Question

I have a dictionary structure, with multiple key value pairs inside.

myDict.Add(key1, value1);
myDict.Add(key2, value2);
myDict.Add(key3, value3);

My dictionary is used as a data source for some control. In the control’s dropdown I see the items are like this:

key1
key2
key3

The order looks identical to my dictionary.
I know Dictionary is not like arrayList – you can get the index or so.
I cannot use sortedDictionary.
Now I need to add one more key value pair to this dictionary at some point of my program and I hope it has the same effect as I do this:

myDict.Add(newKey, newValue);
myDict.Add(key1, value1);
myDict.Add(key2, value2);
myDict.Add(key3, value3);

If I do this, I know newKey will display in my control as first element.

I have an idea to create a tempDict, put each pair in myDict to tempDict, then clear myDict, then add pairs back like this:

myDict.Add(newKey, newValue);
myDict.Add(key1, value1);
myDict.Add(key2, value2);
myDict.Add(key3, value3);

Is there better way than this?

Thanks!

Answer 1

Dictionary<K,V> does not have an ordering. Any perceived order maintenance is by chance (and an artifact of a particular implementation including, but not limited to, bucket selection order and count).

These are the approaches (just using the Base Class Libraries BCL) I know about:

1. Lookup<K,V>
   • .NET4, immutable, can map keys to multiple values (watch for duplicates during building)
2. OrderedDictionary
   • Old, non-generic, expected Dictionary performance bounds (other two approaches are O(n) for “get(key)/set(key)”)
3. List<KeyValuePair<K,V>>
   • .NET2/3 okay, mutable, more legwork, can map keys to multiple values (watch for duplicates in inserts)

Happy coding.

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Creating a hash data-structure that maintains insertion order is actually only a slight modification of a standard hash implementation (Ruby hashes now maintain insertion order); however, this was not done in .NET nor, more importantly, is it part of the Dictionary/IDictionary contract.