String.IsNullOrEmpty(String) 方法

public:
 static bool IsNullOrEmpty(System::String ^ value);

public static bool IsNullOrEmpty (string value);

public static bool IsNullOrEmpty (string? value);

static member IsNullOrEmpty : string -> bool

Public Shared Function IsNullOrEmpty (value As String) As Boolean

參數

傳回

Boolean

如果 true 參數為 value 或空字串 ("")，則為 null，否則為 false。true 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，並判斷每個字串是否具有值、是否為空字串，或為 null 。The 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.

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));

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 是方便的方法，可讓您同時測試 a String null 或其值是否為 String.Empty 。IsNullOrEmpty is a convenience method that enables you to simultaneously test whether a String is null or its value is String.Empty. 它相當於下列程式碼：It is equivalent to the following code:

result = s == nullptr || s == String::Empty;

bool TestForNullOrEmpty(string s)
{
    bool result;
    result = s == null || s == string.Empty;
    return result;
}

string s1 = null;
string s2 = "";
Console.WriteLine(TestForNullOrEmpty(s1));
Console.WriteLine(TestForNullOrEmpty(s2));

// The example displays the following output:
//    True
//    True

result = s Is Nothing OrElse s = String.Empty

您可以使用 IsNullOrWhiteSpace 方法來測試字串是否為 null 、其值為 String.Empty ，或只包含空白字元。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?

null如果未在 c + + 和) Visual Basic 的 (指派值，或已明確指派值給，則為字串 null 。A 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 複合格式化 功能可以正常地處理 null 字串（如下列範例所示），但如果其成員擲回，則嘗試呼叫其中一個 NullReferenceException 。Although 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.

  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?

如果明確地將空字串指派給空字串 ( "" ) 或，字串就會是空的 String.Empty 。A string is empty if it is explicitly assigned an empty string ("") or String.Empty. 空字串 Length 的為0。An empty string has a Length of 0. 下列範例會建立空字串，並顯示其值和長度。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

另請參閱

• IsNullOrWhiteSpace(String)