How to get a value from a generic dictionary using a custom comparer for the key?

Question

I have a generic dictionary of objects and want to use a custom comparer to update the value in the dictionary.

myObjects contains a dictionary of objects and the value is the number of times that object exists. Note that the value may be incremented numerous times using different comparators or removed altogether.

testObject is my custom object.

customComparer is a dynamically changing comparer based on the type of testObject. but all comparers are of the type IEqualityComparer<MyObject>

IDictionary<MyObject, int> myObjects;
var testObject;
var customComparer;

if (myObjects.Keys.Contains(testObject, customComparer))
{
    //get the value, if its > 1 then decrement the value
    //else remove the entry entirely
    //not sure how to get the value based on my custom comparer??

    //this code below does not work because it requires the custom comparer
    //var occurrences = myObjects[testObject];
    //if (occurrences > 1)
    //    myObjects[testObject]--;
    //else
    //    myObjects.Remove(testObject);
}
else
{
    myObjects.Add(testObject, 1);
}

I can use Keys.Contains to determine if the object exists with custom comparer but then i'm not sure how to update the value?

If you change the key comparer dynamically, the state of your dictionary could become invalid (it could contain duplicate keys that were not considered duplicates with another comparer). So it doesn't seem to be a very good idea... — Thomas Levesque, Dec 05 '11 at 17:28
Additionally, if you change the comparer, you probably want to change the hash algorithm, too -- but there's no good mechanism for doing so. — mqp, Dec 05 '11 at 17:54
@Thomas: The business logic defines how it should work. Who's to say two objects are NOT the same with two different comparer's? It's a code question, not a question on logistics. — David, Dec 05 '11 at 18:32

Mark Byers · Answer 1 · 2011-12-05T17:48:20.887

4

When you create the dictionary you should provide your custom IEqualityComparer in the constructor. You can't change the equality comparer after the dictionary is constructed.

You could iterate over the key value pairs until you find a key that matches according to your custom comparer, but then you aren't taking advantage of the features that a Dictionary provides.

edited Dec 05 '11 at 17:48

answered Dec 05 '11 at 17:27

Mark Byers

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There would be no reason to pass an IEqualityComparer into the constructor if I require to change the comparer dynamically. – David Dec 06 '11 at 08:47
You can't change the equality comparer of a dictionary dynamically. Are you sure you are looking for a dictionary? Perhaps a list of key value pairs would suit your needs better? – Mark Byers Dec 06 '11 at 09:52
I wasn't adding the equality comparer into the constructor in the first place, rather using the .Contains method and using the overloaded method to pass in a comparer. Alternatively I have created a wrapper equality comparer than internally contains my list of other comparers. This wrapper I do insert into the constructor and use that as the basis to compare objects. Am having a little trouble with different hash codes at the moment but will work it out. – David Dec 06 '11 at 10:25

score 1 · Accepted Answer · answered Dec 06 '11 at 13:43

There was no quick imlpementation for this. Instead, I created a wrapper IEqualityComparer that housed numerous internal equality comparers. The wrapper overwrote the Equals and GetHashCode methods which picked the appropriate internal dictionary of equality comparers based on a property from the comparison object.

public class WrapperComparer : IEqualityComparer<MyObject>
{
    private IDictionary<string, IEqualityComparer<MyObject>> _myComparerList;

    public bool Equals(MyObject x, MyObject y)
    {
        var comparer = _myComparerList[x.SomeProperty];
        return comparer.Equals(x, y);
    }

    public bool GetHashCode(MyObject obj)
    {
        var comparer = _myComparerList[obj.SomeProperty];
        return comparer.GetHashCode(obj);
    }
}

Then the comparison works...

var testObject;
var customComparer = new WrapperComparer(list of comparers here);
IDictionary<MyObject, int> myObjects = 
    new Dictionary<MyObject, int>(customComparer);

if (myObjects.ContainsKey(testObject))
{
    var occurrences = myObjects[testObject];
    if (occurrences > 1)
      myObjects[testObject]--;
    else
      myObjects.Remove(testObject);
}
else
{
    myObjects.Add(testObject, 1);
}

How to get a value from a generic dictionary using a custom comparer for the key?

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