Why do my tests fail when run together, but pass individually?

Question

When I write a test in Visual Studio, I check that it works by saving, building and then running the test it in Nunit (right click on the test then run).

The test works yay... so I Move on...

Now I have written another test and it works as I have saved and tested it like above. But, they dont work when they are run together.

Here are my two tests that work when run as individuals but fail when run together:

using System; using NUnit.Framework; using OpenQA.Selenium.Support.UI; using OpenQA.Selenium;  namespace Fixtures.Users.Page1 {     [TestFixture]     public class AdminNavigateToPage1 : SeleniumTestBase     {         [Test]         public void AdminNavigateToPage1()         {             NavigateTo<LogonPage>().LogonAsCustomerAdministrator();             NavigateTo<Page1>();             var headerelement = Driver.FindElement(By.ClassName("header"));              Assert.That(headerelement.Text, Is.EqualTo("Page Title"));             Assert.That(Driver.Url, Is.EqualTo("http://localhost/Page Title"));         }          [Test]         public void AdminNavigateToPage1ViaMenu()         {             NavigateTo<LogonPage>().LogonAsCustomerAdministrator();             Driver.FindElement(By.Id("menuitem1")).Click();             Driver.FindElement(By.Id("submenuitem4")).Click();             var headerelement = Driver.FindElement(By.ClassName("header"));              Assert.That(headerelement.Text, Is.EqualTo("Page Title"));             Assert.That(Driver.Url, Is.EqualTo("http://localhost/Page Title"));         }     } } 

When the second test fails because they have been run together

Nunit presents this:

Sse.Bec.Web.Tests.Fixtures.ManageSitesAndUsers.ChangeOfPremises.AdminNavigateToChangeOfPremises.AdminNavigateToPageChangeOfPremisesViaMenu: OpenQA.Selenium.NoSuchElementException : The element could not be found

And this line is highlighted:

var headerelement = Driver.FindElement(By.ClassName("header")); 

Does anyone know why my code fails when run together, but passes when run alone?

Any answer would be greatly appreciated!

Answer 1

Such a situation normally occurs when the unit tests are using shared resources/data in some way.

1. It can also happen if your system under test has static fields/properties which are being leveraged to compute the output on which you are asserting.
2. It can happen if the system under test is being shared (static) dependencies.