Tutorial :.Net: Empty string is not clear space character?



Question:

I've always use String.IsNullOrEmpty to check for a empty string. It recently come to my attention that a " " count as not a empty string. For example,

 Dim test As String = " "      If String.IsNullOrEmpty(test) Then          MessageBox.Show("Empty String")      Else          MessageBox.Show("Not Emtpy String")      End If  

It will show "Not Empty String". So how do we check for " " or " " in a string?

edit: I wasn't aware that " " count as character.


Solution:1

Try this method to check for blank strings. It is different from the Trim() versions in that it does not allocate a new string. It also uses a more expanded notion of whitespace.

Public Function IsNullOrBlank(ByVal str as String) As Boolean    if String.IsNullOrEmpty(str) Then      Return True    End IF    For Each c In str      IF Not Char.IsWhiteSpace(c) Then        Return False      End If    Next    Return True  End Function  


Solution:2

String.IsNullOrWhiteSpace is in the BCL in .NET 4


Solution:3

An " " is an ASCII 32 value, it is no different from any other ASCII character except it "looks" blank.


Solution:4

The problem is you need to trim the string, however if you call trim() on a null string you will get an error.

string.IsNullOrEmpty(s.Trim())  

This will error out.

You will need to do something like

if (Not string.IsNullOrEmpty(s) AndAlso s.Trim()!=string.Empty)  

This will verify that the string isn't null or empty, if has something it will trim and then verify its not empty.

Edit

Thanks Slough for helping me with my VB syntax. I'm a C# guy need to brush up on vb's syntax


Solution:5

In VB.NET you'll need to use a test like this:

If String.IsNullOrEmpty(test) OrElse String.IsNullOrEmpty(test.Trim()) Then  

The OrElse prevents the NullException from occurring on the test.Trim()


Solution:6

public static bool IsNullOrWhite(string s)  {    if (String.IsNullOrEmpty(s))      return true;      for (int i = 0; i < s.Length; i++)      if (!Char.IsWhiteSpace(s, i))        return false;      return true;  }  


Solution:7

As was mentioned, calling Trim() will throw a NullReferenceException if the string is null. I sometimes use Regex.IsMatch(test, "^ +$") (hope I have the parameter order right) to test for one or more spaces. The ^ and $ make sure you're matching the entire string.


Solution:8

If you really need to treat strings containing only whitespace character the same as empty or null strings, then you could use an extension method like this one (sorry, it's in C#):

namespace ExtensionMethods  {      public static class StringExtensions      {          public static bool IsNullOrEmptyOrWhitespace(this string str)          {              if (string.IsNullOrEmpty(str)) return true;              return (str.Trim().Length == 0);          }      }  }  

This allows you to write:

using ExtensionMethods;    string s1 = "  ";  string s2 = "some string";    s1.IsNullOrEmptyOrWhitespace(); //-> returns true  s2.IsNullOrEmptyOrWhitespace(); //-> returns false  


Solution:9

If test Is Nothing OrElse test.Trim().Length = 0 Then ...  


Solution:10

Perhaps you just want to trim the string (of spaces) before checking it?

Dim test As String = " "  If test.Trim().Length = 0 Then // Best option as long as string is guaranteed not to be null.  //  If test = Nothing OrElse test.Trim().Length = 0 Then // If string can be null.      MessageBox.Show("Empty String")  Else      MessageBox.Show("Not Emtpy String")  End If  


Solution:11

If String.IsNullOrEmpty(str) OrElse str.Trim().Length = 0 Then    ' ..  End If  

The following may not work sometime if the str is null as calling a method on null will result in exception. If String.IsNullOrEmpty(str.Trim()) Then ' exception sometimes ... End If


Solution:12

You can add an extension method such as was discussed here and keep the same ease of use you previously had with IsNullOrEmpty().


Solution:13

Try

Dim test As String = " "  If String.IsNullOrEmpty(test.Trim()) Then      MessageBox.Show("Empty String")  Else      MessageBox.Show("Not Emtpy String")  End If  

Hope that helps.


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