如果字符串在c＃中有空或空格，如何返回false

Question

I cannot return null because my Hlist is a non nullable type. What else can I return in the place of null?

HList findHListentry(string letter)
{
    if (string.IsNullOrWhiteSpace(letter))
    {
        HList result = listentry.Find(delegate(HList bk)
                       {
                           return bk.letter == letter;
                       });
        return result;
    }
    else
    {
        return ?;
    }
}

Answer 1

Use a Nullable<HList> instead?

HList? findHListentry(string letter)
{
    ///
    return null;
}

Answer 2

There are several approaches of dealing with non-nullable value types:

• Using Nullable<HList> (the shorthand name for it is HList? ), or
• Defining an "empty" HList entry, similar to how Microsoft defined one for Guid
• Make your method return bool instead of HList , and return HList through an out parameter, the way it is done in Dictionary.TryGetValue

Using a special value:

struct HList {
    public static HList Empty;
    ...
}

if (...) {
    return HList.Empty;
}

Returning bool :

bool findHListentry(string letter, out HList res) {
    ...
}

Answer 3

If the input to the method truly shouldn't be null or an empty string, you could throw an exception:

HList findHListentry(string letter)
{
    if (string.IsNullOrWhiteSpace(letter))
        throw new ArgumentNullException("letter");

    HList result = listentry.Find(
        delegate(HList bk)
        {
            return bk.letter == letter;
        }
    );

    return result;
}

(Note that I also reversed the conditional logic, since it sounded like you were looking for the opposite from the title of the question.)

You can also look into Code Contracts to verify preconditions for methods instead of manually checking the input and throwing an exception.

Answer 4

You have a few options:

If HList is a struct:

1. Make it into a class intead. Then you can return null.
2. Change the method signature to HList? findHListentry(string letter) HList? findHListentry(string letter)

Answer 5

你可能想看看N的Nullable

Answer 6

If you didnt want to return a null, you could create a static instance of HList that's used to indicate it's an 'empty' value.

Similar to EventArgs.Empty

public static readonly HList EmptyHList = new Hlist() { /* initialise */ };

Answer 7

One implementation is to provide an Empty instance of the non-nullable type, and return that in place of null. Take string as an example...Whilst String is a nullable type in .NET, it contains a built in, readonly field called Empty, so with String you can do this:

if(mystring == null)
{
    //My String Is Null
}

or, you could do this

if(mystring == String.Empty)
{
    //My String is Empty
}

Whilst it is probably not the best approach, you could add an empty instance of HList to your class/struct. eg

HList findHListentry(string letter)
{
    if (string.IsNullOrWhiteSpace(letter))
    {
        HList result = listentry.Find(delegate(HList bk) { return bk.letter == letter; });
        return result;
    }
    else
    {
        return HList.Empty;
    }
}

public struct HList
{
    public const HList Empty;
}

Now you can do this in place of null

if(myHList == HList.Empty)
{
    //My HList is Empty
}