I have a collection of type objects which I know they are type Employee. I'd like to perform some linq operations on them using the employee some thing similar to this:
var options = (from e in m_Employees
select (e as Employee).DepartmentCode).Distinct();
But the as employee is not surprisingly giving an error. is there a way around it?
Changing the collection is not really an options since I,m maintaining the code and I want to avoid big changes.
You can use either
from e in m_Employees.Cast<Employee>()
select e.DepartmentCode
or
from e in m_Employees.OfType<Employee>()
select e.DepartmentCode
Cast thows an error if you can not cast each item to Employee, but OfType will filter out those objects not matching the type.
Cast<T> in a query expression is to specify the type of the range variable explicitly.Rather than using as Employee it would be better to make the compiler basically insert a call to Cast<T> using an explicitly typed range variable:
var options = (from Employee e in m_Employees
select e.DepartmentCode).Distinct();
Or alternatively and equivalently:
var options = m_Employees.Cast<Employee>()
.Select(e => e.DepartmentCode)
.Disinct();
However, I still wouldn't have expected your original code to fail, if the array really does only include Employee references... If you were getting a NullReferenceException then either one of the values was null, or it was a non-null reference to a non-Employee object. These will both still give you an error with the above code, but you'll be able to see which one based on whether you still get a NullReferenceException or an InvalidCastException.
In general you should only use as when you're going to use the result conditionally. If you're sure that every value is really of the right type, you should use a cast instead - so that if you're wrong, you'll get the code blowing up with an exception instead of propagating a null reference to the rest of the code, where it could cause harm later on and make it hard to spot the source of the error.
If you were getting a compile-time error then there are a number of possible causes, based on what exception you were seeing.
EDIT: Okay, so it was an IEnumerable causing a compile-time error... Cast<T>() and OfType<T>() are both extension methods on just IEnumerable instead of on IEnumerable<T>.
IEnumerable) as pointed out in my answer. In that case, his code is simply illegal.object[] or something similar (object[] sounds pretty likely based on the question title). But yes, if it's an untyped collection that would explain it. Basically we don't know...
One option would be:
(from e in m_Employees
let x = e as Employee
select x.DepartmentCode).Distinct();
Use this:
var options = (from e in m_Employees.Cast<Employee>()
select e.DepartmentCode).Distinct();
The important part is the Cast<Employee>. My answer assumes that m_Employees is an IEnumerable like ArrayList instead of an IEnumerable<Employee> like List<Employee>.
You could try this:
var options = m_Employees
.Cast<Employee>()
.Select(item => item.DepartmentCode)
.Distinct();
