trigger CountyLookupByZip on Contact (before insert) {
    for (Contact c : Trigger.New) {
        if(c.MailingPostalCode != null) {
        	List<Zip_Code__c> contactCounty = new List<Zip_Code__c>();
            contactCounty = [SELECT Id,
                                County__c,
                                Postal_Code__c
         				 FROM   Zip_Code__c 
        			     WHERE  Postal_Code__c = :c.MailingPostalCode];
            if(contactCounty.size() != 0){
            	c.Mailing_County__c = contactCounty[0].County__c;
            } else {
                c.MailingPostalCode.addError('Postal Code not found. Please ensure the postal code is a valid Virginia zip code.');
            }
        }
	}
}

@isTest
public class TestCountyLookupByZip {

    @isTest
    public static void TestContactWithNullZip() {
        // Test data setup
        // Create a contact without a Mailing Postal Code
        // and check to ensure County is also null
        Contact contactNullZip = new Contact(LastName = 'Test');
        insert contactNullZip;
        
        // Perform Test
        System.assertEquals(contactNullZip.Mailing_County__c, null);
    }

    @isTest
    public static void TestContactWithValidZip() {

        // Test data setup
        // Create a contact with a valid VA Mailing Postal Code
        // and check to ensure County is updated correctly
        
    	Contact contactValidZip = new Contact(LastName = 'Test', MailingPostalCode = '20101');
        insert contactValidZip;
        
        // Perform Test
        System.assertEquals(contactValidZip.Mailing_County__c, 'Loudoun');
    }   
    
    @isTest
    public static void TestContactWithInvalidZip() {
        // Test data setup
        // Create a contact with an invalid Mailing Postal Code
        // and throw an error
        Test.startTest();
        try {
       		Contact contactInvalidZip = new Contact(LastName = 'Test', MailingPostalCode = '17602');
        	insert contactInvalidZip;
        // Perform Test
        } catch(Exception e) {
			System.Assert(e.getMessage().contains('FIELD_CUSTOM_VALIDATION_EXCEPTION'));
            System.Assert(e.getMessage().contains('Postal Code not found. Please ensure the postal code is a valid Virginia zip code.'));
        }
        Test.stopTest();
    }
}