"Object reference not set to an instance of an object": why can't .NET show more details?

Question

"Object reference not set to an instance of an object"

Why does the exception not also show the name of the object reference field, or at least its type?

This is probably one of the most common run-time errors in .NET. Although the System.Exception has a stack trace, there are no other helpful details.

Over the course of a year I spend hours sifting through stack traces (often in code I did not write), hoping there is a line number from a ".pdb" file, then finding the line in the code, and even then it is often not obvious which reference on the line was null. Having the name of the reference field would be very convenient.

If System.ArgumentNullException instances can show the name of the method parameter ("Value cannot be null. Parameter name: value"), then surely System.NullReferenceException instances could include the name of the null field (or its containing collection).

Answer 1

The difference between ArgumentNullException and NullReferenceException is that ArgumentNullException is always thrown explicitly like so:

if (parameter == null)
  throw new ArgumentNullException("parameter");

Had a quick look at ILDASM output, the local variables are indeed present inside a function's IL. There is still, however, no API to retrieve those names programatically. My understanding is that it would be fairly complex as you would basically need to build a parse tree that represents a function with scopes, variables, statements etc.

It's further complicated by the fact that it's not just simple variables that can throw NullReferenceException, but result of a function call, a property or an expression. I could get pretty complicated pretty fast.

Imagine this:

internalObject.OtherProperty = myObject.GetOtherObject().ThirdObject.SomeProperty == "value"
 ? myObject.OtherProperty
 : myObject.GetSomethingElse();

There are multiple points of failure there and building up a string representing what is actually null could be tricky.