MVC C# simple question context.Instructors.AddOrUpdate

Could someone tell me what this line of code does?

       instructors.ForEach(s => context.Instructors.AddOrUpdate(p => p.LastName, s));
            context.SaveChanges();


  var instructors = new List<Instructor>
            {
                new Instructor { FirstMidName = "Kim",     LastName = "Abercrombie",
                    HireDate = DateTime.Parse("1995-03-11") },
                new Instructor { FirstMidName = "Fadi",    LastName = "Fakhouri",    
                    HireDate = DateTime.Parse("2002-07-06") },
                new Instructor { FirstMidName = "Roger",   LastName = "Harui",      
                    HireDate = DateTime.Parse("1998-07-01") },
                new Instructor { FirstMidName = "Candace", LastName = "Kapoor",      
                    HireDate = DateTime.Parse("2001-01-15") },
                new Instructor { FirstMidName = "Roger",   LastName = "Zheng",      
                    HireDate = DateTime.Parse("2004-02-12") }
            };
            instructors.ForEach(s => context.Instructors.AddOrUpdate(p => p.LastName, s));
            context.SaveChanges();
yguyon28Asked:
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chaauConnect With a Mentor Commented:
This is a new feature of C# called "lambda expression".
The statement:
instructors.ForEach(s => context.Instructors.AddOrUpdate(p => p.LastName, s));
context.SaveChanges(); 

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is an equivalent of:
foreach(Instructor s in instructors)
{
    foreach(var p in context.Instructors)
    {
        context.Instructors[p.LastName] = s;
    }
}
context.SaveChanges(); 

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