String.IsNullOrEmpty(String) 方法
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);
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。The string to test.
傳回
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.