C# constructor and constants mystery

Question

why would a static constructor throw exception when it references to a const string in another class.

 class MyClass
 {  
      static MyClass() 
      { 
           ExamineLog();   
      }

      static ExamineLog()  
      {
          FilePath = HttpContext.Current.Server.MapPath(Helper.LogConfiguration);                
      }
}

class Helper
{  
      public const string LogConfiguration= "\rootpath\counters.txt";
}

The exception thrown is object reference not set to an instance of an object. The stack trace points to the line where attempt is made to read the constant value. Any thoughts?

phoog · Accepted Answer · 2012-03-19T19:23:41.003

6

Thoughts:

HttpContext might be null
HttpContext.Current might be null
HttpContext.Current.Server might be null

Further thoughts:

Current is a static property of the HttpContext class, so HttpContext is not an object reference, and it cannot be null. If you want to simplify your debugging, you can change the code like this (I'm assuming that ExamineLog should have been declared as a void method):

static void ExamineLog()   
{
    var context = HttpContext.Current;
    var server = context.Server;
    FilePath = server.MapPath(Helper.LogConfiguration);                 
}

edited Mar 19 '12 at 19:23

answered Mar 19 '12 at 19:11

phoog

42,068
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Well, not to be picky, but `HttpContext` is a class, so it can't be `null`. – SuperOli Mar 19 '12 at 19:24
1

@SuperOli I just finished editing the answer to reflect that. – phoog Mar 19 '12 at 19:25

score 0 · Answer 2 · answered Mar 19 '12 at 19:13

0

my first bet is a bad string...

"\rootpath\counters.txt" // => "\r" is carriage return

So MapPath fails.

answered Mar 19 '12 at 19:13

Jarrett Meyer

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According to MSDN MapPath can't throw such exception – sll Mar 19 '12 at 19:15

score 0 · Answer 3 · answered Mar 19 '12 at 19:23

My guess is that the HttpContext.Current is null in the context of a static constructor. It's been a while since I was knee-deep in ASP.NET, but IIRC, the HttpContext.Current won't be setup unless you're in the request-response life cycle of a page. I don't know when static constructors are necessarily executed in an ASP.NET application (technically, should be when first accessed by code) and in your case, it could easily be in a context outside of this page life cycle.

I doubt the null reference is coming from your const reference: const reference is inserted as a literal value/string at compile-time so there should be no run-time null reference exception thrown.

C# constructor and constants mystery

3 Answers3