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

Answer 1

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.