初始化属性的良好做法？

Question

I have a class property that is a list of strings, List. Sometimes this property is null or if it has been set but the list is empty then count is 0. However elsewhere in my code I need to check whether this property is set, so currently my code check whether it's null and count is 0 which seems messy.

if(objectA.folders is null)
{
    if(objectA.folders.count == 0)
    {
      // do something
    }
}

Any recommendation on how this should be handled? Maybe I should always initialise the property so that it's never null?

Answer 1

When I have List as a property, I usually have something that looks like the following (this is not a thread safe piece of code):

public class SomeObject
{
    private List<string> _myList = null;

    public List<string> MyList
    {
        get
        {
            if(_myList == null)
                _myList = new List<string>();
            return _myList;
        }
    }
}

Your code would then never have to check for null because the Property would be initialized if used. You would then only have to check for the Count.

Answer 2

现在你的代码将始终抛出空指针异常，你正在检查Null，如果它是空的 - 你正试图访问一个不存在的对象。

Answer 3

If for your application the collection being a null reference never has a different meaning than the collection being empty, then yes, I would say you should always initialize it and this way remove the null checks from the remaining code.

This approach only makes sense if the property setter does not allow to change it to a null reference after initialization.

Answer 4

You could always initialize the property so it's an empty List. Then you can just check the count property.

List<String> Folder = Enumerable.Empty<String>();

I once wrote an extension method for ICollection objects that checked if they were null or empty

public static Boolean IsNullOrEmpty<T>(this ICollection<T> collection)
{
    return collection == null ? true : collection.Count() == 0;
}

public static Boolean IsPopulated<T>(this ICollection<T> collection)
{
    return collection != null ? collection.Count() > 0 : false;
}

Answer 5

You have three options (and you need to decide based on your project):

1. Create a method to check for NullOrNoElements. Pro: Allows both null and no entries. Con: You have to call it everywhere you want to use the property.
2. Preinitialize with a list. Pro: Thread-save and very easy. Con: will use memory even when not used (depending on how many instances you have this may be a problem)
3. Lazy initialize Pro: Does only use memory when really used. Con: NOT thread save.

 private List<string> lp = null; public List<string> ListProp { get { if(lp == null) lp = new List<string>(); return lp; } } 

Answer 6

You could do this in a single IF

if(objectA.folders is null || objectA.folders.count == 0)

Or you could create a boolean property in the class which checks this status for you and returns a result

public bool objectA.FolderIsNullOrEmpty
{
    get { return objectA.folders is null || objectA.folders.count == 0;}
}

If it does not make a difference to your application, I would rather recomend initializing the List to start with.

Answer 7

You could handle this by initializing the object in the constructor. This is usually where this type of thing is done. Although I see nothing wrong with your current code. No point in initializing stuff that doesn't exist yet, it just wastes memory.

Answer 8

Its a good question. I would add a method to objectA FoldersNullOrEmpty() that you can use eg

public virtual FoldersNullOrEmpty()
{
   return (folders == null || folders.count == 0)
}

Answer 9

I almost always initialize lists and even make sure they can't be set to null if exposed by any setters. This makes using them much easier.