See the below example I am adding 4 items to list with 1 duplicate and calling these methods, output written with comments side by method call.
Base b1 = new Base() { Id = 1, Name = "base1" };
Base b2 = new Base() { Id =2, Name = "base2" };
Base b3 = new Base() { Id = 3, Name = "base3" };
List<Base> lst = new List<Base> { b1, b2, b3, b1 };
Base b4 = new Base { Id = 4, Name = "base4" };
var findfirst= lst.Find(x => x.Id == 9);
var firstElement = lst.First(x => x.Id == 9);//exception
var firstOrDefaultTest = lst.FirstOrDefault(x => x.Id == 90); //null, default value;
var singleOrdefaulttest = lst.SingleOrDefault(x => x.Id == 1);// exception because found more than 1 element for Id=1. no exception In-case not found.
var singleTest = lst.Single(x => x.Id == 90);//exception if not found also if found duplicate.
