers
            Console.WriteLine("Header Name: {0}, Value: {1}", h.Name, h.Raw)
        Next h
    End If 
End Sub