What is a NullReferenceException, and how do I fix it?

I have some code and when it executes, it throws a `NullReferenceException`, saying:

>Object reference not set to an instance of an object.

What does this mean, and what can I do to fix this error?

Adding a case when the class name for entity used in entity framework is same as class name for a web form code-behind file.

Suppose you have a web form Contact.aspx whose codebehind class is Contact and you have an entity name Contact.

Then following code will throw a NullReferenceException when you call context.SaveChanges()

Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

For the sake of completeness DataContext class

public class DataContext : DbContext
{
public DbSet<Contact> Contacts {get; set;}
}

and Contact entity class. Sometimes entity classes are partial classes so that you can extend them in other files too.

public partial class Contact
{
public string Name {get; set;}
}

The error occurs when both the entity and codebehind class are in same namespace.
To fix this, rename the entity class or the codebehind class for Contact.aspx.

**Reason**
I am still not sure about the reason. But whenever any of the entity class will extend System.Web.UI.Page this error occurs.

For discussion have a look at

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Another general case where one might receive this exception involves mocking classes during unit testing. Regardless of the mocking framework being used, you must ensure that all appropriate levels of the class hierarchy are properly mocked. In particular, all properties of `HttpContext` which are referenced by the code under test must be mocked.

See "[NullReferenceException thrown when testing custom AuthorizationAttribute](

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; for a somewhat verbose example.

It means your code used an object reference variable that was set to null (i.e. it did not reference an actual object instance).

To prevent the error, objects that could be null should be tested for null before being used.

if (myvar != null)
{
// Go ahead and use myvar
myvar.property = ...
}
else
{
// Whoops! myvar is null and cannot be used without first
// assigning it to an instance reference
// Attempting to use myvar here will result in NullReferenceException
}

You are using the object that contains the null value reference. So it's giving a null exception. In the example the string value is null and when checking its length, the exception occurred.

Example:

string value = null;
if (value.Length == 0) // <-- Causes exception
{
Console.WriteLine(value); // <-- Never reached
}

The exception error is:

> Unhandled Exception:
>
> System.NullReferenceException: Object reference not set to an instance
> of an object. at Program.Main()

A `NullReferenceException` is thrown when we are trying to access Properties of a null object or when a string value becomes empty and we are trying to access string methods.

For example:

1. When a string method of an empty string accessed:

string str = string.Empty;
str.ToLower(); // throw null reference exception

2. When a property of a null object accessed:

Public Class Person {
public string Name { get; set; }
}
Person objPerson;
objPerson.Name /// throw Null refernce Exception