Rust

Add a Custom Header to a Related Item

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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Rust

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

let email = chilkat::Email::new();
email.set_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.
let Ok(cid) = email.add_related_string("styles.css", "body { color: black; }", "utf-8") else {
    println!("{}", email.last_error_text());
    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.
let sb_html = chilkat::StringBuilder::new();
let _ = sb_html.append("<html><head><link rel=\"stylesheet\" href=\"cid:PLACEHOLDER_CID\"/></head><body>Styled content.</body></html>");
let num_replaced = sb_html.replace("PLACEHOLDER_CID", &cid);
email.set_html_body(&sb_html.get_as_string().unwrap_or_default());

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

// The custom header now appears in the related item's MIME part.
println!("{}", email.get_mime().unwrap_or_default());