C#: String named format parameters.

StringUtils.cs

// Modified based on http://stackoverflow.com/questions/1322037/

public static class StringUtils
{
    private static Regex FormatPattern =
        new Regex(@"(\{+)([^\}]+)(\}+)", RegexOptions.Compiled);

    public static string Format(this string pattern, object template)
    {
        if (template == null)
        {
            throw new ArgumentNullException("template");
        }

        var type = template.GetType();
        var cache = new Dictionary<string, string>();

        return FormatPattern.Replace(pattern, match =>
        {
            var lCount = match.Groups[1].Value.Length;
            var rCount = match.Groups[3].Value.Length;

            if ((lCount % 2) != (rCount % 2))
            {
                throw new InvalidOperationException("Unbalanced braces");
            }

            var lBrace = lCount == 1 ? "" : new string('{', lCount / 2);
            var rBrace = rCount == 1 ? "" : new string('}', rCount / 2);

            var key = match.Groups[2].Value;
            string value;

            if (lCount % 2 == 0)
            {
                value = key;
            }
            else
            {
                if (!cache.TryGetValue(key, out value))
                {
                    var prop = type.GetProperty(key);
                    if (prop == null)
                    {
                        throw new ArgumentException("Not found: " + key, " pattern");
                    }

                    value = Convert.ToString(prop.GetValue(template, null));
                    cache.Add(key, value);
                }
            }

            return lBrace + value + rBrace;
        });
    }
}