When using the insertOne function in the C# driver, in case we initialize an array with {{new[]

}}.  The _t Discriminator is missing in the DB.

for example:

collection.InsertOne(new Parent { Children = new[] { new Child() } });

will save to DB:

{"_id":"5bf6aef6c0beccc414b70d45","Child":[{}]}

The problem does not happen when using List

collection.InsertOne(new Parent { Children = new List<IChild> { new Child() } });

will insert into DB:

{    "_id":"5bf6b074c0beccc414b70dc2",    "Children":[{"_t":"Child"}]}

for further information please check this StackOverflow question:

https://stackoverflow.com/questions/53432370/inserting-an-array-into-a-child-collection-in-mongodb-is-omitting-the-t-discrim/53544804?noredirect=1#comment94153188_53544804