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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 var

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