Class design for filter expressions

Question

I have a couple of C# business class as follows:

    public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string DepartmentCode { get; set; }
    public string HireDate { get; set; }
    public DateTime DateOfBirth { get; set; }
    public DateTime Gender { get; set; }
}

public class EmployeeManagement
{
    public List<Employee> GetEmployees(FilterExpression criteria)
    {
        throw new System.NotImplementedException();
    }
}

The goal is to get the list of employees by passing various filter conditions e.g. all employees belonging to particular department, or hired after a particular date or all female employees or a combination of criterias (involving AND or OR conditions). I don't wish to implement many of the overloaded methods for each of the filter criteria since the end users are defining the filter criteria and i can not assume all possible cases.

Can you please help in designing the "FilterExpression" class that i can pass to GetEmployees method? Any other alternate approach suggestion would be welcome.

Thanks.

More details: The employee data is stored in a DB table. I don't want to bring in all the employees in List<Employee> and then filter the data. My goal is to generate the SQL "where clause" from the filter expression so that from the database itself, I get the filtered Employee dataset. I am struggling with the class design of the FilterExpression class as asked above.

Answer 1

Why not use an Expression<Func<Employee,bool>> instead? Although it's typically associated with LINQ it doesn't have to be. You could pass a lambda and examine the expression tree at runtime that way.

var employees = GetEmployees(x => x.HireDate > DateTime.Today.AddDays(-7))

Answer 2

You must tell us what kind of expression/filter you want to pass to the method. Otherwise I'll give a cheat answer:

    public List<Employee> GetEmployees(Expression<Func<Employee,bool>> predicate)
    {
        return AllEmployees.Where(predicate.Compile()).ToList();
    }

you can use:

var bob = GetEmployees(emp=>emp.Name.Equals("Bob")).FirstOrDefault;
var ITStaff = GetEmployees(emp=>emp.DepartmentCode.Equals("IT"));