在C＃中，将字符串与null进行比较以及“”返回true的最佳方法是什么

Question

I have the following code (as i am trying to detect changes to a field)

 if (person.State != source.State)
 {
      //update my data . .
  }

the issue is I am having cases where person.State is NULL and source.State is "" and thus returning true.

If one is null and the other is an empty string, I want to treat them as equal and don't update my data. What is the cleanest way of doing that? Do i need to create my own Comparer object as this seems like a generic problem

Answer 1

You could, if you really need to, do:

if ((person.State ?? string.Empty) != (source.State ?? string.Empty))
{
    // ...
}

However, depending on your needs, a better solution might be to modify your person.State property to never return null values.

public class Person
{
    string _state = string.Empty;
    public string State
    {
        get { return _state; }
        set { _state = value ?? string.Empty; }
    }
}

Answer 2

Personally, I'd sanitize/normalize upstream, but if I had to do it here:

// check different, treating null & "" as equivalent
if ((person.State ?? "") != (source.State ?? ""))

Answer 3

While the other answers are good, I would pull them out into their own method to make it clearer to the reader:

public static bool StatesEqual(string first, string second)
{
  return first ?? "" == second ?? "";
}

This will be beneficial if you compare these states in multiple places, or allow you to handle other odd cases if there are any. (Examples might be to change it to be case insensitive, or if two states are textually different but one is an abbreviation of another, ie you want "WI" to be equal to "Wisconsin".

Answer 4

You'd think there would be a StringComparison enum value to handle this with String.Equals... or a CompareOptions enum value to handle it with String.Compare... but there is not.

In any case, I think you should still be using String.Equals as a best practice.

string s1 = null;
string s2 = string.Empty;

bool areEqual = string.Equals(s1 ?? string.Empty, s2 ?? string.Empty);

// areEqual is now true.

And like this you can add case or culture string compare options easily...

bool areEqual = string.Equals(s1 ?? string.Empty, s2 ?? string.Empty, StringComparison.OrdinalIgnoreCase);

Answer 5

Do i need to create my own Comparer object as this seems like a generic problem

It should be clear by now from the good answers here, that you don't, but if you are doing this sort of comparison over and over, or want to use the states as keys, then:

public class NullEmptStringComparer : IComparer<string>
{
  public Equals(string x, string y)
  {
    return (x ?? string.Empty) == (y ?? string.Empty);
  }
  public int GetHashCode(string str)
  {
    return (str ?? string.Empty).GetHashCode();
  }
}

Or to base it on another comparison, in case the default == comparison isn't appropriate (which it rarely is, really):

public class NullEmptCustStringComparer : IComparer<string>
{
  private readonly IComparer<string> _baseCmp;
  public NullEmptCustStringComparer(IComparer<string> baseCmp)
  {
    _baseCmp = baseCmp;
  }
  public Equals(string x, string y)
  {
    return _baseCmp.Equals(x ?? string.Empty, y ?? string.Empty);
  }
  public int GetHashCode(string str)
  {
    return _baseCmp.GetHashCode(str ?? string.Empty);
  }
}

Answer 6

This sounds like a perfect solution for a extension method.

    public static bool IsEqualNoNulls(this String str, string cmp) //bad name, but you get the point
    {
        return (str ?? "") == (cmp ?? "");
    }

.... or just using the body of the extension method, which I'd probably prefer, as I don't see this as too much of a style problem.

Answer 7

The String class has a function "IsNullOrEmpty" which takes in a string.

http://msdn.microsoft.com/en-us/library/system.string.isnullorempty.aspx

From the documentation:

IsNullOrEmpty is a convenience method that enables you to simultaneously test whether a String is null or its value is Empty . It is equivalent to the following code:

result = s == null || s == String.Empty;

For example:

if (!(string.IsNullOrEmpty(person.State) && string.IsNullOrEmpty(source.State)))
{
      //update your data . .
}

Alternatively you could use an extension method, similar to what is outlined by @Earlz

You can learn more about them here http://msdn.microsoft.com/en-us/library/bb383977.aspx

Therefore, assuming I had an extension method like the following:

public static bool IsBlank(this string str)
{
    return string.IsNullOrEmpty(str);
}

This will allow you to do something like

if(!(person.State.IsBlank() && source.State.IsBlank())
{
     //do something
}

The reason this works, even if person.State or source.State is null is because the extension method, while looking like a method of the string class, is actually converted to a static method with the string variable as it's argument (as per the documentation), so it'll happily work even if the string variable isn't set to an instance of string.

