new.cs

 SimSet setInst = (dynamic)new ObjectInstance("SimSet", "TestObject1")
 {
    Dynamics =
    {
       Fish1 = "TestVal"
    }
 };

 SimObject inst = (dynamic)new SimObjectInstance("TestObject2")
 {
    Dynamics =
    {
       Fish1 = "TestVal2"
    }
 };
 setInst.add(inst);
 omni.Util._echo(setInst["Fish1"] + " " + inst["Fish1"] + " " + ((SimObject)setInst.getObject(0))["Fish1"]);

old.cs

 ObjectCreator objectCreator1 = new ObjectCreator("SimSet", "TestObject6");
 objectCreator1["Fish1"] = "TestVal2";
 ObjectCreator objectCreator2 = new ObjectCreator("SimObject", "TestObject7");
 objectCreator2["Fosj1"] = "TestVal2";
 objectCreator1["#object1"] = objectCreator2;
 setInst = objectCreator1.Create();
 omni.Util._echo(setInst["Fish1"] + " " + ((SimObject)setInst.getObject(0))["Fish1"]);