Initialize List<T> with index--is this a bug?

Question

In C# you can initialize Dictionary<TKey,TValue> like this:

var dictionary = new Dictionary<int, string>()
{
  [0] = "Hello",
  [1] = "World"
};

You can also initialize List<T> like this:

var listA = new List<string>()
{
  "Hello",
  "World"
};

The two examples listed above compile and run with no exceptions.

However, the following example compiles, but throws an exception:

var listB = new List<string>()
{
  [0] = "Hello",
  [1] = "World"
};
// System.ArgumentOutOfRangeException: Index was out of range.
// Must be non-negative and less than the size of the collection.
// (Parameter 'index') at System.Collections.Generic.List`1.set_Item(Int32 index, T value)

Is this an unfinished feature, since it compiles but throws an exception? If not, shouldn't this be a compilation error?

I'm using C# 12 and ASP.NET 8

Answer 1

var listB = new List<string>()
{
  [0] = "Hello",
  [1] = "World"
};

is compiled to:

List<string> list = new List<string>();
list[0] = "Hello";
list[1] = "World";

From the Docs:

The preceding sample generates code that calls the Item[TKey] to set the values.

Of course, the exception makes sense because when you initialize the list, it has zero elements. Elements at index 0 and 1 don't exist.

Answer 2

To complete gunr2171's answer, the first example with the dictionary does not use the indexer as in the 2nd example with the list(which tries to assign strings to list-indexes which don't exist because the list is empty). But this ...

var dictionary = new Dictionary<int, string>()
{
  [0] = "Hello",
  [1] = "World"
};

... is the same as(which adds 2 new entries because it's using this property):

var dictionary = new Dictionary<int, string>();
dictionary[0] = "Hello"; // adds a new dictionary entry with the key 0 and the value "Hello"
dictionary[1] = "World"; // adds a new dictionary entry with the key 1 and the value "World"