How to add duplicate keys into the Dictionary

Question

I have some lines from text files that i want to add into the Dictionary.I am using Dictionary for the first time.While adding up starting lines it was Ok but suddenly i got error:

An item with the same key has already been added

Here in my code there are duplicate keys which i can not change.Here is my code in c#

Dictionary<string, string> previousLines = new Dictionary<string, string> { };
previousLines.Add(dialedno, line);

Here dialedno is the key and line is the textfile line. Here is the code from which i am retrieving the given line based on key.

string tansferOrginExt = previousLines[dialedno];

So my concern is how to allow to add duplicate keys in Dictionary if possible and if not how can i get similar functionality.

As other have said, it is not possible to add duplicate key in dictionary. How about using Dictionary>? Then check if the key exist then add the line in the list value for that key and if the key does not exist then create a new entry in the dictionary. — samar, Mar 10 '14 at 10:31
Possible duplicate of [Duplicate keys in .NET dictionaries?](https://stackoverflow.com/questions/146204/duplicate-keys-in-net-dictionaries) — RBT, Mar 15 '18 at 12:21

Sergey Berezovskiy · Accepted Answer · 2019-04-15T12:41:07.533

31

how to allow to add duplicate keys in Dictionary

It is not possible. All keys should be unique. As Dictionary<TKey, TValue> implemented:

Every key in aDictionary<TKey, TValue> must be unique according to the dictionary's equality comparer.

Possible solutions - you can keep collection of strings as value (i.e. use Dictionary<string, List<string>>), or (better) you can use Lookup<TKey, TValue> instead of dictionary.

how to check for duplicate keys and delete previous value from Dictionary?

You can check if the key exists with previousLines.ContainsKey(dialedno) but if you always want to hold the last line, then just replace whatever dictionary had for the key, or add the new key if it is not in the dictionary:

previousLines[dialedno] = line;

edited Apr 15 '19 at 12:41

answered Mar 10 '14 at 10:18

Sergey Berezovskiy

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how to check for duplicate keys and delete previous value from Dictionary? – Ram Mar 10 '14 at 10:53
@Ram see updated solution. You also can use Linq to create dictionary from last values only – Sergey Berezovskiy Mar 10 '14 at 11:41
It's good to note that a lookup does not implement ICollection. So there are no `Add`, `Remove`, `Clear` `Contains`, or `CopyTo` methods. Or `Count` property. – Douglas Gaskell Mar 01 '18 at 21:39
1

Why not just use 'previousLines[dialedno] = line' without ContainsKey check? – Rob Hinchliff Apr 15 '19 at 11:27
@RobHinchliff 101% agree with you – Sergey Berezovskiy Apr 15 '19 at 12:38

score 16 · Answer 2 · answered Sep 17 '15 at 12:42

We can Use a List of Key Value Pair

List<KeyValuePair<string, string>> myduplicateLovingDictionary= new List<KeyValuePair<string, string>>();
KeyValuePair<string,string> myItem = new KeyValuePair<string,string>(dialedno, line);
myduplicateLovingDictionary.Add(myItem);

Sadique · Answer 3 · 2014-03-10T10:20:38.243

7

Its not possible to add duplicate items to a Dictionary - an alternative is to use the Lookup class.

Enumerable.ToLookup Method

Creates a generic Lookup from an IEnumerable.

edited Mar 10 '14 at 10:20

answered Mar 10 '14 at 10:18

Sadique

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Note that this class does not implement `ICollection` – Douglas Gaskell Mar 01 '18 at 21:41

score 1 · Answer 4 · answered Oct 11 '18 at 13:20

Example:

class Program
        {
             private static List<KeyValuePair<string, int>> d = new List<KeyValuePair<string, int>>();

            static void Main(string[] args)
            {
                 d.Add(new KeyValuePair<string, int>("joe", 100));
                 d.Add(new KeyValuePair<string, int>("joe", 200));
                 d.Add(new KeyValuePair<string, int>("jim", 100));
                 var result = d.Where(x => x.Key == "joe");
                foreach(var q in result)
                    Console.WriteLine(q.Value   );
                Console.ReadLine();
            }
         }

score 0 · Answer 5 · edited Feb 20 '19 at 07:19

0

List< KeyValuePair < string, string>> listKeyValPair= new List< KeyValuePair< string, string>>();
KeyValuePair< string, string> keyValue= new KeyValuePair< string, string>("KEY1", "VALUE1");
listKeyValPair.Add(keyValue);

edited Feb 20 '19 at 07:19

Shree

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answered Feb 20 '19 at 07:04

Nagesh Hugar

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2

This is almost the same code as in [this answer](https://stackoverflow.com/a/32630573/7404943) – barbsan Feb 20 '19 at 07:22

score 0 · Answer 6 · answered Feb 21 '19 at 14:48

If your question is if you can add the same key twice, the answer is No. However if you want to just iterate through the item and then increase the count of the value for the particular Key, you can achieve that by using "TryAdd" method.

var dict = new Dictionary<int, int>();
        foreach (var item in array)
        {
            dict.TryAdd(item, 0);
            dict[item]++;
        }

The same thing we are trying to achieve with if else, can be achieved with this method.``

https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.tryadd?view=netframework-4.7.2

This worn't help since still the `dict` will have unique keys. Consider updating your answer as per the question `Dictionary`. — Vinod Srivastav, Aug 02 '19 at 06:36

How to add duplicate keys into the Dictionary

6 Answers6

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