Ytdyklly

Since I couldn't find any good answers I made my own class called DuplicateDictionary for personal usage. I would like some tips to improve it.

public class DuplicateDictionary<TKey, TValue>: List<KeyValuePair<TKey, TValue>>

{

    public DuplicateDictionary()

    {



    }



    public DuplicateDictionary(List<KeyValuePair<TKey, TValue>> list)

    {

        foreach(KeyValuePair<TKey, TValue> kvp in list)

        {

            this.Add(kvp);

        }

    }



    public DuplicateDictionary(Dictionary<TKey, TValue> dictionary)

    {

        foreach(KeyValuePair<TKey, TValue> kvp in dictionary)

        {

            this.Add(kvp);

        }

    }



    public TValue this[TKey index]

    {

        get

        {

            this.ContainsKey(index);

            return this[index];

        }

        set

        {

            this[index] = value;

        }

    }



    public void Add(TKey key, TValue value)

    {

        this.Add(new KeyValuePair<TKey, TValue>(key, value));

    }



    public bool ContainsKey(TKey key)

    {

        foreach(KeyValuePair<TKey, TValue> kvp in this)

            if(kvp.Key.Equals(key)) return true;



        return false;

    }

}

This class is useful for log things. I use it to log a calculator's past functions and numbers, and once equals is pressed, adds all the past numbers with the functions.

Such a data structure already exists, and is called ILookup<TKey, TElement>. It can be created using the ToLookup extension.

The ToLookup<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>) method returns a Lookup<TKey, TElement>, a one-to-many dictionary that maps keys to collections of values. A Lookup<TKey, TElement> differs from a Dictionary<TKey, TValue>, which performs a one-to-one mapping of keys to single values.

var list = new List<Tuple<string, int>>

{

    Tuple.Create("a", 1),

    Tuple.Create("a", 2),

    Tuple.Create("a", 3),

    Tuple.Create("b", 4),

    Tuple.Create("c", 5)

};



var lookup = list.ToLookup(t => t.Item1, t => t.Item2);



foreach(var kv in lookup)

{

    Console.Write(kv.Key);

    Console.WriteLine(" - " + string.Join(", ", kv));

}



// prints

// a - 1, 2, 3

// b - 4

// c - 5

As Vince pointed out, your DuplicateDictionary will suffer from slow access. Lookup won't.

First I second what Vince Panuccio stated in his answer

I don't think this class should have a reason to exist. A key is just that, a key. If you have duplicate keys and duplicate values what you're essentially after is a grouping or a dictionary or with a set or list as its value.

Bug alert

This will break with an StackOverflowException

public TValue this[TKey index]

{

    get

    {

        this.ContainsKey(index);

        return this[index];

    }

    set

    {

        this[index] = value;

    }

}  

by calling this.ContainsKey() which again refers to the Item (this[TKey]) property and if we would omit this call, it would just break again with a StackOverflowException based on the return this[index]; which is reffering the property getter itself again.

Setting aside this big bug, an argument named index usually indicates some kind of numerical type. So a better parameter name would be in this case just key.

The ctor

public DuplicateDictionary(List<KeyValuePair<TKey, TValue>> list)

{

    foreach(KeyValuePair<TKey, TValue> kvp in list)

    {

        this.Add(kvp);

    }

}  

could be easily improved by using the AddRange() method of the List<T> like so

public DuplicateDictionary(List<KeyValuePair<TKey, TValue>> list)

{

    this.AddRange(list);

}

but that isn't really needed in this way because you also have a ctor which takes an Dictionary<TKey, TValue> as a parameter. I suggest to replace both ctors by a different one which only takes an IEnumerable<KeyValuePair<TKey, TValue>> like so

public DuplicateDictionary(IEnumerable<KeyValuePair<TKey, TValue>> items)

{

    this.AddRange(items);

}  

which can be called using a Dictionary and a List.

This looks good but we can still do better by using the ctor of the List instead like so

public DuplicateDictionary(IEnumerable<KeyValuePair<TKey, TValue>> items)

    : base(items)

{  } 

It might help if you explain the problem you are trying to solve first, but I'll have a crack at this.

I don't think this class should have a reason to exist. A key is just that, a key. If you have duplicate keys and duplicate values what you're essentially after is a grouping or a dictionary with a set or list as its value.

What you have is a list of KeyValuePairs which is not the same as a Dictionary :-)

Dictionaries benefit from having unique keys by giving you fast lookups, I'm not sure what benefit you gain by having a DuplicateDictionary.

I think you want to have some kind of a multi value single key dictionary. So I suggest to use for example a List<> to hold the values.

Here's an example:

public class DuplicateDictionary<TKey, TValue> : Dictionary<TKey, List<TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>

{

    public new IEnumerable<KeyValuePair<TKey, TValue>> this[TKey key]

    {

        get

        {

            List<TValue> values;

            if (!TryGetValue(key, out values))

            {

                return Enumerable.Empty<KeyValuePair<TKey, TValue>>();

            }



            return values.Select(v => new KeyValuePair<TKey, TValue>(key, v));

        }

        set

        {

            foreach (var _value in value.Select(kvp => kvp.Value))

            {

                Add(key, _value);

            }

        }

    }







    public void Add(TKey key, TValue value)

    {

        List<TValue> values;

        if (!TryGetValue(key, out values))

        {

            values = new List<TValue>();

            Add(key, values);

        }

        values.Add(value);

    }



    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()

    {

        foreach (var item in ((Dictionary<TKey, List<TValue>>)this))

        {

            foreach (var value in item.Value)

            {

                yield return new KeyValuePair<TKey, TValue>(item.Key, value);

            }

        }

    }

}

static void Main(string args)

{

    var dupDic = new DuplicateDictionary<string, string>();



    dupDic.Add("abc", "123");

    dupDic.Add("abc", "456");

    dupDic.Add("xyz", "789");

    dupDic.Add("xyz", "098");

    dupDic.Add("xyz", "290");



    foreach (var kvp in dupDic)

    {

        Console.WriteLine("Key = "{0}" Value = "{1}"", kvp.Key, kvp.Value);

    }



    Console.ReadKey();

}

You can List of KeyValuePair to store duplicates

Example :

List<KeyValuePair<string, string>> listKeyValPair= new List<KeyValuePair<string, string>>();



KeyValuePair<string, string> keyValue= new KeyValuePair<string, string>("KEY1", "VALUE1");



listKeyValPair.Add(keyValue);