Caught exception is null itself !

问题:

I have an ASP.NET applications. Everything was fine, but recently I get exceptions that are null themselves:

try
{
    // do something
}
catch (Exception ex)
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

Sometimes ex is null itself !

Any idea?

回答1:

For anyone ending up here, I've found an instance where this is possible (If only detectable in the debugger). VS2013 Update 4.

Broken:

try
{
    // do something
}
catch (WebException ex) // <- both variables are named 'ex'
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}
catch (Exception ex) // <- this 'ex' is null
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

The solution is to name your exception variables differently.

Fixed:

try
{
    // do something
}
catch (WebException webEx) // <- all good in the hood
{
    Logger.Log("Error while tried to do something. Error: " + webEx.Message); // <-
}
catch (Exception ex) // <- this 'ex' correctly contains the exception
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

回答2:

That cannot happen.

If you throw null, you'll get a NullReferenceException from the throw; the exception in the catch block can never be null.

You have something else that's null.

回答3:

In my case, the cause was a StackOverflowException. Such exceptions normally don't reach the catch block at all, but this time, for some reason I don't understand, it did reach the catch block, but the exception was null.

回答4:

I just ran into an issue where someone was passing ex.InnerException to a method, where ex was the root. Since the parameter was also called ex it led to some confusion in the debugger when I looked at the originally caught exception. This was likely the result of some careless refactoring.

e.g.:

public void MyMethod(string input)
{
    try {
        Process(input);
    } catch (Exception ex) { // <- (2) Attempting to view ex here would show null
        _logger.log(ex);
        LogInner(ex.InnerException);
    }
}

private void LogInner(Exception ex)
{
    _logger.log(ex); // <- (1) NullReferenceExeption thrown here
    if(ex.InnerException != null)
        LogInner(ex.InnerException);
}

This was refactored as such:

public void MyMethod(string input)
{
    try {
        Process(input);
    } catch (Exception ex) {
        LogExceptionTree(ex);
    }
}

private void LogExceptionTree(Exception exception)
{
    _logger.log(exception);
    if(exception.InnerException != null)
        LogExceptionTree(exception.InnerException);
}

回答5:

I met the same problem, and the reason is: the exception is a NullReferenceException, so you can not use ex.Message, and you should try the flowing:

try
 {     // do something } 

catch (NullReferenceException)
{
  Logger.Log("Error while tried to do something. Error: Null reference");
}

catch (Exception ex) 
{     
  Logger.Log("Error while tried to do something. Error: " + ex.Message); 
}