CODEWITHSUNDEEP
c#
Andriy Tylychko
Andriy Tylychko

Reputation: 16256

Adding to or updating a dictionary with not reference type value

Before flagging this as a duplicate please double check.

I'm accumulating some stats in dictionary.

Dictionary<string, int> stats;

When an event happens I update stats:

if (!stats.ContainsKey(eventName))
    stats.Add(eventName, eventValue);
else
    stats[eventName] = stats[eventName] + eventValue;

The problem is this code does 2 lookups for the first occurrence of an event and 3 lookups otherwise. The first occurrence is not a big deal. But those 3 lookups for updating are really bad. How could I improve this?

Upvotes: 1

Views: 111

Answers (1)

Jon Skeet
Jon Skeet

Reputation: 1500185

You can fetch the value when you check for containment. In fact, in this case you can make it unconditional:

int value;
stats.TryGetValue(eventName, out value);
stats[eventName] = value + eventValue;

This uses the fact that value will be 0 if there's no entry for eventName. You could make it explicit if you want:

int value;
if (!stats.TryGetValue(eventName, out value))
{
    stats.Add(eventName, eventValue);
}
else
{
    stats[eventName] = value + eventValue;
}

Either way, you're now down to one fetch and one set.

Alternatively, you could use ConcurrentDictionary and its AddOrUpdate method.

Upvotes: 4

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