Unable to update the EntitySet - because it has a DefiningQuery and no element exist

When you come across the above error while inserting into the database table using LINQ the reason for that is your Database table doesn't have a primary key I was trying to insert the data into the table which does not have any primary key created on it the solution is to create the primary key on that table.

No 'Access-Control-Allow-Origin' header is present on the requested resource.

When we create a web API and it works fine in our local application or the same application but if we want that same API to use in the different application then we come across an error known as the 'Access-Control-Allow-Origin'

Please look at the below image

Failed to load http://localhost:4815/api/v1/user/hello: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access.

To solve that error we need to do the following thing.By setting the cors we are allowing the other users to access our web API we can set the restriction also on the controllers or on the methods of our web API so that users can only access the content which we want them to access. Please find the ways how to enable cors in your application.

First, install the library  Nuget Package Link

Then add the below namespace in your application

using System.Web.Http.Cors;

After adding the namespace you will able to access the EnableCors attribute

[RoutePrefix("API/v1/user")]

    [EnableCors("*", "*", "GET,POST")] //add this line

    public class UserController : ApiController

    {

        [HttpGet]

        [Route("Hello")]

        public HttpResponseMessage Hello()

        {

            return Request.CreateResponse(HttpStatusCode.OK,"I'm from WebAPI");

        }

    }

Also, go to the web API config and add this lines

public static class WebApiConfig

    {

        public static void Register(HttpConfiguration config)

        {

            // Web API configuration and services

            // Web API routes

            config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(

                name: "DefaultApi",

                routeTemplate: "api/{controller}/{id}",

                defaults: new { id = RouteParameter.Optional }

            );

            EnableCorsAttribute co = new EnableCorsAttribute("*", "*", "GET,POST"); //add this line

            config.EnableCors(co); //add this line

        }

    }

Class Files inside the App_Code Folder not accessible

Just click on the properties of the class and change it's Build Action from content to compile.Please see the below images for clear idea.

Change to

The object has not yet been initialized. Ensure that HttpConfiguration.EnsureInitialized() is called in the application's startup code after all other initialization code.

When you get the above exception after running your WebAPI follow the below steps to resolve the error
method write the below line of code.

just open the Global.asax.cs class and under the Application_Start()

public class WebApiApplication : System.Web.HttpApplication

    {

        protected void Application_Start()

        {

            GlobalConfiguration.Configure(WebApiConfig.Register);

            GlobalConfiguration.Configuration.EnsureInitialized();  //add this line

        }

    } 

.CS file not visible in Visual Studio

None of the .cs files are opening in the visual studio the solution for that is locate to the below folder at the location
C:\Users\My PC\AppData\Local\Microsoft\VisualStudio\12.0
Folder Name: -
ComponentModelCache
and delete the folder.If the message comes access is denied then check if any instance of the visual studio is running close it then delete the folder and check it worked for me.