Question

I’m testing the UNION method to merge to dictionaries (of type Dictionary). It works fine with TValue type is string or int or even object. But if TValue type is a collection (tested with List and object[]) an exception is thrown : “ArgumentException: An item with the same key has already been added.”

Here is my code :

Dictionary<int,string> _dico1 = new Dictionary<int, string>()
{
    {0, "zero"},
    {1, "one"}
};

Dictionary<int,string> _dico2 = new Dictionary<int,string>()
{
    {1 , "one"},
    {2 , "two"},
    {3 , "three"},
    {4 , "four"},
    {5 , "five"},
    {6 , "six"}
};

Dictionary<int, List<string>> _dico3 = new Dictionary<int, List<string>>()
{
    {0, new List<string>{"zero"}},
    {1, new List<string>{"one"}}
};

Dictionary<int, List<string>> _dico4 = new Dictionary<int, List<string>>()
{
    {1, new List<string>{"one"}},
    {2, new List<string>{"two"}},
    {3, new List<string>{"three"}},
    {4, new List<string>{"four"}},
    {5, new List<string>{"five"}},
    {6, new List<string>{"six"}},
};

    // works fine
    var mergeDico = _dico1.Union(_dico2).ToDictionary(key => key.Key, value => value.Value);

    // throw an ArgumentException : An item with the same key has already been added
    var mergeDico2 = _dico3.Union(_dico4).ToDictionary(key => key.Key, value => value.Value);

Why the behavior is not the same ? And How to resolve this problem ?

Thank you !

Answer 1

In the first case, the Union is discarding the duplicate keys because the key/value pairs themselves are equal. In the second case they’re not, because a List<String>{"one"} isn’t equal to another List<string>{"one"}.

I suspect you want your Union call to use an IEqualityComparer which only takes account of the keys within the dictionary.