KeyedCollection with enum key type will not index by int

Question

I have classes

public class FooKeyedCollection : KeyedCollection<FooType,Foo>
{
}

public enum FooType
{
   FooType1,
   FooType2,
   FooType3
}

public Foo
{
   public FooType Type { get; set; }
   public string Value { get; set; }
}

When I add items to my FooKeyedCollection, I can index them by FooType, but when I attempt to index them int, the int is interpretted as the FooType enum. As a result, it leads to a confusing error, i.e.

public static void main()
{
   FooKeyedCollection fkc = new FooKeyedCollection();

   Foo myFoo = new Foo();
   myFoo.Type = FooType.FooType3;
   myFoo.Value = "someValue";

   foo.Add( myFoo );

   Foo myFooByType = foo[FooType.FooType3];
   Foo myFooByIndex = foo[0]; // <-- exception thrown here
}

The result of executing this code is an exception when attempting to retrieve the item by integer index. I hope to expose the FooKeyedCollection as part of a public API, and I want to protect the consumers of this API from this error. Is there any way that I can modify the FooKeyedCollection class to get proper behavior?

score 3 · Accepted Answer · answered May 06 '09 at 19:22

You might consider using the "new" keyword here:

public new Foo this[int index]
{
   get
   {
      IList<Foo> self = this;
      return self[index];
   }
   set
   {
       (IList<Foo>)[index] = value;
   }
}

This will allow you to index by integer value. Using the enum explicitly will fall back to the KeyedCollection implementation.

score 3 · Answer 2 · answered May 06 '09 at 19:28

3

Here's a helpful extension method to avoid these cases where the two indexers overlap - it also helps if the key is int:

static class CollectionUtils
{
    public static TValue GetByIndex<TKey,TValue>
        (this KeyedCollection<TKey, TValue> collection, int index)
    {
        return collection[index];
    }
}

then call fkc.GetByIndex(0) and it will work fine. Inside the generic method it can only resolve index to the int indexer.

answered May 06 '09 at 19:28

Marc Gravell

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Personally I think this is the best solution. Aside from anything else, it means you won't go wrong if you end up just using it as a KeyedCollection (where Jeff's solution falls down). – Jon Skeet May 06 '09 at 19:42
@Jon - my thoughts exactly ;-p – Marc Gravell May 06 '09 at 20:00

score 2 · Answer 3 · answered May 06 '09 at 19:24

2

The problem is that 0 is implicitly convertible to FooType. If you use any other number, it'll be fine. If you use this code, it should work too:

int index = 0;
Foo myFooByIndex = foo[index];

EDIT: Unfortunately casting doesn't help here - the compiler still recognises it as a constant 0.

answered May 06 '09 at 19:24

Jon Skeet

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1

I tried casting to (int), but it still resolved as a FooType and threw the exception. – JadeMason May 06 '09 at 19:25

KeyedCollection with enum key type will not index by int

3 Answers3

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