C＃“ ??”运算符-怎么了？

Question

// EWS Microsoft.Exchange.WebServices.Data.Folder
private Folder _historyFolder;

_historyFolder = GetHistroyFolder(exchangeService) ?? CreateHistortFolder(exchangeService);

public Folder GetHistroyFolder(ExchangeService service)
{
    //if found the folder I want - return it , otherwise returns null
}

public Folder CreateHistortFolder(ExchangeService service)
{
    var folder = new Folder(service);
    folder.DisplayName = _historyFolderName;   // "who cares" :)
    folder.FolderClass = _historyFolderClass; // "IPF.Note"
    folder.Save(WellKnownFolderName.MsgFolderRoot);
    return folder;
}

For some reason _historyFolder is always null , although GetHistroyFolder(exchangeService) does return a folder. Why is that?

UPDATE I

I have updated proof for the unbelievers ! If I separate it to 2 lines (without ??) it's OK! but I still wanna understand why the first approach doesn't work... why down vote? mean ppl..

UPDATE II

Thanks for the all warm people who down vote the question / vote for "close" this thread.

And Thanks for the true warm people who are trying ...

I used the workaround approach, split it to 2 lines

    _historyFolder = GetHistroyFolder(exchangeService);
    if (_historyFolder == null) _historyFolder = CreateHistortFolder(exchangeService); 

you know what funny? Resharper suggests me to return it to how it was before...

Yea , this is a workaround and not real answer for WHY and WTF ... ( .net 4.5 )

UPDATE III

if GetHistroyFolder(..) is return null ( when foreach doesn't find ... ) , CreateHistoryFolder does return Folder object , and _historyFolder is getting the value

Answer 1

Instead of a field why dont you use a property with a backing field. This doesn't exactly solve the problem but at least this makes it easier to debug.

Folder historyFolder 
{
    get{
        if(_historyFolder != null)
            return _historyFolder;
        _historyFolder = GetHistroyFolder(exchangeService); 
        if(_historyFolder == null)
            _historyFolder = CreateHistortFolder(exchangeService) 
        if(_historyFolder == null)
            throw new NullReferenceException("history folder still null");
        return _historyFolder;
    }
}

Answer 2

There's no reason for _historyFolder to be NULL, if GetHistroyFolder() is returning an object eg

namespace ConsoleApplication1
{
    class Program
    {
        // EWS Microsoft.Exchange.WebServices.Data.Folder
        private static object _historyFolder;

        static void Main(string[] args)
        {
            _historyFolder = GetHistroyFolder(null) ?? CreateHistortFolder(null);

            Console.WriteLine(_historyFolder == null);
        }

        public static object GetHistroyFolder(object service)
        {
            return new object();
            //if found the folder I want - return it , otherwise returns null
        }

        public static object CreateHistortFolder(object service)
        {
            return null;
        }
    }
}

I can only imagine that _historyFolder is being set to NULL after GetHistroyFolder() has been called. Your code looks incomplete, are you running in an ASP.NET or something?

EDIT:

In your code, where you call FindFolders(new FolderView()) do this:

FindFolders(new FolderView()).ToList()

Because FindFolders returns an IEnumerable , I think you should call ToList() to ensure that all the folders are returned in one go, not just yielded.

Answer 3

可能是“ CreateHistoryFolder”，返回null