using System;

using System.Text.RegularExpressions;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var keyValue = new KeyValuePair<string,string>("FullName", "John");

        string docText = "Dear {{FullName}}";

        string result = docText.Replace("{{" + keyValue.Key + "}}", keyValue.Value);

        Console.WriteLine(result);

    }

}