using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{

    private static Regex _regex = new Regex(@"(?<={)(\w+)");

​

    public static void Main()

    {

        string line = "My values are {first} and {second}";

        var dict = new Dictionary<string, object>()

        {

            ["first"] = 1,

            ["second"] = 2

        };

        string formatted = Format(line, dict);

        Console.WriteLine(formatted);

    }

​

    public static string Format(string rawMsg, IReadOnlyDictionary<string, object> dict) =>

​

        _regex.Replace(rawMsg, match =>

        {

            string placeHolder = match.ToString();

            return dict.TryGetValue(placeHolder, out object placeHolderValue) 

                        ? placeHolderValue.ToString() : null;

        });

}