Swap Key and Value in Concurrent Dictionary in C#

Question

Is there any way to swap key and value in Concurrent Dictionary in C#? I know for Dictionary type, it can be something like

dictionary = dictionary.ToDictionary(x => x.Value, x => x.Key);

Anything similar in ConcurrentDictionary ?

Answer 1

You cannot cast or convert Dictionary to ConcurrentDictionary directly. Buy you can create a ConcurrentDictionary with a Dictionary, check this constructor

public ConcurrentDictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection);

And look the extension method of LINQ that you say

  public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
        this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector,
        Func<TSource, TElement> elementSelector);

So if Dictionary has compatibility wiht IEnumerable<KeyValuePair<TKey, TValue>>

Solution 1

Just use ToDictionary extension method of LINQ

var concurrentDict = new ConcurrentDictionary<string, string>();
/* Add some items */
bool firstItem = concurrentDict.TryAdd("1", "First");  //returns true
bool secondItem = concurrentDict.TryAdd("2", "Second");  //returns  true

/* Swaping Value <-> Key to new Dictionary */
Dictionary<string,string> normalDict = concurrentDict.ToDictionary(x => x.Value, x => x.Key);

/* creating ConcurrentDictionary */
var newConcurrentDict = new ConcurrentDictionary<string, string>(normalDict);

Solution 2

What about thread safe?

You can use a custom extension method (like LINQ)

var concurrentDict = new ConcurrentDictionary<string, string>();
/* Add some items */
bool firstItem = concurrentDict.TryAdd("1", "First");  //returns true
bool secondItem = concurrentDict.TryAdd("2", "Second");  //returns  true

/* Swaping Value <-> Key to new Dictionary */
Dictionary<string,string> normalDict = concurrentDict.ToDictionary(x => x.Value, x => x.Key);

/* creating ConcurrentDictionary */
var newConcurrentDict = normalDict.ToConcurrentDictionary();

Extension method

Ref https://stackoverflow.com/a/27064366/1669574

public static class ConcurrentDictionaryExtensions
{
    public static ConcurrentDictionary<TKey, TElement> ToConcurrentDictionary<TSource, TKey, TElement>(
        this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector,
        IEqualityComparer<TKey> comparer)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (keySelector == null) throw new ArgumentNullException("keySelector");
        if (elementSelector == null) throw new ArgumentNullException("elementSelector");

        ConcurrentDictionary<TKey, TElement> d = new ConcurrentDictionary<TKey, TElement>(comparer);
        foreach (TSource element in source)
            d.TryAdd(keySelector(element), elementSelector(element));

        return d;
    }

    public static ConcurrentDictionary<TKey, TSource> ToConcurrentDictionary<TSource, TKey>(
        this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        return ToConcurrentDictionary<TSource, TKey, TSource>(source, keySelector, IdentityFunction<TSource>.Instance,
            null);
    }

    public static ConcurrentDictionary<TKey, TSource> ToConcurrentDictionary<TSource, TKey>(
        this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
    {
        return ToConcurrentDictionary<TSource, TKey, TSource>(source, keySelector, IdentityFunction<TSource>.Instance,
            comparer);
    }

    public static ConcurrentDictionary<TKey, TElement> ToConcurrentDictionary<TSource, TKey, TElement>(
        this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
    {
        return ToConcurrentDictionary<TSource, TKey, TElement>(source, keySelector, elementSelector, null);
    }

    internal class IdentityFunction<TElement>
    {
        public static Func<TElement, TElement> Instance
        {
            get { return x => x; }
        }
    }
}