Check if data exist in List Generic Collection using C# .Net

i want to check my collection so i can add a new customer as long as the username does not exist in the collection i do not want 2 customers to be able to have the same username

i have been trying many different things to try to get this to work but i am not exactly sure what i should be doing I have a aspforum and collection C# class of the list

here is some things that i have tried

CustomerExists()      i also have a collection of List<Customers>

//HERE IS MY EXISTS METHOD
public static Boolean IfCustomerExists(string login)
    {
        List<Customer> theGuy = Collections.Customers;

        bool exist = false;
        foreach (Customer customer in theGuy)
        {
            if (customer.UserName == login)
            {
                exist = true;
                break;
            }           
        }
        return exist;
    }
//THIS IS MY FORUM TRYING TO ADD CUSTOMER TO COLLECTION IF ALL IS WELL
if(customer.UserName != CustomerOptions.IfCustomerExists(Collections.Customers))
        { 
            Collections.Customers.Add(customer);
        }