Implementing partialFilterExpression in subdocument field C# driver

Working with Data
#1

sample doc:-

{
“ClientId” : “Tenant1”,
“email” : “em1@em.com”,
“CredentialItems” : [
{
“CardNumber” : “1002”
}
]
}

I have mongo createIndex command:-

db.Identities.createIndex({“ClientId” : 1, “CredentialItems.CardNumber” : 1},{“unique”:true, “partialFilterExpression”:{“CredentialItems.CardNumber”:{ $exists: true } } })

the same command I want to do from C# mongo driver some thing like below

var createIndexOptions = new CreateIndexOptions()
{
Name = “ClientIdCard_Index”,
Unique = true,
Background = false,
PartialFilterExpression= Builders.Filter.Exists(i => i.CardNumber)
};

await collection.Indexes.CreateOneAsync(Builders.IndexKeys.Ascending(“ClientId”).Ascending(“CredentialItems.CardNumber”), createIndexOptions);

but I am getting error from c#

public class Identity
{
public string ClientId {get;set;}
public string email {get;set;}
public List { get; set; }
}
public class CredentialItems
{
public string CardNumber { get; set; }
public string CardType { get; set; }
}

so how to implement PartialFilterExpression for subdocument field in C#

with regards
AG