Question

I have to work on some code that’s using generic lists to store a collection of custom objects.

Then it does something like the following to check if a given object’s in the collection and do something if so:

List<CustomObject> customObjects; //fill up the list List<CustomObject> anotherListofCustomObjects; //fill it up  //...  foreach (CustomObject myCustomObject in customObjects) {    if (anotherListofCustomObjects.Contains(myCustomObject))    {       //do stuff    } } 

Problem is is taking forever to process 7000 objects like that.

This is not my code – I am just trying to come up options to improve it – Looks to me it would be much faster to use a dictionary to get the stuff by key instead of looping through the whole collection like the above.

Suggestions?

Answer 1

Well, you seem to have answered it yourself? If you need fast query against a set of data, then a dictionary may be better than a flat list (for largish data sizes, which yours is).

You could, for example, use the object as its own key –

Dictionary<CustomObject,CustomObject> ... 

Note that the meaning of equality depends on the context. If you are passing in the original reference, then that is fine – ContainsKey would do the job. If you have a different but similar-for-the-purposes-of-equality object to compare to, then you’ll need to implement your own GetHashCode(), Equals(), and ideally IEquatable<CustomObject>. Either in CustomObject itself, or in a custom IEqualityComparer<CustomObject>.