I'd guess that you're looking for something like the ExpandoObject in C#. See this question for details on how to use it. Basically, it allows you to add properties to objects at runtime in a manner similar to Javascript. Please note, however, that idiomatic C# doesn't make much use of it.
The analogous C# code for your JavaScript example would be something like this:
dynamic house = new ExpandoObject();
house.kitchen = 1;
house.bedroom = 2;
house.livingroom = 3;