c# - Object reference not set to an instance of an object.Why doesn't .NET show which object is `null`?

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Regarding this .NET unhandled
exception
message:

Object reference not set to an instance of an
object.

Why
doesn't .NET show which object is
null?

I know that I can
check for null and resolve the error. However, why doesn't .NET
help pointing out which object has a null-reference and which expression triggered the
NullReferenceException?

Answer

(For information about the new exception
helper in Visual Studio 2017 see the end of this
answer)

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Consider this
code:

String s =
            null;
Console.WriteLine(s.Length);

This
will throw a NullReferenceException in the second line and you
want to know why .NET doesn't tell you that it was s that was
null when the exception was
thrown.

To understand why you don't
get that piece of information you should remember that it is not C# source that executes
but rather IL:

IL_0001: ldnull
            
IL_0002: stloc.0 // s
IL_0003: ldloc.0 // s
IL_0004:
            callvirt System.String.get_Length
IL_0009: call
            System.Console.WriteLine

It
is the callvirt opcode that throws the
NullReferenceException and it does that when the first argument
on the evaluation stack is a null reference (the one that was loaded using
ldloc.0).

If .NET
should be able to tell that it was s that was a null reference
it should in some way track that the first argument on the evaluation stack originated
form s. In this case it is easy for us to see that it is
s that was null but what if the value was a return value from
another function call and not stored in any variable? Anyway, this kind of information
is not what you want to keep track of in a virtual machine like the .NET virtual
machine.

---

To
avoid this problem I suggest that you perform argument null checking in all public
method calls (unless of course you allow the null
reference):

public void Foo(String
            s) {
 if (s == null)

 throw new
            ArgumentNullException("s");

            Console.WriteLine(s.Length);
}

If
null is passed to the method you get an exception that precisely describes what the
problem is (that s is
null).

---

Four
years later Visual Studio 2017 now has a new exception helper that will try to tell what
is null when a NullReferenceException is thrown. It is even
able to give you the required information when it is the return value of a method that
is null:

href="https://i.stack.imgur.com/Mv5gE.png" rel="noreferrer"> src="https://i.stack.imgur.com/Mv5gE.png" alt="Visual Studio 2017 exception
helper">

Note that this only works
in a DEBUG build.