C# Dictionary ContainsKey

Question

My problem is ContainsKey is always returning false even when they key has been added and .Equals evaluates to true.

I have the following class:

public class StatisticsFilter 
{
    private String someString1;
    private String someString2;
    .....

    public override string ToString()
    {
        return string.Format("{0}-{1}-{2}-{3}-{4}", someString1, someString2, ...)
    }

    public override bool Equals(object obj)
    {            
        return obj.ToString().Equals(ToString());
    }

    public override int GetHashCode()
    {
        return ToString().GetHashCode();
    }
}

I then have a dictionary that looks like this:

private readonly IDictionary<StatisticsFilter, Statistics> _filteredStatisticsDict =
            new Dictionary<StatisticsFilter, Statistics>();

....

{
    // ALWAYS EVALUATES TO FALSE!
    if (_filteredStatisticsDict.ContainsKey(statisticsFilter) == false)
    {
         _filteredStatisticsDict.Add(statisticsFilter, new Statistics());
    }
}

The problem must lie in your `GetHashCode` then. Does it always return the same value for the same instance? — Etienne de Martel, May 25 '11 at 19:07
Wait, does `ContainsKey(statisticsFilter)` evaluate to false, or `ContainsKey(statisticsFilter) == false` evaluate to false? — Ben Voigt, May 25 '11 at 19:20

score 5 · Accepted Answer · answered May 25 '11 at 19:15

Unable to reproduce with the code you've given us.

using System;
using System.Collections.Generic;

public class StatisticsFilter 
{
    private String someString1;
    private String someString2;

    public StatisticsFilter(string x, string y)
    {
        this.someString1 = x;
        this.someString2 = y;
    }

    public override string ToString()
    {
        return string.Format("{0}-{1}xyz", someString1, someString2);
    }

    public override bool Equals(object obj)
    {            
        return obj.ToString().Equals(ToString());
    }

    public override int GetHashCode()
    {
        return ToString().GetHashCode();
    }
}

class Test
{
    static void Main()
    {
        var dict = new Dictionary<StatisticsFilter, int>();

        var sf1 = new StatisticsFilter("hello", "there");
        var sf2 = new StatisticsFilter("hello", "there");

        dict[sf1] = 10;
        Console.WriteLine(dict.ContainsKey(sf2)); // Prints true
    }
}

You are correct. The issue was the StatisticsFilters were actually the same object and was to do with updating the reference etc. Very convoluted and confusing in our code. Thanks! — Luke Belbina, May 25 '11 at 19:25
'@jon Skeet see a similar problem i have here http://stackoverflow.com/questions/27921694/how-to-implement-comparison-on-a-class-that-can-take-any-generic-type, please help. thanks. — Charles Okwuagwu, Jan 13 '15 at 14:54

score 0 · Answer 2 · answered May 25 '11 at 19:21

Are expecting the 'ToString()' to be the key? I think you will get the desired result by changing the Dictionary declaration to: Dictionary<string, Statistics>

// not always be false
if (_filteredStatisticsDict.ContainsKey(statistics.ToString() == false)
{
    _filteredStatisticsDict.Add(statisticsFilter.ToString(), newStatisitcs());
}

If I understand what you are trying to accomplish, this should work. With this method the dictionary key is based on the content of the filter.

yeah, that would work but it then is not to what actually is the dictionary so less readable. — Luke Belbina, May 25 '11 at 22:25

C# Dictionary ContainsKey

2 Answers2

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