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

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.