What's a good way of doing string templating in .NET?

Here is the version for those of you who can use a new version of C#:

// add $ at start to mark string as template
var template = $"Your job finished at {FinishTime} and your file is available for download at {FileURL}."

In a line - this is now a fully supported language feature (string interpolation).

Use a templating engine. StringTemplate is one of those, and there are many.

Example:

using Antlr.StringTemplate;
using Antlr.StringTemplate.Language;
 
StringTemplate hello = new StringTemplate("Hello, $name$", typeof(DefaultTemplateLexer));
hello.SetAttribute("name", "World");
Console.Out.WriteLine(hello.ToString());

You can use the "string.Format" method:

var user = GetUser();
var finishTime = GetFinishTime();
var fileUrl = GetFileUrl();
var signature = GetSignature();
string msg =
@"Dear {0},

Your job finished at {1} and your file is available for download at {2}.

Regards,

--
{3}";
msg = string.Format(msg, user, finishTime, fileUrl, signature);

It allows you to change the content in the future and is friendly for localization.