Question

I’ve used VS profilier and noticed that ~40% of the time program spends in the lines below.
I’m using title1 and color1 because either Visual Studio or Resharper suggested to do so. Are there any perfomance issues in the code below?

Dictionary<Item, int> price_cache = new Dictionary<Item, int>();
....


string title1 = title;
string color1 = color;
if (price_cache.Keys.Any(item => item.Title == title && item.Color == color))
{
    price = price_cache[price_cache.Keys.First(item => item.Title == title11  && item.Color == color1)];

Answer 1

The problem is that your Keys.Any method iterates through all keys in your dictionary to find if there is a match. After that, you use the First method to do the same thing again.

Dictionary is suited for operations when you already have the key and want to get the value fast. In that case, it will calculate the hash code of your key (Item, in your case) and use it to “jump” to the bucket where your item is stored.

First, you need to make your custom comparer to let the Dictionary know how to compare items.

class TitleColorEqualityComparer : IEqualityComparer<Item>
{
     public bool Equals(Item a, Item b) 
     {
          // you might also check for nulls here
          return a.Title == b.Title && 
              a.Color == b.Color;
     }

     public int GetHashCode(Item obj)
     {
          // this should be as much unique as possible,
          // but not too complicated to calculate
          int hash = 17;
          hash = hash * 31 + obj.Title.GetHashCode();
          hash = hash * 31 + obj.Color.GetHashCode();
          return hash;
     }
}

Then, instantiate your dictionary using your custom comparer:

Dictionary<Item, int> price_cache = 
    new Dictionary<Item, int>(new TitleColorEqualityComparer());

From this point on, you can simply write:

Item some_item = GetSomeItem();
price_cache[some_item] = 5; // to quickly set or change a value

or, to search the dictionary:

Item item = GetSomeItem();
int price = 0;
if (price_cache.TryGetValue(item, out price))
{
    // we got the price
}
else
{
    // there is no such key in the dictionary
}

[Edit]

And to emphasize again: never iterate the Keys property to look for a key. If you do that, you don’t need a Dictionary at all, you can simply use a list and get same (even slightly better performance).