Argument Exception "Item with Same Key has already been added"

Question

I keep getting an error with the following code:

Dictionary<string, string> rct3Features = new Dictionary<string, string>();
Dictionary<string, string> rct4Features = new Dictionary<string, string>();

foreach (string line in rct3Lines) 
{
    string[] items = line.Split(new String[] { " " }, 2, StringSplitOptions.None);

    rct3Features.Add(items[0], items[1]);

    ////To print out the dictionary (to see if it works)
    //foreach (KeyValuePair<string, string> item in rct3Features)
    //{
    //    Console.WriteLine(item.Key + " " + item.Value);
    //}

}

The error throws an ArgumentException saying,

"An item with the same key has already been added."

I am unsure after several Google searches how to fix this.

Later in the code I need to access the dictionary for a compare function:

Compare4To3(rct4Features, rct3Features);

public static void Compare4To3(Dictionary<string, string> dictionaryOne, Dictionary<string, string> dictionaryTwo)
{
    //foreach (string item in dictionaryOne)
    //{

    //To print out the dictionary (to see if it works)
    foreach (KeyValuePair<string, string> item in dictionaryOne)
    {
        Console.WriteLine(item.Key + " " + item.Value);
    }

        //if (dictionaryTwo.ContainsKey(dictionaryOne.Keys)
        //{
        //    Console.Write("True");
        //}
        //else
        //{
        //    Console.Write("False");
        //}
    //}
}

This function isn't completed, but I am trying to resolve this exception. What are the ways I can fix this exception error, and keep access to the dictionary for use with this function? Thank you

Stop trying to add multiple items with the same key. Perhaps you should add a check when adding to make sure the key doesn't already exist. Otherwise we have no way of knowing what the correct course of action would be since you have given almost no information about what you want to happen if there are duplicate items. — eddie_cat, Oct 22 '14 at 20:48
Why do you have duplicate keys? If you need more than one value per key, you should use `Dictionary>`. — Erik Philips, Oct 22 '14 at 20:50
Check my answer below for more details, but you could replace your Add with a different syntax. The following will just overwrite an existing value for a key if the key already exists: `rct3Features[items[0]] = items[1];` — Rufus L, Oct 22 '14 at 21:35
Related: [How to update the value of a key in a dictionary, but only if the key already exists, without hashing the key twice?](https://stackoverflow.com/questions/72837813/how-to-update-the-value-of-a-key-in-a-dictionary-but-only-if-the-key-already-ex) — Theodor Zoulias, Dec 06 '22 at 19:15

score 44 · Accepted Answer · edited Oct 12 '16 at 15:03

This error is fairly self-explanatory. Dictionary keys are unique and you cannot have more than one of the same key. To fix this, you should modify your code like so:

Dictionary<string, string> rct3Features = new Dictionary<string, string>();
Dictionary<string, string> rct4Features = new Dictionary<string, string>();

foreach (string line in rct3Lines) 
{
    string[] items = line.Split(new String[] { " " }, 2, StringSplitOptions.None);

    if (!rct3Features.ContainsKey(items[0]))
    {
        rct3Features.Add(items[0], items[1]);
    }

    ////To print out the dictionary (to see if it works)
    //foreach (KeyValuePair<string, string> item in rct3Features)
    //{
    //    Console.WriteLine(item.Key + " " + item.Value);
    //}
}

This simple if statement ensures that you are only attempting to add a new entry to the Dictionary when the Key (items[0]) is not already present.

score 12 · Answer 2 · answered Nov 09 '18 at 03:07

If you want "insert or replace" semantics, use this syntax:

A[key] = value;     // <-- insert or replace semantics

It's more efficient and readable than calls involving "ContainsKey()" or "Remove()" prior to "Add()".

So in your case:

rct3Features[items[0]] = items[1];

Rufus L · Answer 3 · 2014-10-22T21:54:00.360

As others have said, you are adding the same key more than once. If this is a NOT a valid scenario, then check Jdinklage Morgoone's answer (which only saves the first value found for a key), or, consider this workaround (which only saves the last value found for a key):

// This will always overwrite the existing value if one is already stored for this key
rct3Features[items[0]] = items[1];

Otherwise, if it is valid to have multiple values for a single key, then you should consider storing your values in a List<string> for each string key.

For example:

var rct3Features = new Dictionary<string, List<string>>();
var rct4Features = new Dictionary<string, List<string>>();

foreach (string line in rct3Lines)
{
    string[] items = line.Split(new String[] { " " }, 2, StringSplitOptions.None);

    if (!rct3Features.ContainsKey(items[0]))
    {
        // No items for this key have been added, so create a new list
        // for the value with item[1] as the only item in the list
        rct3Features.Add(items[0], new List<string> { items[1] });
    }
    else
    {
        // This key already exists, so add item[1] to the existing list value
        rct3Features[items[0]].Add(items[1]);
    }
}

// To display your keys and values (testing)
foreach (KeyValuePair<string, List<string>> item in rct3Features)
{
    Console.WriteLine("The Key: {0} has values:", item.Key);
    foreach (string value in item.Value)
    {
        Console.WriteLine(" - {0}", value);
    }
}

score 5 · Answer 4 · answered Oct 22 '14 at 20:55

To illustrate the problem you are having, let's look at some code...

Dictionary<string, string> test = new Dictionary<string, string>();

test.Add("Key1", "Value1");  // Works fine
test.Add("Key2", "Value2");  // Works fine
test.Add("Key1", "Value3");  // Fails because of duplicate key

The reason that a dictionary has a key/value pair is a feature so you can do this...

var myString = test["Key2"];  // myString is now Value2.

If Dictionary had 2 Key2's, it wouldn't know which one to return, so it limits you to a unique key.

score 2 · Answer 5 · answered Oct 22 '14 at 20:55

That Exception is thrown if there is already a key in the dictionary when you try to add the new one.

There must be more than one line in rct3Lines with the same first word. You can't have 2 entries in the same dictionary with the same key.

You need to decide what you want to happen if the key already exists - if you want to just update the value where the key exists you can simply

rct3Features[items[0]]=items[1]

but, if not you may want to test if the key already exists with:

if(rect3Features.ContainsKey(items[0]))
{
    //Do something
} 
else 
{
    //Do something else
}

score 1 · Answer 6 · answered Dec 06 '22 at 15:13

I suggest .NET's TryAdd: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.tryadd?view=net-7.0

I suggest a extension method for environments where .NET's TryAdd is not available:

public static class DictionaryUtils
{
    /// <summary>
    ///    Prevents exception "Item with Same Key has already been added".
    /// </summary>
    public static void TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue value)
    {
        if (!dictionary.ContainsKey(key))
        {
            dictionary.Add(key, value);
        }
    }
}

Tokelo · Answer 7 · 2015-04-28T14:59:51.530

-5

Clear the dictionary before adding any items to it. I don't know how a dictionary of one object affects another's during assignment but I got the error after creating another object with the same key,value pairs.

NB: If you are going to add items in a loop just make sure you clear the dictionary before entering the loop.

edited Apr 28 '15 at 14:59

answered Apr 28 '15 at 14:05

Tokelo

1
2

4

`new Dictionary()` [returns an empty dictionary](http://msdn.microsoft.com/en-us/library/x525za90), so clearing it after construction is not necessary. Clearing it before every call to `Add` *would* stop the exceptions, but would result in only the last item ever being present in the dictionary. Are you sure this is a good suggestion? – Wai Ha Lee Apr 28 '15 at 14:27

Argument Exception "Item with Same Key has already been added"

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