Well, the code you've given is invalid to start with - List is a generic type, and it has an Add method instead of add etc.
But you could do something like:
List<Person> list = new List<Person>
{
new person{ID=1,Name="jhon",salary=2500},
new person{ID=2,Name="Sena",salary=1500},
new person{ID=3,Name="Max",salary=5500}.
new person{ID=4,Name="Gen",salary=3500}
};
// The "Where" LINQ operator filters a sequence
var highEarners = list.Where(p => p.salary > 3000);
foreach (var person in highEarners)
{
Console.WriteLine(person.Name);
}
If you want to learn details of what all the LINQ operators do, and how they can be implemented in LINQ to Objects, you might be interested in my Edulinq blog series.