asp.net String.IsNullOrEmpty 方法

此方法在 .NET Framework 2.0 版中是新增的。
指示指定的 String 對象是 Null 參考（在 Visual Basic 中為 Nothing） 還是 Empty 字串。
這個方法在VB，VB.Net, C#,C++,JScript,J#中都有，很好很強大的一個方法。

String.Empty 欄位
表示Null 字元串。此欄位為唯讀。
命名空間:System
程式集:mscorlib（在 mscorlib.dll 中）
文法
Visual Basic（聲明）
Public Shared ReadOnly Empty As String
Visual Basic（用法）
Dim value As String
value = String.Empty
C#
public static readonly string Empty
C++
public:
static initonly String^ Empty
J#
public static final String Empty
JScript
public static final var Empty : String
備忘
此欄位的值為零長度字串 ""。
樣本
下面的程式碼範例示範如何使用 Empty 欄位。
在第一個樣本中，如果另一個欄位的值為 Null 參考（在 Visual Basic 中為 Nothing），則 Empty 字串作為預設值返回。
Visual Basic 複製代碼 複製代碼 代碼如下:Dim myBinding As DataBinding = DataBindings("Text")
If Not (myBinding Is Nothing) Then
Return myBinding.Expression
End If
Return [String].Empty
End Get

C# 複製代碼 複製代碼 代碼如下:DataBinding myBinding = DataBindings["Text"];
if (myBinding != null)
{
return myBinding.Expression;
}
return String.Empty;

C++ 複製代碼 複製代碼 代碼如下:DataBinding^ myBinding = DataBindings[ "Text" ];
if ( myBinding != nullptr )
{
return myBinding->Expression;
}
return String::Empty;

J# 複製代碼 複製代碼 代碼如下:DataBinding myBinding = get_DataBindings().get_Item("Text");
if (myBinding != null) {
return myBinding.get_Expression();
}
return("");

在第二個樣本中，Compare 中使用了 Empty 字串來測試子字串。
Visual Basic 複製代碼 複製代碼 代碼如下:Dim myString As String = "abc"
Dim test1 As Boolean = String.Compare(myString.Substring(2, 1), "c") = 0 ' This is true.
myString.Substring(3, 1) ' This throws ArgumentOutOfRangeException.
Dim test2 As Boolean = String.Compare(myString.Substring(3, 0), String.Empty) = 0 ' This is true.

C# 複製代碼 複製代碼 代碼如下:String myString = "abc";
bool test1 = String.Compare(myString.Substring(2, 1), "c") == 0; // This is true.
myString.Substring(3, 1); // This throws ArgumentOutOfRangeException.
bool test2 = String.Compare(myString.Substring(3, 0), String.Empty) == 0; // This is true.

C++ 複製代碼 複製代碼 代碼如下:String^ myString = "abc";
bool test1 = String::Compare( myString->Substring( 2, 1 ), "c" ) == 0; // This is true.
myString->Substring( 3, 1 ); // This throws ArgumentOutOfRangeException.
bool test2 = String::Compare( myString->Substring( 3, 0 ), String::Empty ) == 0; // This is true.

J# 複製代碼 複製代碼 代碼如下:String myString = "abc";
// This is true.
boolean test1 = String.Compare(myString.Substring(2, 1), "c") == 0;
myString.Substring(3, 1); // This throws ArgumentOutOfRangeException.
// This is true.
boolean test2 = String.Compare(myString.Substring(3, 0), " ") == 0;

JScript 複製代碼 複製代碼 代碼如下:var myString : String = "abc";
var test1 : boolean = String.Compare(myString.Substring(2, 1), "c") == 0; // This is true.
myString.Substring(3, 1); // This throws ArgumentOutOfRangeException.
var test2 : boolean = String.Compare(myString.Substring(3, 0), String.Empty) == 0; // This is true.

