String.IsNullOrEmpty(String) 方法

定義

表示指定的字串是否為 null 或空字串 ("")。

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

value
String

要測試的字串。

傳回

Boolean

如果 true 參數為 value 或空字串 (""),則為 null,否則為 false

範例

下列範例會檢查三個字串,並判斷每個字串是否有值、是空字串或 為 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.
let test (s: string): string =
    if String.IsNullOrEmpty(s)
    then "is null or empty"
    else $"(\"{s}\") is neither null nor empty"

let s1 = "abcd"
let s2 = ""
let s3 = null

printfn "String s1 %s" (test s1)
printfn "String s2 %s" (test s2)
printfn "String s2 %s" (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.

備註

IsNullOrEmpty 是一種方便的方法,可讓您同時測試 是 String null 或其值是否為 String.Empty 。 它相當於下列程式碼:

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
let testForNullOrEmpty (s: string): bool =
    s = null || s = String.Empty

let s1 = null
let s2 = ""

printfn "%b" (testForNullOrEmpty s1)
printfn "%b" (testForNullOrEmpty s2)

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

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。

什麼是 Null 字串?

如果字串尚未在 C++ 中 (指派值,則為 , null Visual Basic) 或明確指派 的值 null 為 。 雖然 複合格式 設定功能可以正常處理 Null 字串,如下列範例所示,如果其成員擲回 NullReferenceException ,就會嘗試呼叫 。

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.
let (s: string) = null

printfn "The value of the string is '%s'" s

try
    printfn "String length is %d" s.Length
with
    | :? NullReferenceException as ex -> printfn "%s" ex.Message

// 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?

如果字串明確指派空字串 (「」) 或 String.Empty ,則字串是空的。 空字串的 值為 Length 0。 下列範例會建立空字串,並顯示其值及其長度。

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.
let s = ""
printfn "The length of '%s' is %d." s s.Length

// The example displays the following output:
//       The length of '' is 0.

適用於

另請參閱