System.ArgumentException: An item with the same key has already been added

Problem

Dictionaries require keys to be unique. When you try to add a key/value to a dictionary and the key already exists, you get the following exception:

System.ArgumentException: An item with the same key has already been added.

This can happen when you use Dictionary.Add() or when you initialize a dictionary using the “curly brace” syntax. The following two code snippets show an example that results in this ArgumentException:

using System.Collections.Generic;

var dictionary = new Dictionary<string, int>();

dictionary.Add("Bob", 1);

dictionary.Add("Bob", 2); //duplicate key => exception
Code language: C# (cs)

using System.Collections.Generic;

var dictionary = new Dictionary<string, int>()
{
    { "Bob", 1 },
    { "Bob", 2 } //duplicate key => exception
};
Code language: C# (cs)

If you accidently initialized the dictionary with duplicate keys, go ahead and remove the duplicates in the initializer. Otherwise, I’ll explain three options for solving this below.

Option 1 – Use Dictionary.TryAdd()

The simplest solution is to use Dictionary.TryAdd() to add the key/value to the dictionary. This adds the key if it doesn’t exist, otherwise it does nothing (instead of throwing an exception).

using System.Collections.Generic;

var dictionary = new Dictionary<string, int>();

dictionary.TryAdd("Bob", 1);
dictionary.TryAdd("Bob", 2); //does nothing

Console.WriteLine($"Value={dictionary["Bob"]}"); //outputs "Value=1"
Code language: C# (cs)

Note: TryAdd() returns true if it was able to add the key. Check that return value if you want.

TryAdd() is a shortcut for checking if the key exists with Dictionary.ContainsKey() and then adding with Dictionary.Add().

Option 2 – Insert or update key

Another option is to add the key/value using the indexer syntax (i.e. dictionary[key] = value). This inserts the key/value or updates the existing key’s value in the dictionary (instead of throwing an exception). Here’s an example:

using System.Collections.Generic;

var dictionary = new Dictionary<string, int>();

dictionary["Bob"] = 1;
dictionary["Bob"] = 2;

Console.WriteLine($"Value={dictionary["Bob"]}"); //outputs "Value=2"
Code language: C# (cs)

This is a nice approach when you simply want the key to have the latest value and don’t want to explicitly check if the key exists every time.

Option 3 – Use a dictionary of lists if you need multiple values per key

Keys are mapped to exactly one value. When you want a key to have multiple values, you can use a dictionary of lists:

• Create the dictionary with a value of type List<T>.
• If the key doesn’t exist, add it with a new list.
• Add the value to the list.

Here’s an example of how to do this:

using System.Collections.Generic;

var dictionary = new Dictionary<string, List<int>>();

if (!dictionary.ContainsKey("Bob"))
    dictionary["Bob"] = new List<int>();

dictionary["Bob"].Add(1);
dictionary["Bob"].Add(2);
Code language: C# (cs)

Note: You can use TryGetValue(), but I find ContainsKey() to be easier to understand at a glance.