在第三個樣本中，XPathNavigator 對象的決策塊中使用了 Empty 字串來作出有關 XML 分析的決策。
Visual Basic 複製代碼 複製代碼 代碼如下:public shared sub RecursiveWalk(nav as XPathNavigator)
select case nav.NodeType
case XPathNodeType.Element
if (nav.Prefix=String.Empty)
Console.WriteLine("<{0}>", nav.LocalName)
else
Console.Write("<{0}:{1}>", nav.Prefix, nav.LocalName)
Console.WriteLine(" "+ nav.NamespaceURI)
end if
case XPathNodeType.Text
Console.WriteLine(" " + nav.Value)
end select
if ( nav.MoveToFirstChild() )
do
RecursiveWalk(nav)
loop while ( nav.MoveToNext() )
nav.MoveToParent()
if (nav.NodeType = XPathNodeType.Element)
Console.WriteLine("</{0}>", nav.Name)
end if
end if
end sub

C# 複製代碼 複製代碼 代碼如下:public static void RecursiveWalk(XPathNavigator nav)
{
switch (nav.NodeType){
case XPathNodeType.Element:
if (nav.Prefix==String.Empty)
Console.WriteLine("<{0}>", nav.LocalName);
else
Console.Write("<{0}:{1}>", nav.Prefix, nav.LocalName);
Console.WriteLine("\t"+ nav.NamespaceURI);
break;
case XPathNodeType.Text:
Console.WriteLine("\t" + nav.Value);
break;
}
if ( nav.MoveToFirstChild() )
{
do{
RecursiveWalk(nav);
} while ( nav.MoveToNext() );
nav.MoveToParent();
if (nav.NodeType == XPathNodeType.Element)
Console.WriteLine("</{0}>", nav.Name);
}
}

C++ 複製代碼 複製代碼 代碼如下:static void RecursiveWalk( XPathNavigator^ nav )
{
switch ( nav->NodeType )
{
case XPathNodeType::Element:
if ( nav->Prefix == String::Empty )
Console::WriteLine( "< {0}>", nav->LocalName );
else
Console::Write( "< {0}: {1}>", nav->Prefix, nav->LocalName );
Console::WriteLine( "\t {0}", nav->NamespaceURI );
break;
case XPathNodeType::Text:
Console::WriteLine( "\t {0}", nav->Value );
break;
}
if ( nav->MoveToFirstChild() )
{
do
{
RecursiveWalk( nav );
}
while ( nav->MoveToNext() );
nav->MoveToParent();
if ( nav->NodeType == XPathNodeType::Element )
Console::WriteLine( "</ {0}>", nav->Name );
}
}

J# 複製代碼 複製代碼 代碼如下:public static void RecursiveWalk(XPathNavigator nav)
{
switch (nav.get_NodeType()) {
case XPathNodeType.Element:
if (nav.get_Prefix().Equals("")){
Console.WriteLine("<{0}>", nav.get_LocalName());
}
else {
Console.Write("<{0}:{1}>", nav.get_Prefix(),
nav.get_LocalName());
}
Console.WriteLine("\t" + nav.get_NamespaceURI());
break;
case XPathNodeType.Text:
Console.WriteLine("\t" + nav.get_Value());
break;
}
if (nav.MoveToFirstChild()) {
do {
RecursiveWalk(nav);
} while (nav.MoveToNext());
nav.MoveToParent();
if (nav.get_NodeType() .Equals(XPathNodeType.Element)) {
Console.WriteLine("</{0}>", nav.get_Name());
}
}
} //RecursiveWalk

平台
Windows 98、Windows 2000 SP4、Windows CE、Windows Millennium Edition、Windows Mobile for Pocket PC、Windows Mobile for Smartphone、Windows Server 2003、Windows XP Media Center Edition、Windows XP Professional x64 Edition、Windows XP SP2、Windows XP Starter Edition
.NET Framework 並不是對每個平台的所有版本都提供支援。有關受支援版本的列表，請參見系統要求。
版本資訊
.NET Framework
受以下版本支援：2.0、1.1、1.0
.NET Compact Framework
受以下版本支援：2.0、1.0