Unexplained null pointer reference, I'm blaming ghosts

Question

I've got the following code:

public void DrawInput(string ChatCurrent){

        int uCursorTop;
        int uCursorLeft;
        uCursorLeft = Console.CursorLeft;
        uCursorTop = Console.CursorTop;
        Console.SetCursorPosition(0, uCursorTop);
        Console.Write("> "+ChatCurrent+" ");
        Console.SetCursorPosition(ChatCurrent.Length, uCursorTop);
    }

Except for the final line, it behaves properly. The final line throws System.NullReferenceException: Object reference not set to an instance of an object. The weird thing? Specifically, accessing ChatCurrent.Length is what's making it fail. The line immediately before, which echoes the string's contents, works just fine.

What's going on?

How are you calling this function, what is the source of ChatCurrent? — Mark Hall, May 28 '12 at 00:01
It's passed by reference from the main thread, which in turn initializes it in a separate class. I figured out the problem, it's stupid, please see my answer below so you know why to mail flaming dog waste to my doorstep. — Winfield Trail, May 28 '12 at 00:03
And what does the line before output? String concatenation does not throw on `null` values. — CodesInChaos, May 28 '12 at 00:03

Tim Schmelter · Answer 1 · 2012-05-27T23:48:30.650

9

String concatenation will treat null values as empty string

In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.

http://msdn.microsoft.com/en-us/library/ms228504.aspx

But even if you would pass null directly to Console.Write, it won't throw an exception, it will just write nothing.

http://msdn.microsoft.com/en-us/library/zcwe8sfx%28v=vs.80%29.aspx

If value is a null reference (Nothing in Visual Basic), nothing is written and no exception is thrown.

edited May 27 '12 at 23:48

answered May 27 '12 at 23:43

Tim Schmelter

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That's the ridiculous thing. `Console.Write` actually writes out text when I invoke it but comment out the line containing `ChatCurrent.Length`. I'm going insane. – Winfield Trail May 27 '12 at 23:55
@sudowned: of course it writes text: "`> `". Whereas `ChatCurrent.Length` throws an exception because you cannot access a property of a null reference. – Tim Schmelter May 27 '12 at 23:58
Yeah, it was writing the text **and the text I'd typed that was supposed to be in the buffer.** This is stupidity, this is entirely my own stupidity, but it's not what you think. See the answer I left. – Winfield Trail May 28 '12 at 00:01

score 1 · Accepted Answer · edited Jul 14 '14 at 13:35

1

Here's what was happening:

I got the error as described above. When I commented out the Length measurement, I typed stuff in to see if the text would cause the error too. When it didn't, I put it back and tried other experiments.

I was consistently not typing anything while I tested, which in turn meant that the variable was declared but not initialized. I have been trying to fix this problem for two hours.

To wit, in case someone else has this problem: Make sure that your variable is initialized. It's as important as declaring it in this context.

edited Jul 14 '14 at 13:35

ForceMagic

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answered May 28 '12 at 00:00

Winfield Trail

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So the essence of this answer is that you should initialize a variable to avoid a `NullReferenceException`? – Tim Schmelter May 28 '12 at 00:03
The essence of the answer is that one should initialize a variable to avoid a NullReferenceException, __and__ that just because you think a variable is initialized doesn't mean it is. – Winfield Trail May 28 '12 at 00:04
Local variables must be initialized or the code doesn't compile, and fields get initialized implicitly. This means it is impossible to read an uninitialized variable in C#. – CodesInChaos May 28 '12 at 00:05
The variable wasn't local. It was passed by reference from the main thread which set, and then loaded it from, another class. – Winfield Trail May 28 '12 at 00:06
1

The essence of the answer is that one should sleep more. – Vasyl Boroviak May 28 '12 at 03:18
That's the honest to God truth. I'll take that advice. – Winfield Trail May 28 '12 at 04:24

Unexplained null pointer reference, I'm blaming ghosts

2 Answers2