This is an extension of the previous post Get Random Element From C# List in which I retrieve a random element from List<T>. This time, though, I will cache the list, and retrieve a random element from the cache, and made sure that on browser refresh the element is retrieved from the cache and not from the list. And this time, instead of a Console project, I will use a Web Application project and a Generic Handler(.ashx file), similar to what I did in my previous post Display Image as ByteArray Using C# Generic Handler and JQuery. Check out that post to see how to set up ASP.NET Web Application project and Generic Handler.

Caching a generic collection in .NET is commonly used for image processing. For this simple demo though, I will simply cache a hardcoded list, just to show how I do it using System.Web.HttpRuntime.Cache (In my future post, I will show how to use this list to store info from the file system, cache the list, and use this cache to display random images to the browser).

Source Code

Download source code here. This project was created in Visual Studio Community 2019.

Application Demo

When I run my program, I call Handler.ashx directly in my browser. And, this is what I get.

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On Refresh, I may get the same string again. Or I may get a different one. This is because I only have three things to choose from. For a list with thousands or even hundreds of elements, I may get a different one everytime.

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Program Setup

In Visual Studio, create the following:

1. A web application, ASP.NET Web Application (.NET Framework). I called my application, GetRandomCaching.
2. A generic handler, and simply call it Handler.ashx.
3. A class called DirInfoCacher.cs - this one is not necessary though. I just wanted the caching logic separate from the handler.

My project looks like this.

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The Handler

My handler is simple:

public class Handler1 : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        
        DirInfoCacher infoCacher = new DirInfoCacher();

        string randomEl = infoCacher.GetRandom();

        context.Response.Write(randomEl);
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

Brief Explanation

First, I declare that I’ll be returning a plain text to the browser as my response,

context.Response.ContentType = "text/plain";

I instantiate my DirInfoCacher class, like so,

DirInfoCacher infoCacher = new DirInfoCacher();

and calls a method called GetRandom()

string randomEl = infoCacher.GetRandom();

which returns a string. I, then, send that string out to the browser.

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The DirInfoCacher class

My main logic is in the DirInfoCacher.cs and it looks like this,

public class DirInfoCacher
{
    public DirInfoCacher()
    {
    }

    public string GetRandom()
    {
        object cacheList = HttpRuntime.Cache.Get("biology") as List<string>;
        string returnedString = string.Empty;

        if (cacheList == null)
        {
            // Initialize our list
            List<string> myList = new List<string>(new string[] { "biology-1", "biology-2", "biology-3" });

            // Cache the 
            HttpRuntime.Cache.Insert("biology", myList, null, DateTime.Now.AddMinutes(60d), System.Web.Caching.Cache.NoSlidingExpiration);

            Random R = new Random();

            // get random number from 0 to 2. 
            int someRandomNumber = R.Next(0, myList.Count());
            returnedString = myList.ElementAt(someRandomNumber);
            return returnedString;
        }
        else
        {
            IList<string> objectCache = (IList<string>)cacheList;

            Random R = new Random();

            // get random number from 0 to 2. 
            int someRandomNumber = R.Next(0, objectCache.Count());

            returnedString = objectCache.ElementAt(someRandomNumber);

            return returnedString;
        }

    }
}

Brief Explanation

Notice that it is divided into two major logical parts.

if(cacheList == null)
{
    // Build the cache & get a random string from the list
    // return the string
}
else
{
    // Get a random string from the cache
    // return the string
}

First part builds the cache when the application is run for the first time. The second part just reads from the cache.

As you may have thought, the first part is called only once - the first time we run the application. On refresh, we always execute the second part.

The following code retrieves the cache named “biology” if it exists,

object cacheList = HttpRuntime.Cache.Get("biology") as List<string>;

If cache does not exist, then initialize the list.

List<string> myList = new List<string>(new string[] { "biology-1", "biology-2", "biology-3" });

Then, I cache the list soon after that using HttpRuntime.Cache.Insert method.

HttpRuntime.Cache.Insert("biology", myList, null, DateTime.Now.AddMinutes(60d), System.Web.Caching.Cache.NoSlidingExpiration);

In the snippet above, I am creating a cache and naming it as “biology”. Then, I pass my list object to it. I also tell it that my cache expires in 60 minutes(absoluteExpiration). Note that if you specified an absolute expiration, the last parameter must be NoSlidingExpiration. The code below produces a random element from the list(same code I used from my old post).

Random R = new Random();               
int someRandomNumber = R.Next(0, myList.Count());
returnedString = myList.ElementAt(someRandomNumber);

On refresh, because the cache object already exists, the code executes the else part. Then we just read our cache like so,

IList<string> objectCache = (IList<string>)cacheList;

we now cast it as IList<string> because we created it previously as object type, to enable us to process it as a list. Then, as we did previously we just need to get a random element from that list, like so,

Random R = new Random();         
int someRandomNumber = R.Next(0, objectCache.Count());
returnedString = objectCache.ElementAt(someRandomNumber);
return returnedString;

And That’s It!

I hope it helps you a little in your projects.