Escape text for HTML

using System.Web;

var encoded = HttpUtility.HtmlEncode(unencoded);

Also, you can use this if you don't want to use the System.Web assembly:

var encoded = System.Security.SecurityElement.Escape(unencoded)

Per this article, the difference between System.Security.SecurityElement.Escape() and System.Web.HttpUtility.HtmlEncode() is that the former also encodes apostrophe (') characters.

You can use actual html tags <xmp> and </xmp> to output the string as is to show all of the tags in between the xmp tags.

Or you can also use on the server Server.UrlEncode or HttpUtility.HtmlEncode.

For a simple way to do this in Razor pages, use the following:

In .cshtml:

@Html.Raw(Html.Encode("<span>blah<span>"))

In .cshtml.cs:

string rawHtml = Html.Raw(Html.Encode("<span>blah<span>"));