Unit Test BulkWriteAsync using C#

I am having trouble writing a unit test for a method that uses Mongo Driver BulkWriteAsync().
The return for BulkWriteAsync() is BulkWriteResult which is an abstract class and I am not able to mock it. So I create a concrete class call MockBulkWriteResult which inherits BulkWriteResult.

Here is a sample code:
var response = new MockBulkWriteResult(5);

        var mockCollection = new Mock<IMongoCollection<Dto>>();
            .Setup(x => x.BulkWriteAsync(

Any suggestions?

I am just pasting code which might help

 var returnBulkResponse =(BulkWriteResult<HierarchyDto>) new BulkWriteResult<HierarchyDto>.Acknowledged(200,0,0,0,
                0,new List<WriteModel<HierarchyDto>>(), new List<BulkWriteUpsert>());

                    => s.BulkWriteAsync(It.IsAny<IEnumerable<WriteModel<HierarchyDto>>>(), null,
                        new CancellationToken()))

Outstanding. Thanks very much, as I had hit a similar issue and was puzzling how to create the BulkWriteResult for the unit test.