String.IsNullOrEmpty(String) String.IsNullOrEmpty(String) String.IsNullOrEmpty(String) String.IsNullOrEmpty(String) Method

定義

表示指定的字串是否為 null 或空字串 ("")。Indicates whether the specified string is null or an empty string ("").

public:
 static bool IsNullOrEmpty(System::String ^ value);
public static bool IsNullOrEmpty (string value);
static member IsNullOrEmpty : string -> bool
Public Shared Function IsNullOrEmpty (value As String) As Boolean

參數

value
String String String String

要測試的字串。The string to test.

傳回

如果 true 參數為 value 或空字串 (""),則為 null,否則為 falsetrue if the value parameter is null or an empty string (""); otherwise, false.

範例

下列範例會檢查三個字串,並判斷每個字串的值、 為空字串,或為nullThe following example examines three strings and determines whether each string has a value, is an empty string, or is null.

using namespace System;
String^ Test( String^ s )
{
   if (String::IsNullOrEmpty(s))
      return "is null or empty";
   else
      return String::Format( "(\"{0}\") is neither null nor empty", s );
}

int main()
{
   String^ s1 = "abcd";
   String^ s2 = "";
   String^ s3 = nullptr;
   Console::WriteLine( "String s1 {0}.", Test( s1 ) );
   Console::WriteLine( "String s2 {0}.", Test( s2 ) );
   Console::WriteLine( "String s3 {0}.", Test( s3 ) );
}
// The example displays the following output:
//       String s1 ("abcd") is neither null nor empty.
//       String s2 is null or empty.
//       String s3 is null or empty.
using System;

class Sample 
{
    public static void Main() 
    {
    string s1 = "abcd";
    string s2 = "";
    string s3 = null;

    Console.WriteLine("String s1 {0}.", Test(s1));
    Console.WriteLine("String s2 {0}.", Test(s2));
    Console.WriteLine("String s3 {0}.", Test(s3));
    }

    public static String Test(string s)
    {
    if (String.IsNullOrEmpty(s)) 
        return "is null or empty";
    else
        return String.Format("(\"{0}\") is neither null nor empty", s);
    }
}
// The example displays the following output:
//       String s1 ("abcd") is neither null nor empty.
//       String s2 is null or empty.
//       String s3 is null or empty.
Class Sample
   Public Shared Sub Main()
      Dim s1 As String = "abcd"
      Dim s2 As String = ""
      Dim s3 As String = Nothing
      
      Console.WriteLine("String s1 {0}.", Test(s1))
      Console.WriteLine("String s2 {0}.", Test(s2))
      Console.WriteLine("String s3 {0}.", Test(s3))
   End Sub
   
   Public Shared Function Test(s As String) As String
      If String.IsNullOrEmpty(s) Then
         Return "is null or empty"
      Else
         Return String.Format("(""{0}"") is neither null nor empty", s)
      End If
   End Function 
End Class  
' The example displays the following output:
'       String s1 ("abcd") is neither null nor empty.
'       String s2 is null or empty.
'       String s3 is null or empty.

備註

IsNullOrEmpty 是一種便利方法,可讓您以同時測試是否Stringnull或其值為String.EmptyIsNullOrEmpty is a convenience method that enables you to simultaneously test whether a String is null or its value is String.Empty. 相當於下列程式碼:It is equivalent to the following code:

result = s == nullptr || s == String::Empty;
result = s == null || s == String.Empty;
result = s Is Nothing OrElse s = String.Empty

您可以使用IsNullOrWhiteSpace方法來測試字串是否null,其值是String.Empty,或它只包含泛空白字元。You can use the IsNullOrWhiteSpace method to test whether a string is null, its value is String.Empty, or it consists only of white-space characters.

什麼是 null 字串?What is a null string?

字串是null如果它尚未指派值 (在C++和 Visual Basic) 或如果已明確地被指派的值nullA string is null if it has not been assigned a value (in C++ and Visual Basic) or if it has explicitly been assigned a value of null. 雖然複合格式功能可以正常處理 null 的字串,如下列範例所示,嘗試呼叫其中一個,如果其成員擲回NullReferenceExceptionAlthough the composite formatting feature can gracefully handle a null string, as the following example shows, attempting to call one if its members throws a NullReferenceException.

using namespace System;

void main()
{
   String^ s;
  
   Console::WriteLine("The value of the string is '{0}'", s);

   try {
      Console::WriteLine("String length is {0}", s->Length);
   }
   catch (NullReferenceException^ e) {
      Console::WriteLine(e->Message);
   }   
}
// The example displays the following output:
//     The value of the string is ''
//     Object reference not set to an instance of an object.
using System;

public class Example
{
   public static void Main()
   {
      String s = null;
      
      Console.WriteLine("The value of the string is '{0}'", s);

      try {
         Console.WriteLine("String length is {0}", s.Length);
      }
      catch (NullReferenceException e) {
         Console.WriteLine(e.Message);
      }
   }
}
// The example displays the following output:
//     The value of the string is ''
//     Object reference not set to an instance of an object.
Module Example
   Public Sub Main()
      Dim s As String

      Console.WriteLine("The value of the string is '{0}'", s)

      Try 
         Console.WriteLine("String length is {0}", s.Length)
      Catch e As NullReferenceException
         Console.WriteLine(e.Message)
      End Try   
   End Sub
End Module
' The example displays the following output:
'     The value of the string is ''
'     Object reference not set to an instance of an object.

什麼是空字串?What is an empty string?

字串是空明確指派為空字串 ("") 或String.EmptyA string is empty if it is explicitly assigned an empty string ("") or String.Empty. 空的字串具有Length為 0。An empty string has a Length of 0. 下列範例會建立空的字串,並顯示它的值,它的長度。The following example creates an empty string and displays its value and its length.

String^ s = "";
Console::WriteLine("The length of '{0}' is {1}.", s, s->Length);
// The example displays the following output:
//       The length of '' is 0.      
String s = "";
Console.WriteLine("The length of '{0}' is {1}.", s, s.Length);
// The example displays the following output:
//       The length of '' is 0.      
Dim s As String = ""
Console.WriteLine("The length of '{0}' is {1}.", s, s.Length)
' The example displays the following output:
'        The length of '' is 0.      

適用於

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