What is the best way to create HTML in C# code?

I would use the Html Agility Pack to assemble HTML and then write it out to a text file.

A lot of man-hours went into making the Html Agility Pack robust and HTML-compliant HTML-friendly.

I think it even includes a sample application that generates HTML.

From the home page:

Sample applications:

Page fixing or generation. You can fix a page the way you want, modify the DOM, add nodes, copy nodes, well... you name it.

Is using something like Razor not applicable here? Because if you're doing a lot of html generation using a view engine can make it a lot easier. It was also built to be used outside of ASP.NET.

However sometimes that's not what you need. Have you considered using the TagBuilder class which is part of .net (mvc)? There is also the HtmlWriter in System.Web.UI (for web forms). I would recommend one of these if you are making Controls or Html Helpers.

There are, of course, libraries out there, such as HTML Agility Pack that can assist you in these endeavors.

If you realy don't want to use an existing library, and want simple, down-and-dirty code, I like the idea of abstracting out some of the behaviors like a previous answer stated. I also like the idea of using an underlying StringBuilder, as opposed to a string for a couple reasons:

1. Strings are less efficient than string builder
2. StringBuilder is an indexable data structure,

If I don't need a massively engineered HTML engine, I'd build a simple, intuitive interface

AddLineBreak();
AddSimpleTag(string tagName);
AddSimpleTagAt(string tagName, string content, int index);
Output();

I would use htmltags to create HTML.

Example:

var tag = new HtmlTag("span")
    .Text("Hello & Goodbye")
    .AddClass("important")
    .Attr("title", "Greetings")

And then CSQuery if I want to parse HTML

Example:

dom.Select("div > span")
    .Eq(1)
    .Text("Change the text content of the 2nd span child of each div");

If you do end up using just strings, don't forget to escape all the HTML reserved characters in your output data.

&    &
>    >
<    &lt;
"    &quot;
'    &apos;

I recommend using an HTML-aware class or library instead of working directly with strings, though. HTMLWriter looks like a pretty good start.

You can use any available XML tools to create valid strict XHTML, and (to some extent) HTML4/5. However, in my experience, and as already mentioned HTML Agility Pack will do a job more naturally.

All strings are evil, but very easy and convenient. In a way you should abstract away so much as possible, but it is more work too. It depends on the scale of the system/program, bigger stuff needs more abstraction.