[TestFixture]
public class InterfaceSample
{
    [Test]
    public async Task Test()
    {
        var subject = DriverTestConfiguration.Client.GetDatabase("test").GetCollection<MyEntity>("test");
        await subject.DeleteManyAsync("{}");
        await subject.InsertOneAsync(new MyEntity { Id = 3, Type = "awesome" });

        var entity = await Find(subject, 3);
    }

    private Task<T> Find<T>(IMongoCollection<T> collection, int id) where T : IEntity
    {
        return collection.Find(x => x.Id == id).FirstOrDefaultAsync();
    }
}

public interface IEntity
{
    int Id { get; set; }
}

public class MyEntity : IEntity
{
    public int Id { get; set; }

    public string Type { get; set; }
}