A very strange System.Collections.Generic.KeyNotFoundException in Dictionary

Question

I have a strange error in my code...

Code:

public class PropertyCollection<T> : IDictionary<T, string>
{
        private Dictionary<T, string> dict;
        ...
        public string this[T key]
        {
            get
            {
                bool has_key = this.Keys.Any(x => x == key); 
                return this.dict[key];
            }
            set
            {
                this.dict[key] = value;
            }
        }
        ...
}

The first row

bool has_key = this.Keys.Any(x => x == key);

returns true.

But

return this.dict[key];

throws the error:

System.Collections.ListDictionaryInternal.NodeKeyValueCollection: The given key was not present in the dictionary.

How this could be?

If I change the row, that throws an exception, to

return this.dict[this.Keys.First(x => x == key)];

everything becomes normal and there is now error.

Doing this: `bool has_key = this.Keys.Any(x => x == key);` defeats the purpose of using a dictionary. — Yuval Itzchakov, Oct 15 '15 at 14:14
If you want us to help, post a small, complete, compiling reproduce of your problem. — Yuval Itzchakov, Oct 15 '15 at 14:15
What is `T` in this case? Do you override `Equals` on `T`? Did you also override `==`? Does the `Dictionary` use the default `EqualityComparer` or a different one? There are lots of reasons this could be the case. Some code demonstrating the issue would be useful. — Charles Mager, Oct 15 '15 at 14:16
What is `this.Keys`? You appear to be confusing `this.Keys` with `this.dict.Keys`. They aren't necessarily the same thing. — Alex Walker, Oct 15 '15 at 14:17
Well, in your code you don't use Dictionary collection in a proper way. So google for how to use it and read some information about this matter. If you'll still have some problem - create short but complete example of your problem and post it to SO — Fabjan, Oct 15 '15 at 14:20
Also, why is `x == key` allowed when the right-hand side `key` has type `T` which is a type parameter without any contraints? Are you hiding some constraint from us? — Jeppe Stig Nielsen, Oct 15 '15 at 14:22
First few comments are strange) bool has_key = this.Keys.Any(x => x == key); - are givin just to look what is going on, of cause. — BigMan, Oct 15 '15 at 14:22
@BigMan First few comments must have been posted before the posters have finished reading your question. — Sergey Kalinichenko, Oct 15 '15 at 14:23
@BigMan well show some code that tells us what you are overriding and how you are doing it. It's likely this that causes the difference. — Charles Mager, Oct 15 '15 at 14:27

score 2 · Accepted Answer · edited May 23 '17 at 10:27

2

Assuming that T overrides both Equals and GetHashCode^*, the only way I could thing how this would happen is if your key is mutable, its GetHashCode method uses mutable fields, and you called a mutating method after the key has been inserted into the dictionary.

This would make the linear search this.Keys.Any(x => x == key) produce true, but hash-based search would produce an exception.

Confirming this is very easy: the code below should print false:

var first = dict.Keys.First(x => x == key);
Console.WriteLine(first.GetHashCode() == key.GetHashCode());

^* If T does not override one or both these methods, see this Q&A.

edited May 23 '17 at 10:27

Community

1
1

answered Oct 15 '15 at 14:19

Sergey Kalinichenko

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1

Another possibility is that `==` and `Equals` have different semantics for the relevant type `T`. – Jeppe Stig Nielsen Oct 15 '15 at 14:23
Or the classical one: `T` overrides `Equals`, but it does not override `GetHashCode`, or that override calls `base.GetHashCode` or is incorrect in some other way. – Jeppe Stig Nielsen Oct 15 '15 at 14:31
@JeppeStigNielsen Yes, that's very likely too. I edited to mention a related Q&A. – Sergey Kalinichenko Oct 15 '15 at 14:37
3

Yes, you are right. `T` overrides both `Equals` and `GetHashCode` and even `==` and `!=`, but `GetHashCode` was `return base.GetHashCode() ` My mistake. I am ashamed for this stupid error( – BigMan Oct 15 '15 at 14:51
1

This code overrides `GetHashCode()` but fails from mutability: `using System.Collections.Generic; using System.Linq; sealed class MuatbleInt { public int Val { get; set; } public override bool Equals(object obj) { var other = obj as MuatbleInt; return (object)other != (object)null && other.Val == Val; } public override int GetHashCode() { return Val; } internal static void Main() { var key = new MuatbleInt { Val = 42, }; var dict = new Dictionary { { key, "" }, }; key.Val = 666; var hasKey = dict.Keys.Any(key.Equals); var bang = dict[key]; } }` – Jeppe Stig Nielsen Oct 15 '15 at 15:00

A very strange System.Collections.Generic.KeyNotFoundException in Dictionary

1 Answers1