Использование YamlDo tNet:
var dictionary = new Dictionary<string, string>();
dictionary.Add("key", "value");
dictionary.Add("key2", "value2");
var serializer = new SerializerBuilder().Build();
var yaml = serializer.Serialize(dictionary);
Console.WriteLine(yaml);
Вывод:
key: value
key2: value2
https://dotnetfiddle.net/vadyyf