Template.cs

public class Template
{
    private static Logger _logger = LogManager.GetCurrentClassLogger();

    public readonly string Body;
    private HashSet<string> _keys;
    public const string KeyExpression = @"{(?<key>[^}]+)}";

    public bool RenderEmptyStringForMissingKeys { get; set; }

    public Template(string body)
    {
        Body = body;
        _keys = new HashSet<string>();

        //Collect all the keys from the template
        Regex.Replace(Body, KeyExpression, k => { 
            _keys.Add(k.Groups["key"].Value);
            return "";
        });
    }

    private static Dictionary<string,object> _emptyDefaults = new Dictionary<string, object>();

    public string Render(IDictionary<string,object> args, IDictionary<string,object> defaults = null)
    {
        defaults = defaults ?? _emptyDefaults;
        //Validate(args);
        return Regex.Replace(Body, KeyExpression, m => Replace(m, args, fallback));
    }

    private void Validate(IDictionary<string, object> args)
    {
        var message = String.Format("Rendering template with {0} keys and {1} args", _keys.Count, args.Count);
        _logger.Trace(message);
        foreach (string key in _keys.Except(args.Keys))
        {
            _logger.Warn("Unassigned key: " + key);
        }
        foreach (string key in args.Keys.Except(_keys))
        {
            _logger.Warn("Unused arg: " + key);
        }
    }

    private string Replace(Match match, IDictionary<string,object> args, IDictionary<string,object> defaults)
    {
        string key = match.Groups["key"].Value;

        if (!args.ContainsKey(key)) args = defaults;

        if (args.ContainsKey(key))
        {
            string result = args[key].ToString();
            return result;
        }
        else if (RenderEmptyStringForMissingKeys) return "";
        else return "{" + key + "}";
    }

    public static Template Load(string path)
    {
        string body = File.ReadAllText(path);
        return new Template(body);
    }
}