Searching if value exists in a list of objects using Linq

One option for the follow on question (how to find a customer who might have any number of first names):

List<string> names = new List<string>{ "John", "Max", "Pete" };
bool has = customers.Any(cus => names.Contains(cus.FirstName));

or to retrieve the customer from csv of similar list

string input = "John,Max,Pete";
List<string> names = input.Split(',').ToList();
customer = customers.FirstOrDefault(cus => names.Contains(cus.FirstName));

Using Linq you have many possibilities, here one without using lambdas:

//assuming list is a List<Customer> or something queryable...
var hasJohn = (from customer in list
         where customer.FirstName == "John"
         select customer).Any();

LINQ defines an extension method that is perfect for solving this exact problem:

using System.Linq;
...
    bool has = list.Any(cus => cus.FirstName == "John");

make sure you reference System.Core.dll, that's where LINQ lives.

zvolkov's answer is the perfect one to find out if there is such a customer. If you need to use the customer afterwards, you can do:

Customer customer = list.FirstOrDefault(cus => cus.FirstName == "John");
if (customer != null)
{
    // Use customer
}

I know this isn't what you were asking, but I thought I'd pre-empt a follow-on question :) (Of course, this only finds the first such customer... to find all of them, just use a normal where clause.)