APEX SalesForce Contact Trigger with Tests

RestrictContactByName.apxt

trigger RestrictContactByName on Contact (before insert, before update) {
	
	//check contacts prior to insert or update for invalid data
	For (Contact c : Trigger.New) {
		if(c.LastName == 'INVALIDNAME') {	//invalidname is invalid
			c.AddError('The Last Name "'+c.LastName+'" is not allowed for DML');
		}

	}
}

TestRestrictContactByName.apxc

@isTest
public class TestRestrictContactByName {

	@isTest static void TestDeleteUpdateContactWithValidLastname() {
        // Test data setup
        // Create an account with an opportunity, and then try to delete it
        Contact contact = new Contact(LastName='Test Contact');
        insert contact;

        // Perform test
        Test.startTest();
        Database.DeleteResult result = Database.delete(contact, false);
        Test.stopTest();
        
		// Verify 
		System.assert(result.isSuccess());
    }

	@isTest static void TestUpdateContactWithInvalidLastname() {
        // Create an conact with an invalid last name
        try{	
        	Contact contact = new Contact(LastName='INVALIDNAME');
            insert contact;
        } catch(DmlException e) {
            System.assertEquals('Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, The Last Name "INVALIDNAME" is not allowed for DML: []',
        	e.getMessage());
        }    
    }      
}