No more StaleElementReferenceException



StaleElementReferenceException can be definitely classified as the number 1 nightmare of people who write automated tests with Selenium framework. This exception occurs when given web element with which we are trying to interact is no longer present in DOM tree. This can be caused by multiple factors, the most common being:

  • an element was removed in the meantime
  • an element was replaced with newer content (for example by Ajax)
  • an element was re-rendered by JavaScript view/template framework

In the first case, StaleElementReferenceException indicates the real issue - the app is broken or our automated test case is invalid - whereas the last two cases are mostly caused by UI framework and shouldn’t affect our UI test. All of the solutions that I’ve found in the network are based on applying try-catch-retry pattern around the places where the problem occurs. However, this could degrade readability and maintainability. So how to get rid of StaleElementReferenceException without introducing technical debt and changing the way we interact with web elements?

StableWebElement to the rescue! 🔗︎

I got the idea of StableWebElement – a wrapper for IWebElement which could detect the situation of stale reference and try to find a new reference to the original element (all this happening behind the scenes). The key element of this wrapper is memoization of searching context (parent element) and locator. Let’s see how it works on example. We have a user list which is a collection of list elements and each list element contains a button to deactivate the user. In order to perform user deactivation, we have to find user list, next find list element that represents given user and at the end, locate deactivation button inside the list element. The code for automating it with Selenium can look as follows:

[Test]
publicv void should_be_able_to_deactivate_user(RemoteWebDriver webDriver)
{
    var userList = webDriver.FindElement(By.Id("UserList"));
    var userListElement = userList.FindElement(By.XPath("*[2]"));
    var deactivationButton = userListElement.FindElement(By.CssSelector(".deactivate-button"));
    deactivationButton.Click();
}

The mapping between UI elements and objects which represent them is showed in the diagram below:

stable element relation diagram

The problem of stale reference can occur at any level of object hierarchy: our deactivate button can be re-rendered or either list element or the whole user list can be replaced with the newer version of HTML. No matter which element was refreshed, StableObjectElement should be able to restore all references necessary to perform intended action.

Implementing StableWebElement 🔗︎

I started with defining IStableWebElement that enriches IWebElement with two additional methods: IsStale which detects if the element is affected by stale reference and RegenerateElement which restores the reference to the original element.

public interface IStableWebElement : IWebElement, ILocatable, ITakesScreenshot, IWrapsElement, IWrapsDriver
{
    bool IsStale();
    void RegenerateElement();
}

At first, I thought that extending IWebElement would be enough, but after running my test suit I encountered a few methods from Selenium that accept IWebElement and perform casting to different interfaces that are not in the IWebElement inheritance hierarchy.

Now we can create an implementation of our IStableWebElement interface:

public class StableWebElement: IStableWebElement
{
    private readonly ISearchContext parent;
    private IWebElement element;
    private readonly By locator;

    public StableWebElement(IWebElement elemen, ISearchContext parent, By locator)
    {
        this.parent = parent;
        this.element = element;
        this.locator = locator;
    }
}

All the methods and properties from the interfaces extended by IStableWebElement should be implemented as a proxy for original methods on the element object in the following manner:

public void SendKeys(string text)
{
    Execute(() => element.SendKeys(text));
}

public void Click()
{
    Execute(() => element.Click());
}

public bool Displayed 
{ 
    get 
    { 
        return Execute(() => element.Displayed); 
    } 
}

All original methods were wrapped in the Execute method which is responsible for detecting of StaleElementReferenceException during original method invocation. When the exception occurs, we try to restore element reference by invoking RegenerateElement and retry the operation. There are two Execute methods because we have to wrap methods and functions with them (notice that one returns generic element and the second one returns void):

private T Execute<T>(Func<T> function)
{
    T result = default (T);
    Execute(() => { result = function(); });
    return result;
}

private void Execute(Action action)
{
    var success = RetryHelper.Retry(3, () =>
    {
        try
        {
            action();
            return true;
        }
        catch (StaleElementReferenceException)
        {
            RegenerateElement();
            return false;
        }
    });
    if (success == false)
    {
        throw new WebElementNotFoundException("Element is no longer accessible");
    }
}

And the most interesting part is the RegenerateElement method:

public void RegenerateElement()
{
    var stableParent = parent as IStableWebElement;
    if (stableParent != null && stableParent.IsStale())
    {
        stableParent.RegenerateElement();
    }
    try
    {
        this.element = this.parent.FindElement(locator);
    }
    catch(Exception ex)
    {
        throw new CannotFindElementByException(locator, parent, ex);  
    }
}

At first, we check if our parent (which is our search context) is also affected by stale reference. If this is the case, we perform RegenerateElement method on the parent. This creates recursion call and we are able to restore references to the root element (no matter how many parents are in the dependency chain). After we retrieve the reference to the parent, we also need to find a fresh version of the current element using new parent and memorized locator. If any exception occurs during searching for the current element, that means the element truly disappeared and the StaleReferenceException was an indicator of the true problem. The last missing part is the method that detects if the element has a stale reference.

public bool IsStale()
{
    try
    {
        //INFO: If element is stale accessing any property should throw exception        
        var tagName = this.element.TagName;
        return false;
    }
    catch (StaleElementReferenceException )
    {
        return true;
    }
}

Applying StableWebElement 🔗︎

In order to use StableWebElement we need to add extension method for ISearchContex that searches for given element and wraps it into our StableWebElement proxy:

static class StableElementExtensions
{
    public static IStableWebElement FindStableElement(this ISearchContext context, By locator)
    {
        var element = context.FindElement(locator);
        return new StableWebElement(context, element, locator);
    }    

    public static ReadOnlyCollection<IStableWebElement> FindStableElements(this ISearchContext context, By locator)
    {        
        return  context.FindElements(locator
                .Select(x=> new StableWebElement(context, x, locator))
                .ToList()
                .AsReadOnly();
    }    
}

Now we have to replace all invocation of FindElement with our new extension method. If we don’t need to be explicit about using this new approach, we can tweak FindElement and FindElements methods from our StableWebElement wrapper to intercept the real result and wrap it into StableWebElement:

public IWebElement FindElement(By locator)
{
    var foundElement = Execute(() => element.FindElement(locator));
    return new StableWebElement(this.parent, foundElement, locator);
}

public ReadOnlyCollection<IWebElement> FindElements(By locator)
{
    return Execute(() => element.FindElements(locator))
        .Select(x=> new StableWebElement(this.parent, x, locator))
        .ToList()
        .AsReadOnly();
}

This little trick allows us to introduce StableWebElement almost transparently into existing codebase with minimal effort. I said “almost” because we still need to use FindStableElement extension method on search context which is WebDriver (alternatively, we can create a wrapper for WebDriver that uses the same trick as StableWebElement).

Summary 🔗︎

After applying StableWebElement in my UI Test the problem of StaleObjectReferenceException was completely eliminated. Utilizing the proxy pattern allowed me to introduce the StableWebElement with minimal effort, preserving standard Selenium API and keeping the readability of my test code. An attempt of implementation of StableWebElement can be found in Tellurium project here.


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