What is the meaning of NullReferenceException [duplicate]

Answer 1

This is the most common exception in .NET... it just mean that you try to call a member of a variable that is not initialized (null). You need to initialize this variable before you can call its members

Answer 2

The code below will show you the exception, and a clue.

string s = null;
s = s.ToUpper();

Answer 3

That means that you have tried to use a method or property of an object, when the variable is not yet initialized:

string temp;
int len = temp.Length; // throws NullReferenceException; temp is null

string temp2 = "some string";
int len2 = temp2.Length; // this works well; temp is a string

Answer 4

It means you've tried to access a member of something that isn't there:

string s = null;
int i = s.Length; // boom

Just fix the thing that is null. Either make it non-null, or perform a null-test first.

There is also a corner-case here related to Nullable<T>, generics and the new generic constraint - a bit unlikely though (but heck, I hit this issue!).

Answer 5

Somewhere in your code, you have an object reference, and it's not set to an instance of an object :)

Somewhere you've used an object without calling it's constructor.

what you should do:

MyClass c = new MyClass();

what you've done:

MyClass c;
c.Blah();

Answer 6

It means you're referencing something that's null, for example:

class Test
{

   public object SomeProp
   {
      get;
      set;
   }

}

new Test().SomeProp.ToString()

SomeProp will be null and should throw a NullReferenceException. This is commonly due to code you are calling expecting something to be there that isn't.