.FormatWith() extension method that we use with some of our Umbraco sites

StringExtensions.cs

using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.Web;

namespace MyUmbracoSite.Core.Extensions
{
    public static class StringExtensions
    {
        // http://bendetat.com/the-greatest-string-formatwith-implementation-in-the-world.html

        public static string FormatWith(this string format, params object[] args)
        {
            if (format == String.Empty) return format;

            args = args ?? new object[0];
            string result;
            var numberedTemplateCount = (from object match in new Regex(@"\{\d{1,2}\}").Matches(format) select match.ToString()).Distinct().Count();

            if (numberedTemplateCount != args.Length)
            {
                var argsDictionary = args[0].ToDictionary();

                if (!argsDictionary.Any())
                {
                    throw new InvalidOperationException("Please supply enough args for the numbered templates or use an anonymous object to identify the templates by name.");
                }

                result = argsDictionary.Aggregate(format, (current, o) => current.Replace("{" + o.Key + "}", (o.Value ?? string.Empty).ToString()));
            }
            else
            {
                result = string.Format(format, args);
            }

            if (result == format && result.Contains("{"))
            {
                throw new InvalidOperationException("You cannot mix template types. Use numbered templates or named ones with an anonymous object.");
            }

            return result;
        }
    }
}

"This string is {foo}".FormatWith(new { foo = "bar" };
// returns "This string is bar"