Using LINQ to Objects to find items in one collection that do not match another

Question

I want to find all items in one collection that do not match another collection. The collections are not of the same type, though; I want to write a lambda expression to specify equality.

A LINQPad example of what I'm trying to do:

void Main()
{
    var employees = new[]
    {
        new Employee { Id = 20, Name = "Bob" },
        new Employee { Id = 10, Name = "Bill" },
        new Employee { Id = 30, Name = "Frank" }
    };

    var managers = new[]
    {
        new Manager { EmployeeId = 20 },
        new Manager { EmployeeId = 30 }
    };

    var nonManagers =
    from employee in employees
    where !(managers.Any(x => x.EmployeeId == employee.Id))
    select employee;

    nonManagers.Dump();

    // Based on cdonner's answer:

    var nonManagers2 =
    from employee in employees
    join manager in managers
        on employee.Id equals manager.EmployeeId
    into tempManagers
    from manager in tempManagers.DefaultIfEmpty()
    where manager == null
    select employee;

    nonManagers2.Dump();

    // Based on Richard Hein's answer:

    var nonManagers3 =
    employees.Except(
        from employee in employees
        join manager in managers
            on employee.Id equals manager.EmployeeId
        select employee);

    nonManagers3.Dump();
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Manager
{
    public int EmployeeId { get; set; }
}

The above works, and will return Employee Bill (#10). It does not seem elegant, though, and it may be inefficient with larger collections. In SQL I'd probably do a LEFT JOIN and find items where the second ID was NULL. What's the best practice for doing this in LINQ?

EDIT: Updated to prevent solutions that depend on the Id equaling the index.

EDIT: Added cdonner's solution - anybody have anything simpler?

EDIT: Added a variant on Richard Hein's answer, my current favorite. Thanks to everyone for some excellent answers!

Answer 1

    /// <summary>
    /// This method returns items in a set that are not in 
    /// another set of a different type
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <typeparam name="TOther"></typeparam>
    /// <typeparam name="TKey"></typeparam>
    /// <param name="items"></param>
    /// <param name="other"></param>
    /// <param name="getItemKey"></param>
    /// <param name="getOtherKey"></param>
    /// <returns></returns>
    public static IEnumerable<T> Except<T, TOther, TKey>(
                                           this IEnumerable<T> items,
                                           IEnumerable<TOther> other,
                                           Func<T, TKey> getItemKey,
                                           Func<TOther, TKey> getOtherKey)
    {
        return from item in items
               join otherItem in other on getItemKey(item)
               equals getOtherKey(otherItem) into tempItems
               from temp in tempItems.DefaultIfEmpty()
               where ReferenceEquals(null, temp) || temp.Equals(default(TOther))
               select item;
    }

I don't remember where I found this method.

Answer 2

Have a look at the Except() LINQ function. It does exactly what you need.

Answer 3

var nonmanagers = employees.Select(e => e.Id)
    .Except(managers.Select(m => m.EmployeeId))
    .Select(id => employees.Single(e => e.Id == id));

Answer 4

         var nonManagers = ( from e1 in employees
                             select e1 ).Except(
                                   from m in managers
                                   from e2 in employees
                                   where m.EmployeeId == e2.Id
                                   select e2 );

Answer 5

This is almost the same as some other examples but less code:

employees.Except(employees.Join(managers, e => e.Id, m => m.EmployeeId, (e, m) => e));

It's not any simpler than employees.Where(e => !managers.Any(m => m.EmployeeId == e.Id)) or your original syntax, however.