HasValue1 returns false for ItemList of ListItems

[dermotblairca]

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Describe the bug

I'm not 100% sure if this is a bug or not but I have json which contains an ItemList of ListItems and HasValue1 returns false on the ItemListElement and HasValue3 returns true. HasValue1 = Gets a value indicating whether the value of type IListItem has a value. HasValue3 = Gets a value indicating whether the value of type IThing has a value. Even though ListItem inherits from Thing, should HasValue1 not return true here since it the type of the objects is ListItem?

{
	"@context": "https://schema.org",
	"@type": "ItemList",
	"itemListElement": [
		{
			"@type": "ListItem",
			"description": "test1",
			"url": "http://test1.com",
			"position": 0
		},
		{
			"@type": "ListItem",
			"description": "test2",
			"url": "http://test2.com",
			"position": 1
		}
	]
}

Steps to reproduce

Run the following unit test:

[Fact]
    public void Deserializing_ItemListJsonLd_HasItemListReturnsTrue()
    {
        var json = "{\"@context\":\"https://schema.org\",\"@type\":\"ItemList\",\"itemListElement\":[{\"@type\":\"ListItem\",\"description\":\"test1\",\"url\":\"http://test1.com\",\"position\":0},{\"@type\":\"ListItem\",\"description\":\"test2\",\"url\":\"http://test2.com\",\"position\":1}]}";
        var itemList = SchemaSerializer.DeserializeObject<ItemList>(json);
        Assert.Equal(true, itemList?.ItemListElement.HasValue1);
    }

Expected behaviour

Not 100% sure but I thought HasValue1 would return true here.

Schema objects

https://schema.org/ItemList

[Turnerj]

It is technically behaving as intended though the intended behaviour isn't fantastic. Basically the ValueJsonConverter performs a "right-to-left" type check for any objects it tries to deserialize. The types for itemListElement are ListItem, Text or Thing from "left-to-right". This ordering is just from how we get the data in the schema.json files we process.

Because everything implements IThing, it succeeds with deserializing and doesn't try other types. What we could do is alter the logic to try a different algorithm for best fit - something that demotes Thing to the last possible choice.

It might seem somewhat obvious in this example that we could just switch from "right-to-left" to "left-to-right" however we'd need to confirm we aren't just creating the same problem for other schema properties.

[dermotblairca]

Thanks for looking into it @Turnerj That makes sense. A workaround I used was just to check if the current item "is" an IListItem, if HasValue3 returns true.

if (itemList.ItemListElement.HasValue3)
{
	foreach (var listItem in itemList.ItemListElement.Value3)
	{
		if (listItem is IListItem)
		{
			// do something with list item
		}
	}
}