How to create MongoDB MultiKey index on attribute of items in an array .NET Driver

This is an example how to do it with C#

var indexDefinition = Builders<FooDocument>.IndexKeys.Combine(
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key1),
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key2));

await collection.Indexes.CreateOneAsync(indexDefinition); 

UPDATE

Regarding index within the array, closest what i was able to find is to use "-1" as index whene you building your index key. As i understand from github source code is is a valid option in case of building queries.

var indexDefinition = Builders<FooDocument>.IndexKeys.Combine(
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key1),
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key2[-1].Key));

await collection.Indexes.CreateOneAsync(indexDefinition); 

"-1" is a hardcoded constant in side mongodb C# drivers which means "$" (proof). So this code would try to create index:

{ "Key1": 1, "Key2.$.Key": 1 }

which is fine for querying info from database, but not allowed (will throw an exception "Index key contains an illegal field name: field name starts with '$'") to use in indexes. So i assume it should be changed in mongodb drivers to make it work. Something like "-2" means empty operator. In that case we could use

var indexDefinition = Builders<FooDocument>.IndexKeys.Combine(
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key1),
    Builders<FooDocument>.IndexKeys.Ascending(f => f.Key2[-2].Key));

await collection.Indexes.CreateOneAsync(indexDefinition); 

which would generate index like:

{ "Key1": 1, "Key2.Key": 1 }

So basically i don't think it is possible right now to build index you want with pure Linq without changing mongo C# drivers.

So i think your only option do like this, still C# but without Linq

await collection.Indexes.CreateOneAsync(new BsonDocument {{"name", 1}, {"bars.key", 1}});

This appears to be a requested feature for the C# driver, although it hasn't seen any progress lately. That said, someone did submit a rough-and-ready solution there on the JIRA thread, so perhaps that will do the job for you.

You can create a string index and use nameof() in C# 6:

Indexes.Add(Context.Collection<FooDocument>().Indexes.CreateOne($"{nameof(FooDocument.Bars)}.{nameof(Bars.Key)}"));