Prior to C# 6, you could use a fully braced collection initializer to initialize a Dictionary’s key–value pairs. For example, if you have a Dictionary<string, int> named toolInven-tory, you could create and initialize it as follows:

var toolInventory = new Dictionary<string, int>{
   {"Hammer", 13},
   {"Saw", 17},
   {"Screwdriver", 7}
};

This is shorthand for creating the Dictionary then using its Add method to add each key– value pairs.

C# 6 introduces the index initializers, which enable you to clearly indicate the key and the value in each key–value pair as follows:

var toolInventory = new Dictionary<string, int>{
   ["Hammer"] = 13,
   ["Saw"] = 17,
   ["Screwdriver"] = 7
};

Prior to C# 6, any collection that defined an Add instance method could be initialized with a collection initializer. As of C# 6, the compiler also supports collection initializers for any collection that has an Add extension method.