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

参数

返回

true value如果参数为null或空字符串（“”）;否则，为 false.

示例

以下示例检查三个字符串，并确定每个字符串是否具有值、空字符串还是空 null字符串。

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.

注解

IsNullOrEmpty 是一种方便的方法，可用于同时测试 String 是否为 null 或其值是否为 String.Empty。 它等效于以下代码：

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

可以使用IsNullOrWhiteSpace方法测试字符串是否为null、其值是否为String.Empty或仅由空格字符组成。

什么是 null 字符串？

如果字符串尚未分配值（在 C++ 和 Visual Basic 中），或者它已显式赋值 null，则该字符串为 null。 尽管复合格式设置功能可以正常处理 null 字符串，如以下示例所示，但如果尝试调用其成员之一，会引发 NullReferenceException。

  String s = null;

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

  try 
  {
      Console.WriteLine($"String length is {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 Main3()
        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.

什么是空字符串？

如果已显式分配空字符串（“”）或 String.Empty，则字符串为空。 空字符串的值为 Length 0。 以下示例创建一个空字符串，并显示其值及其长度。

String s = "";
Console.WriteLine($"The length of '{s}' is {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.

另请参阅

• IsNullOrWhiteSpace(String)