Question

I am trying to optimize a piece of .NET 2.0 C# code that looks like this:

Dictionary<myType, string> myDictionary = new Dictionary<myType, string>();
// some other stuff
// inside a loop check if key is there and if not add element
if(!myDictionary.ContainsKey(currentKey))
{
   myDictionary.Add(currentKey, "");
}

Looks like the Dictionary has been used by whoever wrote this piece of code even if not needed (only the key is being used to store a list of unique values) because faster than a List of myType objects for search.
This seems obviously wrong as only the key of the dictionary but I am trying to understand what’s the best way to fix it.

Questions:

1) I seem to understand I would get a good performance boost even just using .NET 3.5 HashSet. Is this correct?

2) What would be the best way to optimize the code above in .NET 2.0 and why?

EDIT:
This is existing code I am trying to optimize, it’s looping through dozens of thousands items and for each one of them is calling a ContainsKey. There’s gotta be a better way of doing it (even in .NET 2.0)! 🙂

Answer 1

I think you need to break this down into 2 questions

Is Dictionary<myType,string> the best available type for this scenario

No. Based on your breakdown, HashSet<myType> is clearly the better choice because it’s usage pattern more accurately fits the scenario

Will switching to Hashset<myType> give me a performance boost?

This is really subjective and only a profiler can give you the answer to this question. Likely you’ll see a very minor memory size improvement per element in the collection. But in terms of raw computing power I doubt you’ll see a huge difference. Only a profiler can tell you if there is one.

Before you ever make a performance related change to your code remember the golden rule.

Don’t make any performance related changes until a profiler has told you precisely what is wrong with your code.

Making changes which violate this rule are just guesses. A profiler is the only way to measure success of a performance fix.