.net有条件不正确评估

Question

I have the following simple block of code

  var assmSpec = util.ASSEMBLYFOLDER + task.AssemblyName; // evals to valid fileSpec
  if (!File.Exists(assmSpec))
      throw new TaskException(string.Format(
          "Assembly [{0}] cannot be located.", assmSpec));

Since the assembly referenced by assmSpec does in fact exist (The File.Exists() evals to true), I expect that the exception would not be thrown. But it is. the code steps into the throw statement. To debug, I modified the code to read:

  var assmSpec = util.ASSEMBLYFOLDER + task.AssemblyName; // evals to valid fileSpec
  var asmExists = File.Exists(assmSpec);
  if (!asmExists)
      throw new TaskException(string.Format(
          "Assembly [{0}] cannot be located.", assmSpec));

Here, asmExists evals to true, and The code still steps into the throw.

I then modified the code to read:

  var assmSpec = util.ASSEMBLYFOLDER + task.AssemblyName; // evals to valid fileSpec
  if (!File.Exists(assmSpec) && File.Exists(assmSpec))
      throw new TaskException(string.Format(
         "Assembly [{0}] cannot be located.", assmSpec));

and again, the code still hits the throw. Something is clearly very wrong here. Has anyone have an explanation ? Am I doing something really obviously stupid here?

fwiw, this code is in a method that also has a try - catch - finally construction, but it is before all of them (before the try) ...

---

The full method is:

  public void StartProcess(Task task)
    {
        log.Write(log.Level.Debug, string.Format(
            "TaskWorker.StartProcess {0} process",
            task.Name), task.Name);
        WorkerMessageManager.MsgArrvdWorkerHndlr += MsgArrvdWorkerHndlr;
        var tskName = task.Name;
        var assmSpec = util.ASSEMBLYFOLDER + task.AssemblyName;
        if (!File.Exists(assmSpec))
            throw new TaskException(string.Format(
                "Assembly [{0}] cannot be located.", assmSpec));

        try
        {
            WorkerMessageManager.NotifyWorker(new ProgressTaskMessage(
                                    tskName, "", tskName + "  starting..."));
            // -------------------------------------------
            Assembly dA;
            try { dA = Assembly.LoadFrom(assmSpec); }
            catch(FileNotFoundException nfX)
            { throw new TaskException(string.Format(
                "Assembly [{0}] cannot be located.", assmSpec), 
                nfX); }
            // -------------------------------------------
            var iTsk = (IExecuteTasks)dA.CreateInstance(task.ClassName);
            if (iTsk == null)
                throw new TaskException(
                    string.Format("Unable to instantiate {0} from {1}",
                        task.ClassName, task.AssemblyName));

            if (iTsk.TaskName != tskName) // do not execute if names do not match
                throw new TaskNameMismatchException(string.Format(
                    "CHECK CONFIGURATION SETTINGS,  Data Task Name Mismatch.{0}" +
                    "Task name defined in TaskScheduler.config [{1}], {0} does " +
                    "not match name [{2}] as defined in Task Logic assembly: {3}.{4}",
                        sNL, tskName, iTsk.TaskName, task.AssemblyName, 
                        task.ClassName),  tskName, iTsk.TaskName);
            // -------------------------------------------
            iTsk.DataImportProgressEvent += OnProgressReport;
            iTsk.ProcessCompletedEvent += OnProcessCompleted;
            iTsk.GeneralEvent += OnGeneralEvent;
            // -----------------------------------
            log.Write(log.Level.Debug, string.Format(
                  "{0} process Started", task.Name),
                  task.Name);
            if (task.JobQueue.HasJobReady)
                iTsk.StartTask(JobQueues.Instance.DeQueue(tskName));
            else iTsk.StartTask(); 
            log.Write(log.Level.Debug, string.Format(
                  "{0} process Completed", task.Name),
                  task.Name);
        }
        catch (TaskNameMismatchException inmX)
        { log.Write(log.Level.Warn, inmX.Message, tskName, inmX); }

        catch (BpaTaskException mX)
        {
            var errMsg = string.Format(
                "Error in Data Import StartProcess(). " + sNL +
                "Exception {0}: {1}, " + sNL +
                "Stack Trace: {2}",
                mX, mX.Message, mX.StackTrace); 
            log.Write(log.Level.Error, errMsg, 
                        task.Name, mX);
        }

        catch(Exception X)
        {
            var errMsg = string.Format(
                "Error in Data Import StartProcess(). " + sNL +
                "Exception {0}: {1}, " + sNL +
                "Stack Trace: {2}",
                X, X.Message, X.StackTrace);
            log.Write(log.Level.Error, errMsg, task.Name, X);

            // WorkerMessageManager.NotifyWorker(new ImportFailMessage(X));
            // This throw instruction causes the Scheduler service to stop alltogether
            // I'm Removing the throw for now, because it seems inappropriate to 
            //          kill the whole service..
            throw;
        }

        finally
        {
            task.IsRunning = false;
            WorkerMessageManager.MsgArrvdWorkerHndlr -= MsgArrvdWorkerHndlr;
        }
    }

Answer 1

Check that there is no text (code or semicolon) following the if () statement on the same line. The most likely cause is that the "throw" is not actually "inside" the if statement, so it's always executed.

Check that you are debugging a DEBUG build - you may get odd values reported in the debugger for a RELEASE build, which may make it look as though a variable is true when in fact it is false.

It's also possible under some circumstances (though only usually with things like references to precompiled dlls or corrupt pdb files) to be looking at different code than you are debugging, giving the impression that changes you are making to the source are being ignored. Do a Build > Clean, check that the assembly you're running is no longer present on disk, and then rebuild it to be sure that it's up to date and in sync with your source code.

Answer 2

I've just tried something similar, and it works fine for me. Have you tried this using a statement block instead of a single throw line?

The compiler could be confused, and may need to be restarted. It's not unheard of. :/

Answer 3

If you look at the internals of the File.Exists method, you find that it will return false under numerous circumstances including:

1. path is null or an empty string
2. The file does not exist
3. The OS thinks the file is actually a directory
4. The account trying to access the file does not have read permissions.
5. Some other internal NotSupportedException, SecurityException, IOException or UnauthorizedAccessException is thrown.

The problem is that all those potential errors are buried. I'd suggest trying the FileInfo class instead:

var fileInfo = new FileInfo( assmSpec );
if ( !fileInfo.Exists )
    throw new TaskException( ...

The constructor on FileInfo will throw one of a handful of exceptions back to you which might give you more information about the issue.