System.nullreferenceexception: 'object reference not set to an instance of an object.'

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Hi ,
I am Working in VS2019 ,First i select a Empty Project and in this i Added a Asp.netCore Web Aplication name T2.Web ,and also i added 3 new project i.e Class library asp.netcore class library .
1 T2.Entity and it's have all Entity Models
2 T2.Database and its content Dbcontex and Migration
3 T3.Services it is database handler between controller class and database

but when i trying to Get data by using T2.services Errorr -

Quote:
System.NullReferenceException: 'Object reference not set to an instance of an object.'

I cant understand which reference is missing here

What I have tried:

in Home Controller

 private HomeServices HSe = new HomeServices();

public IActionResult Index()
       {
           HomeViewModel model = new HomeViewModel();
           model.ShopCatlist = HSe.GetShopCategotiesAll();



           return View();
       }

in T2.Services

private readonly TDbContexxt _db;

       public HomeServices()
       {
       }

       public HomeServices(TDbContexxt db)
       {
           _db = db;
       }


       public async Task<IEnumerable<ShopCategoty>> GetShopCategotiesAll()
       {
           return await _db.ShopCategoties.ToListAsync();



       }

appsetting.json

{
  "ConnectionStrings": {
    "TConnection": "Server=DESKTOP-RSNAD6S;Database=T-Data;Trusted_Connection=True;MultipleActiveResultSets=True"
  },

  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

in startup class

public IConfiguration Configuration { get; }

      // This method gets called by the runtime. Use this method to add services to the container.
      public void ConfigureServices(IServiceCollection services)
      {
          services.AddDbContext<TDbContexxt>(options =>
                  options.UseSqlServer(Configuration.GetConnectionString("TConnection")));
          services.AddControllersWithViews().AddRazorRuntimeCompilation();
          services.AddMvc();
      }

Posted 31-Aug-20 18:15pm

yogesh vaidya

Updated 31-Aug-20 20:10pm

v2

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OriginalGriff · Answer 1 · 2020-08-31T19:53:00

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!

Sandeep Mewara · Answer 2 · 2020-08-31T19:37:00

Details about the error you see: NullReferenceException Class (System) | Microsoft Docs[^]

Quote:
A NullReferenceException exception is thrown when you try to access a member on a type whose value is null

Possibly because:
1. You've forgotten to instantiate a reference type
2. You've forgotten to dimension an array before initializing it.
3. You get a null return value from a method, and then call a method on the returned type.
4. You're using an expression (for example, you're chaining a list of methods or properties together) to retrieve a value
5. You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws
6. Exception is thrown by a method that is passed null

Following will help you learn about debugging:
Tutorial: Debug Visual Basic code - Visual Studio | Microsoft Docs[^]
First look at the debugger - Visual Studio | Microsoft Docs[^]

When you debug, you will be able to get the exact line where the variable is NULL and error is being raised.

For now, in your case, following has high potential:

C#

return await _db.ShopCategoties.ToListAsync();

Either _db is null or _db.ShopCategories is null and thus conversion to list fails. Handle/Resolve on why so.

Try out.

System.nullreferenceexception: 'object reference not set to an instance of an object.'

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