Hoge.cs

using System.Collections.Generic;

public class Hoge
{
  // jsonを読み込んで作られたIDictionaryを使って初期化
  public Hoge (IDictionary dict)
  {
    Dictionary<string, Dictionary<string, int>> fugaMap = dict["fuga"] as Dictionary<string, Dictionary<string, int>>; // nullになる
    int fugaID = fugaMap["fugaID"]; // error!!
  }
}