Essayez ElemMatch :
var thread = threadHelper.Collection.Find(
Query.And(
Query.ElemMatch("Recipients", Query.EQ("UserId", user.Id)),
Query.ElemMatch("Recipients", Query.EQ("UserId", sendToUser.Id))
)
).SingleOrDefault();
Essayez ElemMatch :
var thread = threadHelper.Collection.Find(
Query.And(
Query.ElemMatch("Recipients", Query.EQ("UserId", user.Id)),
Query.ElemMatch("Recipients", Query.EQ("UserId", sendToUser.Id))
)
).SingleOrDefault();