Argument Exception Item with Same Key has already been added

Question

I keep getting an error with the following code   Dictionary lt string  string gt  rct3Features   new Dictionary lt string  string gt     Dictionary lt string  string gt  rct4Features   new Dictionary lt string  string gt      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 lt string  string gt  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 lt string  string gt  dictionaryOne  Dictionary lt string  string gt  dictionaryTwo          foreach  string item in dictionaryOne                 To print out the dictionary  to see if it works      foreach  KeyValuePair lt string  string gt  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

User · Answer

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.

User · Answer

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

User · Answer

If you want  insert or replace  semantics  use this syntax   A key    value          lt -- 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

User · Answer

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

User · Answer

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 lt string  string gt  rct3Features   new Dictionary lt string  string gt     Dictionary lt string  string gt  rct4Features   new Dictionary lt string  string gt      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 lt string  string gt  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

User · Answer

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 lt string gt  for each string key   For example   var rct3Features   new Dictionary lt string  List lt string gt  gt     var rct4Features   new Dictionary lt string  List lt string gt  gt      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 lt string gt    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 lt string  List lt string gt  gt  item in rct3Features        Console WriteLine  The Key   0  has values    item Key       foreach  string value in item Value                Console WriteLine   -  0    value

[c#] Argument Exception "Item with Same Key has already been added"

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