Generating HTML email body in C#

You can use the MailDefinition class.

This is how you use it:

MailDefinition md = new MailDefinition();
md.From = "[email protected]";
md.IsBodyHtml = true;
md.Subject = "Test of MailDefinition";

ListDictionary replacements = new ListDictionary();
replacements.Add("{name}", "Martin");
replacements.Add("{country}", "Denmark");

string body = "<div>Hello {name} You're from {country}.</div>";

MailMessage msg = md.CreateMailMessage("[email protected]", replacements, body, new System.Web.UI.Control());

Also, I've written a blog post on how to generate HTML e-mail body in C# using templates using the MailDefinition class.

Use the System.Web.UI.HtmlTextWriter class.

StringWriter writer = new StringWriter();
HtmlTextWriter html = new HtmlTextWriter(writer);

html.RenderBeginTag(HtmlTextWriterTag.H1);
html.WriteEncodedText("Heading Here");
html.RenderEndTag();
html.WriteEncodedText(String.Format("Dear {0}", userName));
html.WriteBreak();
html.RenderBeginTag(HtmlTextWriterTag.P);
html.WriteEncodedText("First part of the email body goes here");
html.RenderEndTag();
html.Flush();

string htmlString = writer.ToString();

For extensive HTML that includes the creation of style attributes HtmlTextWriter is probably the best way to go. However it can be a bit clunky to use and some developers like the markup itself to be easily read but perversly HtmlTextWriter's choices with regard indentation is a bit wierd.

In this example you can also use XmlTextWriter quite effectively:-

writer = new StringWriter();
XmlTextWriter xml = new XmlTextWriter(writer);
xml.Formatting = Formatting.Indented;
xml.WriteElementString("h1", "Heading Here");
xml.WriteString(String.Format("Dear {0}", userName));
xml.WriteStartElement("br");
xml.WriteEndElement();
xml.WriteElementString("p", "First part of the email body goes here");
xml.Flush();

Updated Answer:

The documentation for SmtpClient, the class used in this answer, now reads, 'Obsolete("SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead")'.

Source: https://www.infoq.com/news/2017/04/MailKit-MimeKit-Official

Original Answer:

Using the MailDefinition class is the wrong approach. Yes, it's handy, but it's also primitive and depends on web UI controls--that doesn't make sense for something that is typically a server-side task.

The approach presented below is based on MSDN documentation and Qureshi's post on CodeProject.com.

NOTE: This example extracts the HTML file, images, and attachments from embedded resources, but using other alternatives to get streams for these elements are fine, e.g. hard-coded strings, local files, and so on.

Stream htmlStream = null;
Stream imageStream = null;
Stream fileStream = null;
try
{
    // Create the message.
    var from = new MailAddress(FROM_EMAIL, FROM_NAME);
    var to = new MailAddress(TO_EMAIL, TO_NAME);
    var msg = new MailMessage(from, to);
    msg.Subject = SUBJECT;
    msg.SubjectEncoding = Encoding.UTF8;
 
    // Get the HTML from an embedded resource.
    var assembly = Assembly.GetExecutingAssembly();
    htmlStream = assembly.GetManifestResourceStream(HTML_RESOURCE_PATH);
 
    // Perform replacements on the HTML file (if you're using it as a template).
    var reader = new StreamReader(htmlStream);
    var body = reader
        .ReadToEnd()
        .Replace("%TEMPLATE_TOKEN1%", TOKEN1_VALUE)
        .Replace("%TEMPLATE_TOKEN2%", TOKEN2_VALUE); // and so on...
 
    // Create an alternate view and add it to the email.
    var altView = AlternateView.CreateAlternateViewFromString(body, null, MediaTypeNames.Text.Html);
    msg.AlternateViews.Add(altView);
 
    // Get the image from an embedded resource. The <img> tag in the HTML is:
    //     <img src="pid:IMAGE.PNG">
    imageStream = assembly.GetManifestResourceStream(IMAGE_RESOURCE_PATH);
    var linkedImage = new LinkedResource(imageStream, "image/png");
    linkedImage.ContentId = "IMAGE.PNG";
    altView.LinkedResources.Add(linkedImage);
 
    // Get the attachment from an embedded resource.
    fileStream = assembly.GetManifestResourceStream(FILE_RESOURCE_PATH);
    var file = new Attachment(fileStream, MediaTypeNames.Application.Pdf);
    file.Name = "FILE.PDF";
    msg.Attachments.Add(file);
 
    // Send the email
    var client = new SmtpClient(...);
    client.Credentials = new NetworkCredential(...);
    client.Send(msg);
}
finally
{
    if (fileStream != null) fileStream.Dispose();
    if (imageStream != null) imageStream.Dispose();
    if (htmlStream != null) htmlStream.Dispose();
}