Question

I came across few articles / explanations on shallow copying and deep copy of hashtables, the more I read, the more I am confused.

Hashtable ht = new Hashtable();
ht.Add("1", "hello");

Hashtable ht2 = new Hashtable();
ht2 = ht;                          // case1: is this shallow copy?
ht2["1"] = "H2";

Hashtable ht3 = new Hashtable(ht); // case2: is this shallow copy?
ht3["1"] = "H3";

Hashtable ht4 = new Hashtable();
ht4 = (Hashtable)ht.Clone();       // case3: is this shallow copy?
ht4["1"] = "H4";

• Case1: result, ht content change become same with ht2.
• Case2: result, ht content different with ht3.
• Case3: result, ht content different with ht4.

If Case2 and Case3 are shallow copying, shouldn’t the result be the same as Case1?

Does this happen to List, ArrayList, etc as well?

Answer 1

In case 1, both ht2 and ht refer to the same instance of Hashtable.

In cases 2 and 3, ht3 and ht4 refer to different objects created by copying the original Hashtable entries.

Note that even when taking a “deep” copy (creating a new mapping) you’d still be copying references. So for example:

var original = new Dictionary<int, StringBuilder>();
original[10] = new StringBuilder();

var copy = new Dictoinary<int, StringBuilder>(original);
copy[20] = new StringBuilder();

// We have two different maps...
Assert.IsFalse(original.ContainsKey(20));

// But they both refer to a single StringBuilder in the entry for 10...
copy[10].Append("Foo");
Assert.AreEqual("Foo", original[10].ToString());