Array items: same type

[mfulton26]

How can a schema be defined where an array's items can be of any type but they must all be of the same type?

[gregsdennis]

You're thinking of an open generic array type. In C#, this would be something like List<T>.

I wrote a blog post on a way to support generic types in JSON Schema a few months ago. It involves $dynamicAnchor and $dynamicRef, so you'll need 2020-12.

There's not a way to leave it open (not define the T) and validate that all of the items are the same, unfortunately. You have to define what the item looks like at some point.

[gregsdennis]

Another option, if you have a limited set of things an item could be is to use an anyOf with a nested items subschema for each kind of thing you have.

{
  "type": "array",
  "anyOf": [
    { "items": { "$ref": "#/$defs/item1" } },
    { "items": { "$ref": "#/$defs/item2" } },
    { "items": { "$ref": "#/$defs/item3" } },
    // ...
  ],
  "$defs": {
    "item1": { /* ... */ },
    "item2": { /* ... */ },
    "item3": { /* ... */ },
    // ...
  }
}

This would ensure that all items are the same type, but it could be any of the listed types.