(this == null) in C#!

Question

Due to a bug that was fixed in C# 4, the following program prints true. (Try it in LINQPad)

void Main() { new Derived(); } class Base { public Base(Func<string> valueMaker) { Console.WriteLine(valueMaker()); } } class Derived : Base { string CheckNull() { return "Am I null? " + (this == null); } public Derived() : base(() => CheckNull()) { } }

In VS2008 in Release mode, it throws an InvalidProgramException. (In Debug mode, it works fine)

In VS2010 Beta 2, it doesn't compile (I didn't try Beta 1); I learned that the hard way

Is there any other way to make this == null in pure C#?

It's most likely a bug in C# 3.0 compiler. It works the way it should in C# 4.0. — Mehrdad Afshari
– Mehrdad Afshari, Commented Oct 21, 2009 at 13:07
@SLaks: Problem with bugs is that you can expect them to be fixed at some point so finding them "useful" is probably not wise. — AnthonyWJones
– AnthonyWJones, Commented Oct 21, 2009 at 13:13

2 revs · Accepted Answer · 2017-05-23 12:34:32Z

75

This observation has been posted on StackOverflow in another question earlier today.

Marc's great answer to that question indicates that according to the spec (section 7.5.7), you shouldn't be able to access this in that context and the ability to do so in C# 3.0 compiler is a bug. C# 4.0 compiler is behaving correctly according to the spec (even in Beta 1, this is a compile time error):

§ 7.5.7 This access

A this-access consists of the reserved word this.

this-access:
this 
A this-access is permitted only in the block of an instance constructor, an instance method, or an instance accessor.

edited May 23, 2017 at 12:34

community wiki

2 revs
Mehrdad Afshari

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3 Comments

quetzalcoatl Over a year ago

I do not see, why in the code presented in this question, usage of keyword "this" is invalid. The method CheckNull is a normal instance method, nonstatic. Using "this" is 100% valid in such method, and even comparing this to null is valid. The error is in the base init line: it is the attempt to pass instance-bounded delegate as a parameter to the base ctor. This is the bug (a hole in sematic checks) in the compiler: it should NOT be possible. You are not allowed to write : base(CheckNull()) if CheckNull is not static, and alike you should not be able to inline an instance-bound lambda.

Mehrdad Afshari Over a year ago

@quetzalcoatl: this in CheckNull method is legal. What is not legal is the implicit this-access in () => CheckNull(), essentially () => this.CheckNull(), which is running outside the block of an instance constructor. I agree that the part of spec I cite is mostly focused on the syntactic legality of this keyword, and probably another part addresses this issue more precisely, but it is easy to conceptually extrapolate from this part of spec as well.

quetzalcoatl Over a year ago

Sorry, I disgree. While I know that (and written that in the comment above) and you also know that - you have not mentioned the actual cause of the problem in your (accepted) answer. The answer is accepted - so seemingly the author grapsed it too. But I doubt that all readers will be as bright and fluent in lambdas to recognize a instancebound-lambda versus static-lambda at first sight and map that to 'this' and problems with emitted IL :) This is why I added my three cents. Aside of that, I agree with everything else what was found, analyzed and described by you and others:)

SLaks · Accepted Answer · 2009-10-21 16:24:39Z

The raw decompilation (Reflector with no optimizations) of the Debug mode binary is:

private class Derived : Program.Base { // Methods public Derived() { base..ctor(new Func<string>(Program.Derived.<.ctor>b__0)); return; } [CompilerGenerated] private static string <.ctor>b__0() { string CS$1$0000; CS$1$0000 = CS$1$0000.CheckNull(); Label_0009: return CS$1$0000; } private string CheckNull() { string CS$1$0000; CS$1$0000 = "Am I null? " + ((bool) (this == null)); Label_0017: return CS$1$0000; } }

The CompilerGenerated method doesn't make sense; if you look at the IL (below), it's calling the method on a null string (!).

 .locals init ( [0] string CS$1$0000) L_0000: ldloc.0 L_0001: call instance string CompilerBug.Program/Derived::CheckNull() L_0006: stloc.0 L_0007: br.s L_0009 L_0009: ldloc.0 L_000a: ret

In Release mode, the local variable is optimized away, so it tries to push a non-existant variable on to the stack.

