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

Definizione

Indica se la stringa specificata è null o una stringa vuota ("").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

Parametri

value
String String String String

Stringa da testare.The string to test.

Restituisce

true se il parametro value è null o una stringa vuota (""); in caso contrario, false.true if the value parameter is null or an empty string (""); otherwise, false.

Esempi

Nell'esempio seguente vengono esaminate tre stringhe e viene determinato se ogni stringa ha un valore, è una stringa vuota o nullè.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.

Commenti

IsNullOrEmptyè un metodo pratico che consente di verificare simultaneamente se un String è null o il suo valore String.Emptyè.IsNullOrEmpty is a convenience method that enables you to simultaneously test whether a String is null or its value is String.Empty. Equivale al codice seguente: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));
result = s Is Nothing OrElse s = String.Empty

È possibile utilizzare il IsNullOrWhiteSpace metodo per verificare se una stringa è null, il valore è String.Emptyoppure solo uno spazio vuoto.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.

Che cos'è una stringa null?What is a null string?

Una stringa è null se non è stato assegnato un valore (in C++ e Visual Basic) o se è stato assegnato esplicitamente un valore di 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. Sebbene la funzionalità di formattazione composita possa gestire normalmente una stringa null, come illustrato nell'esempio seguente, provare a chiamarne una se i relativi membri NullReferenceExceptiongenerano un'eccezione.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.

Che cos'è una stringa vuota?What is an empty string?

Una stringa è vuota se viene assegnata in modo esplicito una stringa vuota ("" String.Empty) o.A string is empty if it is explicitly assigned an empty string ("") or String.Empty. Una stringa vuota ha un Length valore pari a 0.An empty string has a Length of 0. Nell'esempio seguente viene creata una stringa vuota e ne vengono visualizzati il valore e la lunghezza.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.      

Si applica a

Vedi anche