Question

Please see my code:

IDictionary dictionary = new Hashtable();
const string key = "key";
const string value = "value";
dictionary[key] = null; // set some trigger here

// set value
IDictionaryEnumerator dictionaryEnumerator = dictionary.GetEnumerator();
while (dictionaryEnumerator.MoveNext())
{
    DictionaryEntry entry = dictionaryEnumerator.Entry;
    if (entry.Value == null) // some business logic check; check for null value here
    {
        entry.Value = value; // set new value here
        break;
    }
}

Assert.AreEqual(value, dictionary[key]); // I have Fail here!

I wonder :

1. What is correct approach for set new value for IDictionary when
   I do not know the corresponding key.

2. Why my example is not working? As I understand I have set new value
   for DictionaryEntry by value (and value here is a reference) but
   it was not affected in source IDictionary. Why?

Answer 1

DictionaryEntry does not hold a direct reference to the actual values, the internal datastructure is quite different. Therefore, setting the value on a DictionaryEntry will do nothing to the values actually in the Hashtable.

To set a value, you must use the indexer. You can enumerate on the keys rather on the key-value pairs. This code is equivalent of what you tried with DictionaryEntry:

IDictionary dictionary = new Hashtable();
const string key = "key";
const string value = "value";
dictionary[key] = null; // set some trigger here

foreach(var k in dictionary.Keys.OfType<object>().ToArray()) 
{
    if(dictionary[k] == null) 
        dictionary[k] = value;
}