Demonstrates how to implement a custom collection initializer by implement the IEnumerable<T> interface and the Add method.

CustomCollectionInitializer.cs

private void OnPresetSelected(object userdata)
{
	// We want to quickly define some string collections as "presets".
	foreach (var preset in presetsOld)
	{
		foreach (var prop in preset)
			Debug.Log(prop);
	}

	// But it's much nicer if each collection has a name.
	foreach (var preset in presets)
	{
		foreach (var prop in preset.Properties)
			Debug.Log(preset.Name + " " + prop);
	}
}

// Jagged arrays are good enough for simple collections, but don't have names.
// Initializing them is pretty sleek though:
private readonly string[][] presetsOld =
{
	new[] { "A", "B", "C" },
	new[] { "D", "E", "F" }
};

// Our custom data type can be shortened a little:
private readonly Preset[] presetsA =
{
	// Object initializer + assigning List with collection initializer:
	new Preset("Simple") { Properties = { "A", "B", "C" } },
	new Preset("Advanced") { Properties = { "D", "E", "F" } }
};

// But this would be even more concise:
private readonly Preset[] presets =
{
	new Preset("Simple") { "A", "B", "C" },
	new Preset("Advanced") { "D", "E", "F" }
};

private class Preset : IEnumerable<string>
{
	public readonly string Name;
	public readonly List<string> Properties;

	public Preset(string name)
	{
		this.Name = name;
		this.Properties = new List<string>();
	}

	public void Add(string property)
		=> Properties.Add(property);

	public IEnumerator<string> GetEnumerator()
		=> Properties.GetEnumerator();

	IEnumerator IEnumerable.GetEnumerator()
		=> Properties.GetEnumerator();
}