 L_0000: ldloc.0 L_0001: call instance string CompilerBug.Program/Derived::CheckNull() L_0006: ret

(Reflector crashes when turning it into C#)

EDIT: Does anyone (Eric Lippert?) know why the compiler emits the ldloc?

Community · Accepted Answer · 2017-02-08 14:16:27Z

11

I have had that! (and got proof too)

alt text

edited Feb 8, 2017 at 14:16

CommunityBot

11 silver badge

answered Oct 21, 2009 at 13:08

leppie

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2 Comments

leppie Over a year ago

Was late, was a sign I should stop coding :) Was hacking our with DLR stuff IIRC.

user90843 Over a year ago

make a debugger visualizer (DebuggerDisplay) for whatever 'this' is, and make that fool you that's null? :D just sayin'

user1228 · Accepted Answer · 2009-10-21 13:09:22Z

10

This isn't a "bug". This is you abusing the type system. You are never supposed to pass a reference to the current instance (this) to anyone within a constructor.

I could create a similar "bug" by calling a virtual method within the base class constructor as well.

Just because you can do something bad doesn't mean its a bug when you get bit by it.

answered Oct 21, 2009 at 13:09

user1228

10 Comments

SLaks Over a year ago

It is a compiler bug. It generates invalid IL. (Read my answer)

leppie Over a year ago

The context is static, so you should not be allowed an instance method reference at that stage.

Mehrdad Afshari Over a year ago

@Will: It's a compiler bug. The compiler is supposed to generate valid, verifiable code for that code snippet or spit out an error message. When a compiler does not behave according to the spec, it is buggy.

SLaks Over a year ago

@Will#4: When I wrote the code, I hadn't thought about the implications. I only realized that it didn't make sense when it stopped compiling in VS2010. –

Mehrdad Afshari Over a year ago

By the way, the virtual method call in constructor is a completely valid operation. It's just not recommended. It may result in logical disasters but never an InvalidProgramException.

|

Dan Tao · Accepted Answer · 2009-10-21 12:59:31Z

3

I could be wrong, but I'm pretty sure if your object is null there's never going to be a scenario where this applies.

For instance, how would you call CheckNull?

Derived derived = null; Console.WriteLine(derived.CheckNull()); // this should throw a NullReferenceException

answered Oct 21, 2009 at 12:59

Dan Tao

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5 Comments

SLaks Over a year ago

In a lambda in the constructor argument. Read the entire code snippet. (And try it if you don't believe me)

jpierson Over a year ago

I agree although I do remember faintly something about how in C++ an object didn't have reference within it's constructor and I'm wondering if the (this == null) scenario is used in those cases to check whether a call to a method was made from the object's constructor before exposing a pointer to "this". Though, as far as I know in C#, there shouldn't be any cases where "this" would ever be null, not even in the Dispose or finalization methods.

Dan Tao Over a year ago

I guess my point is that the very idea of this is mutually exclusive of the possibility of being null--sort of a "Cogito, ergo sum" of computer programming. Therefore your desire to use the expression this == null and ever have it return true strikes me as misguided.

Dan Tao Over a year ago

In other words: I did read your code; what I'm saying is that I question what you were trying to accomplish in the first place.

SLaks Over a year ago

This code simply demonstrates the bug, and, as you pointed out, is utterly useless. To see real useful code, read my second answer.

Scott and the Dev Team · Accepted Answer · 2010-05-07 16:16:40Z

Not sure if this is what you are looking for

 public static T CheckForNull<T>(object primary, T Default) { try { if (primary != null && !(primary is DBNull)) return (T)Convert.ChangeType(primary, typeof(T)); else if (Default.GetType() == typeof(T)) return Default; } catch (Exception e) { throw new Exception("C:CFN.1 - " + e.Message + "Unexpected object type of " + primary.GetType().ToString() + " instead of " + typeof(T).ToString()); } return default(T); }

example: UserID = CheckForNull(Request.QueryString["UserID"], 147);

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