What happens to C# Dictionary lookup if the key does not exist?

Assuming you want to get the value if the key does exist, use Dictionary<TKey, TValue>.TryGetValue:

int value;
if (dictionary.TryGetValue(key, out value))
{
    // Key was in dictionary; "value" contains corresponding value
} 
else 
{
    // Key wasn't in dictionary; "value" is now 0
}

(Using ContainsKey and then the indexer makes it look the key up twice, which is pretty pointless.)

Note that even if you were using reference types, checking for null wouldn’t work – the indexer for Dictionary<,> will throw an exception if you request a missing key, rather than returning null. (This is a big difference between Dictionary<,> and Hashtable.)