How to throw a custom exception?

Question

ASP.NET

for example, there is such a custom exception:

public class CustomerExistsException
    {
        public void DisplayException()
        {
            throw new AuthException("The email has already been taken.");
        }
    }

    public class AuthException : Exception
    {
        public AuthException(string message) : base(message) { }
        public AuthException(string message, Exception exception) 
            : base(message, exception) { }
    }

And then how to throw it in the service?

public async Task<CustomerDto> Register(CustomerDto customerModel, string password)
        {
            if (await EmailExists(customerModel.Email))
            {
                //Cannot implicitly convert type CustomerExistsException to System.Exception
                //throw new CustomerExistsException();
            }
        }

Answer 1

Throwing an exception is the most "expensive" of all possible solutions to a problem in terms of performance. In addition, this is not an exception at all, but a completely regular situation. A simple check in the form of a call to bool CustomerExists would look much more appropriate here. In addition, you already have this check.
And so, yes, the class of any exception must be a successor of System.Exception

Answer 2

regular function
new CustomerExistsException().DisplayException();