Why this confusing syntax exists?

If we have property of dictionary type:

public class Test
{
    public Dictionary<string, string> Dictionary { get; set; } = new Dictionary<string, string>
    {
        {"1", "1" },
        {"2", "2" },
    };
}

Then we can construct object and add value to it

 var test = new Test { Dictionary = { { "3", "3" } } };
 Console.WriteLine(test.Dictionary.Count); // 3

And I don't understand the point why such a confusing syntax to add items exists? When looking at someone else code it's very easy to confuse it with very similarly looking

 var test = new Test { Dictionary = new Dictionary<string, string> { { "3", "3" } } };
 Console.WriteLine(test.Dictionary.Count); // 1

I'd be more OK with it if following would be possible (but it's not):

var dictionary = new Dictionary<string, string> { { "1", "1" } };
...
// adding a new value
dictionary = { { "2" , "2"} }; // invalid expression term '{'

So why this form of adding was needed and exists? For interviews?

Solution

The collection initializer syntax is simply a convenient way of initializing collections (including dictionaries) as part of a complex object model using an object initializer. For example:

var model = new SomeModel {
   Name = "abc",
   Id = 42,
   SpecialMaps = {
       { "foo", "bar" },
       { "magic", "science" },
   }
};

If you don't like it: just don't use it; but the equivalent with manual .Add is IMO much less elegant - a lot of things are taken care of automatically, such as only reading the property once. The longer version that actually creates the collection at the same time works very similarly.

Note that there is also an indexer variant now:

var model = new SomeModel {
   Name = "abc",
   Id = 42,
   SpecialMaps = {
       ["foo"] = "bar",
       ["magic"] ="science",
   }
};

This is very similar, but instead of using collection.Add(args); it uses collection[key] = value;. Again, if it confuses you or offends you: don't use it.