【求助大神】如何动态拼接多维索引？

Dictionary<string, string> a = new Dictionary<string, string>();
a.Add("akey","val");
Dictionary<string, Dictionary<string, string>> b = new Dictionary<string, Dictionary<string, string>>();
b.Add("bkey",a);
print(b["bkey"]["akey"]);//正常写法
//想要的过程
string[] keys = new[] { "bkey", "akey" };
foreach (string item in keys)
{
//通过某种形式，想直接拼接成 ["bkey"]["akey"]，
}
//最后对变量b直接进行赋值操作， 例如：b["bkey"]["akey"] = "1234";