Change object-or-collection-initializer to object-and-collection-initializer

[mariusGundersen]

According to this SO answer it's not possible to combine object initializer with collection initializer. I have a situation where this would be useful.

First, lets have a look at just the collection initializer. It uses some simple pattern matching to find an Add() method with the correct signature, and is even able to work with extension methods. This lets me create this very useful extension method:

public static void Add<T>(this ICollection<T> destination, IEnumerable<T> source)
{
    foreach (var item in source)
    {
        destination.Add(item);
    }
}

Using this it's now possible to create a new ICollection and copy the contents of another IEnumerable:

new List<string>
{
    { "Prefix" },
    someOtherList,
    { "Suffix" }
};

With a dictionary I would like to use the index initializer:

var dictionary = new Dictionary<string, string>
{
    someOtherDictionary,
    ["Alpha"] = "one",
    ["Beta"] = "two"
};

Unfortunately this is not possible, since the ["Alpha"] = "one" is using the object initializer. It is possible to do this using the existing collection initializer:

var dictionary = new Dictionary<string, string>
{
    someOtherDictionary,
    { "Alpha", "one"},
    { "Beta", "two"}
}

But this is not as nice.

[svick]

For Dictionary<K, V>, you can already write the following code instead:

var dictionary = new Dictionary<string, string>(someOtherDictionary)
{
    ["Alpha"] = "one",
    ["Beta"] = "two"
};

Is this also "not as nice"?

[mariusGundersen]

That's my point, you can't combine that with a property, which is what I'm asking for.