使用streamreader处理异常时遇到麻烦

Question

I'm having some trouble with StreamReader , I have a settings file where I save settings in. I want to open and close the file on a way that I also can handle exceptions.

When the file can't be loaded I want to return for now false.

I created a function that loads the file for me:

    private bool LoadSettingsFile(out StreamReader SettingsFile)
    {
        try
        {
            SettingsFile = new StreamReader("Settings.txt");
            return true;
        }
        catch
        {
            //Going to solve the exception later, but if I can't I want to return false.
            SettingsFile = new StreamReader(); //You need to assign StreamReader, but you need to open a file for that.
            //'System.IO.StreamReader' does not contain a constructor that takes 0 arguments
            return false;
        }
    } 

I call the function on this way:

StreamReader SettingsFile;

if (!LoadSettingsFile(out SettingsFile))
   return false;

How can I avoid or solve this?

Answer 1

If you are unable to open the file, why would you want to return a StreamReader instance? Surely you would want to return null. Also, it's never really a good idea to do a catch-all in your exception handling, be more specific eg

private bool LoadSettingsFile(out StreamReader settingsFile)
{
    try
    {
         settingsFile = new StreamReader("Settings.txt");
         return true;
    }
    catch (IOException) // specifically handle any IOExceptions       
    {
        settingsFile = null;
        return false;
    }
}

Answer 2

This is arguably bad practise in that, in general, .NET code prefers "throwing exceptions" over "returning failure." The reason for this is that, if you are "returning failure," you rely on the consumer of your code to recognise this and do something about it. If you throw an exception and the consumer of your code ignores it, the application will fail - which is often more desireable than for it to continue in an undefined state.

In your case, the problem is that you're forced to assign to your out parameter even when there is no sensible value to assign there. One obvious suggestion is to assign null instead of trying to fake a StreamReader . Alternatively, you could create an empty MemoryStream and return a reader for that, but this is going to some extreme lengths to cover up the fact that the variable has no meaning in a failure case and should not be set.

Ultimately I'd suggest you allow the exception to bubble rather than returning a bool to indicate failure - or alternatively, return the StreamReader for success and return null in the case of failure.

Answer 3

Just set SettingsFile = null before entering into the Try/Catch block. Presumably by returning false you're handling this condition at a higher level, so SettingsFile will never be used. So your code would look like this:

   private bool LoadSettingsFile(out StreamReader SettingsFile) 
    { 
        SettingsFile = null;
        try 
        { 
            SettingsFile = new StreamReader("Settings.txt"); 
            return true; 
        } 
        catch 
        { 
            //Handle Exception Here
            return false; 
        } 
    }  

Answer 4

You can try

private StreamReader LoadSettingsFile()
{
    try
    {
        return new StreamReader("Settings.txt");
    }
    catch
    {
        return null;
    }
} 

and then

StreamReader sr = LoadSettingsFile();
if (sr == null) return false;