Hi,
we are using xunit and moq for unit testing.
How can I "mock" a method which is taking readonlycollection<employee> and it is updating one of its property and returning readonlycollection<employee>?

I really appreciate your help.

The method is doing following functionality.

public async Task<ReadOnlyCollection<Employee>> CalculateAsync(ReadOnlyCollection<Employee> employees)
        {
            if (employees == null)
            {
                throw new ArgumentNullException(nameof(employees));
            }

            ReadOnlyCollection<int> Ids = employees.Select(x => x.Id).Distinct().ToList().AsReadOnly();

            var Results = (await Repository.GetAsync(Ids));          

            foreach (var r in Results)
            {
                var Employeeid= employee.FirstOrDefault(x => x.Id == r.Id);
                if Employeeid!= null)
                {
                    employee.BillDate = r.StartDate;
                }
            }

            return employee.ToList().AsReadOnly();

        }