NullReferenceException when reading a string while debugging, or running normally

Question

When attempting to read a file while calling DoSomething(); (from Something();) in the TestProgram_Load() method, I encounter a NullReferenceException, which I'm having trouble making sense of.

This happens when attempting to check if a file exists, or even attempting to read it. However, I am able to write to the file without an issue, and reference the string value, even in the debugger.

Here's the problem code:

// No matter the file name, this fails every time.
string fileName = "file.txt";

public void DoSomething()
{
    if (File.Exists(fileName)) // NullReferenceException
    {
        using (StreamReader r = new StreamReader(fileName)) // NullReferenceException
        {

        }
    }
}

And the method that calls it:

public void Something()
{
    // This works fine
    if (!File.Exists(fileName))
    {
        // This works fine
        using (StreamWriter w = new StreamWriter(fileName))
        {
             w.Write("Test");
        }
    }

   // Test to see if there's an issue with this method too...
   // This is fine, but whether or not File.Exists(fileName) is used, DoSomething(); has the same problem.

   if (File.Exists(fileName))
   { 
       DoSomething(); 
   }

}

Here's the TestProgram_Load method:

private void TestProgram_Load(object sender, EventArgs e)
{
    TestClass t = new TestClass();
    t.Something();
}

Here's the stack trace:

at TestProgram.TestClass.DoSomething() in Visual Studio 2015\Projects\Test Program\Test Program\Classes\FileSystem\TestClass.cs:line 39
at TestProgram.Program.Main() in Visual Studio 2015\Projects\Test Program\Test Program\Program.cs:line 19
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

And Line 39:

if (File.Exists(fileName))

This code is being executed in the main program's startup functions: either in the constructor, or the TestProgram_Load() method: both are having the same issue. There should not be any threading at all.

Here are some key details:

1. I attempt to read the file, DoSomething();
2. The string value exists in the debugger, and can be referenced right before File.Exists(fileName)
3. File.Exists(fileName) works in a different method, but not this one.
4. The string is not being updated at all.
5. The string value is hard-coded.
6. There are no additional threads to interfere with this.

I want this to actually detect existence and, of course, open the file. What's going on here?

Answer 1

What's interesting is that the documentation for File.Exists states:

The Exists method returns false if any error occurs while trying to determine if the specified file exists. This can occur in situations that raise exceptions such as passing a file name with invalid characters or too many characters, a failing or missing disk, or if the caller does not have permission to read the file.

My initial inclination was to believe that the write was still completing while you were trying to read, but according to the documentation, this should still just return false. It sounds like an internal .Net bug.

I don't have a clear cut answer, but you may be able to get to the root cause by:

• Putting a Thread.Sleep after the write for 1000ms
• Using a FileInfo instead of File (in case there's a bug in File
• Don't check File.Exists and see if your stream gives you a more descriptive error

If those don't work, and you know the string works the first time, you can try switching to using the FileInfo class completely. Something like:

var file = new FileInfo(fileName);
if (!file.Exists())
{
    using (var writer = file.CreateText())
    {
        writer.Write("test");
    }
}
using (var writer = file.OpenText())
{
    // do stuff
}

Excuse my code if it doesn't build. I don't have visual studio installed on this device.

Answer 2

Never encountered this kind of bug before, but it appears to be a problem with my debugger and Stacktrace not giving the appropriate information. In a large code base, a List<String> object had yet not been instantiated in the beginning.

Normally, my debugger will tell me which object was not instantiated. Unfortunately, it kept complaining about the wrong item, a fileName string (bug?), and anything put inside of it. This was not running in a separate thread either.

After trial and error, it was discovered way down inside of the following line:

if (File.Exists(fileName)
{
      // [...] many lines down. Do lots of stuff.

      // Culprit
      Class.ListStringObject.Add("string");
}

Why it was breaking on this line, I have no clue, but I'd love to know if there's a way around that.

Answer 3

Make sure you have

using System.IO;

What is the runtime context? Is this IIS hosted or a windows execution environment?

In a web hosted environment you will want to do something like this:

var physicalFolder = HostingEnvironment.MapPath(VirtualFolder);
if (File.Exists(physicalFolder + fileName)){
    ...
}

In a windows runtime context you will want this:

var rootOfCurrentPath = Path.GetPathRoot(Environment.CurrentDirectory);
if (File.Exists(rootOfCurrentPath + fileName)){
    ...
}