C# 6 Preview: Index initializers

Dictionary initializer syntax is pretty convenient, but has always been somewhat awkward to use. It doesn’t really feel like you’re working with a dictionary at all. Let’s borrow some sample code from C# Basics: Morse Code Converter Using Dictionaries, one of my early articles at Programmer’s Ranch:

            Dictionary<char, string> morse = new Dictionary<char, string>()
            {
                {'A' , ".-"},
                {'B' , "-..."},
                {'C' , "-.-."},
                {'D' , "-.."},
                {'E' , "."}
                //...
            };

C# 6 offers an alternative syntax just for the sake of making this kind of thing more intuitive:

            Dictionary<char, string> morse = new Dictionary<char, string>()
            {
                ['A'] = ".-",
                ['B'] = "-...",
                ['C'] = "-.-.",
                ['D'] = "-..",
                ['E'] = ".",
                //...
            };

For the purpose of initialising a dictionary, you can pretty much assume that the two syntaxes above are semantically, equivalent, even though that is not entirely true. Scott Allen’s What’s New in C# 6 course on Pluralsight demonstrates that the original syntax is translated into dictionary .Add() calls, while the new one is translated into index assignments.

It is also not permitted to mix the two syntaxes above.

Note: according to the official C# feature descriptions (PDF), index initializers supposedly “do not work in the current CTP”, however the above example worked just fine in Visual Studio 14 CTP 4.

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