How can I store same key in IDictionary?

Question

I'm using C# 4.0. I want to store (string,string) pair using IDictionary. like below:

Dictionary<string, string> _tempDicData = new Dictionary<string, string>();
      _tempDicData.Add("Hello", "xyz");
      _tempDicData.Add("Hello", "aaa");
      _tempDicData.Add("Hello", "qwert");
      _tempDicData.Add("Hello", "foo");
      _tempDicData.Add("Hello", "pqr");
      _tempDicData.Add("Hello", "abc");

but got an error:

An item with the same key has already been added.

So How can I store same key in IDictionary?

The core of this question is very similar to [this other question](http://stackoverflow.com/questions/10962002/dictionary-in-vb-net-obtain-keys-grouped/10962040#10962040) and its answers may help you solve your problem. — O. R. Mapper, Jul 17 '12 at 10:36
if you want to add several values to the same key why wouldn't you use string[] for value instead of string — Maha Khairy, Jul 17 '12 at 10:39

Sergey Kalinichenko · Answer 1 · 2012-07-17T10:44:09.337

You cannot add the same item to IDictionary<K,T> more than once - that's the whole point of a dictionary, an associative container. But you can replace an existing one like this:

_tempDicData.Add("Hello", "xyz");
_tempDicData["Hello"] = "aaa";

and you can make a dictionary of lists if you need multiple items per key:

IDictionary<string,IList<string>> _tempDicData =
    new Dictionary<string,IList<string>>();
IList<string> lst = new List<string>();
lst.Add("xyz");
lst.Add("aaa");
_tempDicData.Add("Hello", lst);

When you do not know for sure if the list for a key exists or not, you can use this pattern to add new items:

IList<string> lst;
if (!_tempDicData.TryGetValue("Hello", out lst)) {
    _tempDicData.Add("Hello", lst);
}
lst.Add("xyz");
lst.Add("aaa");

score 3 · Answer 2 · answered Jul 17 '12 at 10:36

You cannot have two items with the same key in a dictionary. However, you can have one item that is a itself a collection:

Dictionary<string, List<string>> _tempDicData = new Dictionary<string, List<string>>
{
    { "Hello", new List<string> { "xyz", "aa", "foo" },
};

If you go this way, you will have to be careful when accessing items because the underlying lists may or may not have been already created:

// WRONG: _tempDicData["Hello"] might not exist!
_tempDicData["Hello"].Add("bar");

// CORRECT:
if (!_tempDicData.ContainsKey("Hello")) {
    _tempDicData["Hello"] = new List<string>();
}
_tempDicData["Hello"].Add("bar");

score 2 · Answer 3 · answered Jul 17 '12 at 10:33

2

You can not. The dictionary can only hold one item for every key. A dictionary is probably not the data structure you are looking for.

answered Jul 17 '12 at 10:33

Marcus Johansson

2,626
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score 2 · Answer 4 · answered Jul 17 '12 at 10:34

2

Create Dictionary<string,List<string>>.

Dictionary<string,List<string>> dict=new Dictionary<string,List<string>>();
dict.Add("hello",new List<string>());
dict["hello"].Add("foo");
dict["hello"].Add("bar");

answered Jul 17 '12 at 10:34

KV Prajapati

93,659
19
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score 2 · Answer 5 · answered Jul 17 '12 at 10:36

2

You can't do it with a IDictionary. Try using NameValueCollection.

answered Jul 17 '12 at 10:36

Riccardo

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score 2 · Accepted Answer · answered Jul 17 '12 at 10:58

A class where u can add duplicated keys may look like the following:

class TDictionary<K, V>
{
    struct KeyValuePair
    {
        public K Key;
        public V Value;
    }

    private readonly List<KeyValuePair> fList = new List<KeyValuePair>();

    public void Add(K key, V value)
    {
        fList.Add(new KeyValuePair { Key = key, Value = value });
    }

    public List<V> this[K key]
    {
        get { return (from pair in fList where pair.Key.Equals(key) select pair.Value).ToList(); }
    }

    public List<K> Keys
    {
        get { return fList.Select(pair => pair.Key).ToList(); }
    }

    public List<V> Values
    {
        get { return fList.Select(pair => pair.Value).ToList(); }
    }
}

score 1 · Answer 7 · answered Jul 17 '12 at 10:34

1

impossible, create own class with fields key value for this purpose

answered Jul 17 '12 at 10:34

burning_LEGION

13,246
8
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which type of class?Can you please give me hint for that? – Dharmik Bhandari Jul 17 '12 at 10:51
public class MyType{public string Key{get;set;} public string Value{get;set;}} – burning_LEGION Jul 17 '12 at 10:57

score 1 · Answer 8 · answered Jul 17 '12 at 10:35

You can't?

You could use a Dictionary<string, List<string>> or, if you want a read-only solution, you can use the ToLookup extension method.

var lp = new[]
{
    Tuple.Create("Hello", "xyz"),
    Tuple.Create("Hello", "aaa"),
    Tuple.Create("Hello", "qwert"),
    Tuple.Create("Hello", "foo"),
    Tuple.Create("Hello", "pqr"),
    Tuple.Create("Hello", "abc"),
}.ToLookup(p => p.Item1, p => p.Item2);

foreach (var key in lp)
{
    foreach (var value in key)
    {
        Console.WriteLine("{0}: {1}", key.Key, value);
    }
}

score 1 · Answer 9 · answered Jul 17 '12 at 10:35

1

Key has to be unique to search the values.

answered Jul 17 '12 at 10:35

Shailesh

1,178
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score 1 · Answer 10 · edited Feb 01 '14 at 12:45

1

I think you have to create your own class where you can add the same key several times. A dictionary is the wrong way to do this.

edited Feb 01 '14 at 12:45

BenMorel

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answered Jul 17 '12 at 10:35

Tomtom

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score 1 · Answer 11 · answered Jul 17 '12 at 10:37

1

why that you will want to do that. the idea of dictionary is items by key and if it appers more then once that is not the key.

you can create dictionary of > but it's better for you to replan your needs.

answered Jul 17 '12 at 10:37

user1425690

11
1

How can I store same key in IDictionary?

11 Answers11