Keep in mind, though, that doing it this way could trip you up at a later time if you're reading the code and trying to figure out why it works when person.State or source.State is set to null :P

Or, y'know, alternatively I'd just write out a comparison in full :)

Answer 8

All given answers will fail the Turkey Test . Try this one instead:

public static bool StatesEqual(string first, string second)
{
    if (first == null || second == null)
        return false; // You can also use return first == second if you want to compare null values.

    return first.Equals(second, StringComparison.InvariantCulture);
}

Answer 9

Well, empty and null are not the same thing, so you're not referring to a generic problem here. Yours is a domain problem where your business rules demand that particular evaluation as being true. You can always write a method that looks like this:

public static bool AreMyStringsCustomEqual(string s1, string s2) {
    return (s1 == null || s1 == "" && s2 == null || s2 == "");
}

Or something like that. And then call it from everywhere. You could even make it an extension method.

Answer 10

I believe this is a case for the Decorator pattern. You need to decorate a stock StringComparer to do what you want:

public enum Collapse
{
  None                      = 0 ,
  EmptyAndWhitespace        = 1 ,
  NullAndWhitespace         = 2 ,
  NullAndEmpty              = 3 ,
  NullAndEmptyAndWhitespace = 4 ,
}

public class MySpecialStringComparerDecorator : StringComparer
{
  const   string         COLLAPSED_VALUE = "" ;
  private StringComparer instance ;
  private Collapse     rule     ;

  public StringComparer Decorate( StringComparer sc , Collapse equivalencyRule )
  {
    StringComparer instance = new MySpecialStringComparer( sc , equivalencyRule ) ;
    return instance ;
  }

  private MySpecialStringComparerDecorator( StringComparer comparer , Collapse equivalencyRule )
  {
    if ( comparer == null                                  ) throw new ArgumentNullException("comparer") ;
    if ( !Enum.IsDefined(typeof(Collapse),equivalencyRule) ) throw new ArgumentOutOfRangeException("equivalencyRule") ;

    this.instance = comparer ;
    this.rule     = equivalencyRule ;

    return ;
  }

  private string CollapseAccordingToRule( string s )
    {
        string collapsed = s ;
        if ( rule != Collapse.None )
        {
            if ( string.IsNullOrWhiteSpace(s) )
            {
                bool isNull  = ( s == null ? true : false ) ;
                bool isEmpty = ( s == ""   ? true : false ) ;
                bool isWS    = !isNull && !isEmpty ;

                switch ( rule )
                {
                    case Collapse.EmptyAndWhitespace        : if ( isNull||isWS          ) collapsed = COLLAPSED_VALUE ; break ;
                    case Collapse.NullAndEmpty              : if ( isNull||isEmpty       ) collapsed = COLLAPSED_VALUE ; break ;
                    case Collapse.NullAndEmptyAndWhitespace : if ( isNull||isEmpty||isWS ) collapsed = COLLAPSED_VALUE ; break ;
                    case Collapse.NullAndWhitespace         : if ( isNull||isWS          ) collapsed = COLLAPSED_VALUE ; break ;
                    default                                 : throw new InvalidOperationException() ;
                }
            }
        }
        return collapsed ;
    }

  public override int Compare( string x , string y )
  {
    string a     = CollapseAccordingToRule(x) ;
    string b     = CollapseAccordingToRule(y) ;
    int    value = instance.Compare(a,b);
    return value ;
  }

  public override bool Equals( string x , string y )
  {
    string a     = CollapseAccordingToRule(x) ;
    string b     = CollapseAccordingToRule(y) ;
    bool   value = instance.Equals(a,b) ;
    return value ;
  }

  public override int GetHashCode( string obj )
  {
    string s     = CollapseAccordingToRule(obj) ;
    int    value = instance.GetHashCode( s ) ;
    return value ;
  }

}

Usage is simple:

StringComparer sc = new MySpecialStringDecorator( StringComparer.CurrentCultureIgnoreCase , Collapse.NullAndEmptyAndWhitespace ) ;

// go to town