I'm going nuts every time I see the code similar to the following snippet.

/* Given:
 * IDictionary<TKey, TValue> dictionary;
 * TKey key;
 */
if (dictionary.ContainsKey(key) 
{
    var value = dictionary[key];
    // do something with retrieved value
}

You should really use TryGetValue instead:

/* Given:
 * IDictionary<TKey, TValue> dictionary;
 * TKey key;
 */
TValue value;
if (dictionary.TryGetValue(key, out value) 
{
    // do something with retrieved value
}

And here is a couple of reasons why:

• It is more performant as the location of the value calculated only once. And in fact implementation of ContainsKey, Item and TryGetValue is exactly the same.
• In the first example the dictionary could be changed between ContainsKey and Item method calls, and the retrieving of the item could fail in multi-threaded environment.

I was so crazy about this, so couple of month ago I wrote a plugin for ReSharper which can analyze and optimize such subsequent calls.