using System;

using System.Collections;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var goodCollection = new[] {"0","1","2","3"};

        var badCollection = new[] {"1","1","2","3"};

        try {

            Console.Write("goodCollection.ToDictionary");

            goodCollection.ToDictionary(x=>x, y=>y);

            Console.WriteLine(" works");

        }catch {Console.WriteLine(" not work");}

        try {

            Console.Write("badCollection.ToDictionary");

            badCollection.ToDictionary(x=>x, y=>y);

            Console.WriteLine(" works");

        }catch {Console.WriteLine(" not work");}

        try {

            Console.Write("goodCollection.ToDictionarySafe");

            goodCollection.ToDictionarySafe(x=>x, y=>y);

            Console.WriteLine(" works");

        }catch {Console.WriteLine(" not work");}

        try {

            Console.Write("badCollection.ToDictionarySafe");

            badCollection.ToDictionarySafe(x=>x, y=>y);

            Console.WriteLine(" works");

        }catch {Console.WriteLine(" not work");}

    }

}

​

public static class EnumerableExtensions

{

    public static Dictionary<TKey, TElement> ToDictionarySafe<TSource, TKey, TElement>(

        this IEnumerable<TSource> source, 

        Func<TSource, TKey> keySelector, 

        Func<TSource, TElement> elementSelector, 

        IEqualityComparer<TKey> comparer = null)

    {

        var dictionary = new Dictionary<TKey, TElement>(comparer);

​

        if (source == null)

        {

            return dictionary;

        }

​

        foreach (TSource element in source)

        {

            dictionary[keySelector(element)] = elementSelector(element);

        }

​

        return dictionary; 

    }

    public static Dictionary<TDest, TDest> ToDictionarySafe<TSource, TDest>(

        this IEnumerable<TSource> source, 

        Func<TSource, TDest> selector, 

        IEqualityComparer<TDest> comparer = null)

    {

        var dictionary = new Dictionary<TDest, TDest>(comparer);

​

        if (source == null)

        {

            return dictionary;

        }

​

        foreach (TSource element in source)

        {

            var selectedElement = selector(element);

            dictionary[selectedElement] = selectedElement;

        }

​

        return dictionary; 

    }   

}