Create Filter Definition with concatenation of document fields

Hi Team,

I’m using c# driver for mongodb.

Here’s my use case -
I’m trying to filter documents that has concatenated values of 2 fields present in another List.

I’m trying sth. like this -

var concatString  = List<string>{"aabb", "ccdd", "eeff"};
var filter = Builder<T>.Filter.In(x => x.prop1 + "," + x.prop2, concatstring)

But this throws me error - Invalid format x => x.prop1 + "," + x.prop2

Any Idea how can I create a filter by combining 2 fields ?