自定义属性以缓存 C# 中的只读属性值

Question

I am trying to create a Custom Attribute capable of caching the value of a property, the type of the property is IWebElement which is somewhat expensive to create and could throw a NoSuchElement exception.

I have had some success implementing it like this:

protected Lazy<IWebElement> lazyWebElement;
protected virtual IWebElement cachedWebElement
{
    get
    {
        if (!lazyWebElement.IsValueCreated)
        {
            try
            {
                lazyWebElement = new Lazy<IWebElement>(() => driver.FindElement(By.Id("someElement")),
                    LazyThreadSafetyMode.PublicationOnly);
            }
            catch (NoSuchElementException)
            {
                throw new NoSuchElementException("someElement is not present in DOM");
            }
        }

        return lazyModal.Value;
    }
}

And what I would like to do is

[CachedWebElement]
protected virtual IWebElement cachedWebElement => driver.FindElement(By.Id("someElement"));

But Attribute does not allow complex types in it's constructor so I am not able to pass driver.FindElement(By.Id("someElement")) as a parameter.

Having it cached would be great as right now I am using a backing field to save the value and the whole mechanism is lost if I override the property in a derived class.

Thank you.

Answer 1

You probably won't be very successful in your approach. I mean, "just adding an attribute".

After compiling, and during runtime, when you retrieve your object (ie when you do something like cachedWebElement.Displayed ), a get method is called.

When a project is build, all properties get get set accessor methods attached to them. You can find out more here microsoft get keyword . Basically, the { get; set; } { get; set; } { get; set; } get compiled into functions, that you can't really "mess" with. They are the default implementations of the compiler. (if I got that right).

If you really want to follow this approach, take a look at this answer .

Now, to your problem

• If you want to have a "cache" for the elements properties, that's fine. You can create a class that will hold your elements for you, and before each FindElement() will check to see if it already exists.
• If you also want to be able to "click" an element, that's impossible. If you "FindElement" on a button, and then the page changes, and you want to Click() it from cache, that can't happen. The page changed. There is a reason that exception is thrown.

I will assume that you want to "cache" only the properties/information about the element, and not actually interact with it.

Then, we will just have to create a "middle man" that will handle locating each element, but will also cache the elements we find. If we ask them again, the class will give them to us from memory instead of "re-locating" them. Let's do them as extension methods to be a little pretty:

public static class ElementLocator
{
    private static readonly Dictionary<By, IWebElement> _CachedElements;

    public static IWebElement FindElementFromCache(this IWebDriver driver, By by)
    {
        if (_CachedElements.ContainsKey(by))
            return _CachedElements.Single(e => e.Key == by).Value;

        return driver.FindElement(by);
    }

    public static ReadOnlyCollection<IWebElement> FindElementsFromCache(this IWebDriver driver, By by)
    {
        if (_CachedElements.ContainsKey(by))
        {
            List<IWebElement> foundCache = _CachedElements.Where(e => e.Key == by)
                                                          .Select(e => e.Value)
                                                          .ToList();

            return new ReadOnlyCollection<IWebElement>(foundCache);
        }

        return driver.FindElements(by);
    }

    static ElementLocator()
    {
        _CachedElements = new Dictionary<By, IWebElement>();
    }
}

Then, you can simply do driver.FindElementFromCache(By.Id("someElement"));

But this might not be the complete implementation. Because, when searching for multiple elements, maybe a new one was added. Instead of 4 elements you got a minute ago, now a new table row is added, and you have 5 elements. The ElementLocator will try to see if there is anything in cache, will find 4 elements and return them to you without searching for the 5th one.

IMO, creating a caching mechanism MIGHT not be the best solution to your problem. It introduces (excuse my language) a shit-ton of problems that you will face down the road.

If you could present us with what the problem is, we could figure out something that might not involve caching. I mean, don't say "how can I do this cache better" or "my caching has a problem", but let's see why you need a cache for elements to begin with.

Best of luck to you!