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I have using Asp.net web api and the method calling another Class library which is working fine.
For implementing Redis cache, I changed to class to interface. In this case the method is not fired which is in class library.
I am getting the below error
"Object reference not set to an instance of an object.","ExceptionType":"System.NullReferenceException","StackTrace"


What I have tried:

public class ArticleListingController : ApiController
        {
             public IArticleProvider _newsarticleProvider { get; set; }
     [HttpGet]
            [Route("Listing")]
            public IHttpActionResult GetArticlesListing(string sectionName, int RegionId, int Count)
            {
                List<Article> articleList = new List<Article>();
                if (!string.IsNullOrEmpty(RegionId.ToString()))
                {
                    if (articleList != null)
                    {
                        articleList = _newsarticleProvider.GetArticleListBySectionName(sectionName, RegionId, Count, ListingArticleCache);
                        return Ok(articleList);
                    }
                }
                return NotFound();
            }
    }

In Provider Interface Code
 

    public interface IArticleProvider
        {
            List<Article> GetArticleListBySectionName(string sectionName, int RegionId, int Count, int CacheTime);
        }


In Provider code
public class ArticleProvider 
    {
    public List<Article> GetArticleListBySectionName(string sectionName, int RegionId, int Count, int CacheTime)
            {
               return _articleRepositary.GetArticleListBySectionName(sectionName, RegionId, Count, CacheTime);
                
            }
}
Posted
Updated 12-Mar-19 2:55am
v2
Comments
gbalamurugan 28-Feb-18 3:41am
   
I am getting Null reference error in below line

articleList = _newsarticleProvider.GetArticleListBySectionName(sectionName, RegionId, Count, ListingArticleCache);

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
   
Use the debugger and look at the "_newsarticleProvider" variable on the line that throws the error and you'll find it is null. At no point are you assigning a class to the variable, you're never creating an instance of ArticleProvider.

Create a constructor for your controller and in that constructor do

_newsarticleProvider = new ArticleProvider();


Alternatively look to use dependency injection, but that's a fairly advanced technique, maybe one you can come back to later.
   
I have using dependency injection and my problems get solved

public static class DIContainer
    {
        public static UnityContainer container;

        public static IUnityContainer Initialise()
        {
            container = new UnityContainer();
            RegisterTypes(container);
            return container;
        }

        static void RegisterTypes(IUnityContainer container)
        {         
            container.RegisterType<IArticleProvider, ArticleProvider>(new InjectionConstructor(new ResolvedParameter<ICacheProxy>()));
        
        }

    }


Thanks for your valuable answers!!
   
v2

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