[c#] Generating HTML email body in C#

Is there a better way to generate HTML email in C# (for sending via System.Net.Mail), than using a Stringbuilder to do the following:

string userName = "John Doe";
StringBuilder mailBody = new StringBuilder();
mailBody.AppendFormat("<h1>Heading Here</h1>");
mailBody.AppendFormat("Dear {0}," userName);
mailBody.AppendFormat("<br />");
mailBody.AppendFormat("<p>First part of the email body goes here</p>");

and so on, and so forth?

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();

You might want to have a look at some of the template frameworks that are available at the moment. Some of them are spin offs as a result of MVC but that isn't required. Spark is a good one.

A commercial version which I use in production and allows for easy maintenance is LimiLabs Template Engine, been using it for 3+ years and allows me to make changes to the text template without having to update code (disclaimers, links etc..) - it could be as simple as

Contact templateData = ...; 
string html = Template
     .FromFile("template.txt")
     .DataFrom(templateData )
     .Render();

Worth taking a look at, like I did; after attempting various answers mentioned here.

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();
}

As an alternative to MailDefinition, have a look at RazorEngine https://github.com/Antaris/RazorEngine.

This looks like a better solution.

Attributted to...

how to send email wth email template c#

E.g

using RazorEngine;
using RazorEngine.Templating;
using System;

namespace RazorEngineTest
{
    class Program
    {
        static void Main(string[] args)
        {
    string template =
    @"<h1>Heading Here</h1>
Dear @Model.UserName,
<br />
<p>First part of the email body goes here</p>";

    const string templateKey = "tpl";

    // Better to compile once
    Engine.Razor.AddTemplate(templateKey, template);
    Engine.Razor.Compile(templateKey);

    // Run is quicker than compile and run
    string output = Engine.Razor.Run(
        templateKey, 
        model: new
        {
            UserName = "Fred"
        });

    Console.WriteLine(output);
        }
    }
}

Which outputs...

<h1>Heading Here</h1>
Dear Fred,
<br />
<p>First part of the email body goes here</p>

Heading Here

Dear Fred,

First part of the email body goes here

I use dotLiquid for exactly this task.

It takes a template, and fills special identifiers with the content of an anonymous object.

//define template
String templateSource = "<h1>{{Heading}}</h1>Dear {{UserName}},<br/><p>First part of the email body goes here");
Template bodyTemplate = Template.Parse(templateSource); // Parses and compiles the template source

//Create DTO for the renderer
var bodyDto = new {
    Heading = "Heading Here",
    UserName = userName
};
String bodyText = bodyTemplate.Render(Hash.FromAnonymousObject(bodyDto));

It also works with collections, see some online examples.

Emitting handbuilt html like this is probably the best way so long as the markup isn't too complicated. The stringbuilder only starts to pay you back in terms of efficiency after about three concatenations, so for really simple stuff string + string will do.

Other than that you can start to use the html controls (System.Web.UI.HtmlControls) and render them, that way you can sometimes inherit them and make your own clasess for complex conditional layout.

I would recomend using templates of some sort. There are various different ways to approach this but essentially hold a template of the Email some where (on disk, in a database etc) and simply insert the key data (IE: Recipients name etc) into the template.

This is far more flexible because it means you can alter the template as required without having to alter your code. In my experience your likely to get requests for changes to the templates from end users. If you want to go the whole hog you could include a template editor.

If you don't want a dependency on the full .NET Framework, there's also a library that makes your code look like:

string userName = "John Doe";

var mailBody = new HTML {
    new H(1) {
        "Heading Here"
    },
    new P {
        string.Format("Dear {0},", userName),
        new Br()
    },
    new P {
        "First part of the email body goes here"
    }
};

string htmlString = mailBody.Render();

It's open source, you can download it from http://sourceforge.net/projects/htmlplusplus/

Disclaimer: I'm the author of this library, it was written to solve the same issue exactly - send an HTML email from an application.