Add a Custom Header to a Related Item

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Demonstrates the Chilkat Email.AddRelatedHeader method, which adds a custom MIME header field to an existing related item identified by its zero-based index. This example first adds a related style sheet (which becomes index 0) and captures its generated Content-ID, builds an HTML body that references the style sheet by that cid:, then attaches an extra header field to the related item and prints the resulting MIME.

Background: Like attachments, each related item (an inline image, style sheet, etc.) is a MIME part with its own small header block describing that part — Content-Type, Content-ID, Content-Location, and so on. AddRelatedHeader lets you insert additional fields into that per-part block, which is occasionally required for interoperability with clients that look for specific custom headers on embedded resources.

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//  Demonstrates the AddRelatedHeader method, which adds a custom MIME header field to an
//  existing related item, identified by its zero-based index.

Chilkat.Email email = new Chilkat.Email();
email.Subject = "Related item with a custom header";

//  Add a related item first (a style sheet); capture its generated Content-ID.  It becomes
//  related-item index 0.
string cid = email.AddRelatedString("styles.css","body { color: black; }","utf-8");
if (email.LastMethodSuccess == false) {
    Debug.WriteLine(email.LastErrorText);
    return;
}

//  Build the HTML body, referencing the related style sheet by its Content-ID.  A
//  placeholder is used and then replaced with the actual Content-ID.
Chilkat.StringBuilder sbHtml = new Chilkat.StringBuilder();
sbHtml.Append("<html><head><link rel=\"stylesheet\" href=\"cid:PLACEHOLDER_CID\"/></head><body>Styled content.</body></html>");
int numReplaced = sbHtml.Replace("PLACEHOLDER_CID",cid);
email.SetHtmlBody(sbHtml.GetAsString());

//  Add a custom header field to the first related item (index 0).
email.AddRelatedHeader(0,"X-Custom-Related-Header","some value");

//  The custom header now appears in the related item's MIME part.
Debug.WriteLine(email.GetMime());