asp.net String.IsNullOrEmpty 方法

更新時(shí)間：2009年04月14日 13:20:45 作者：

此方法在 .NET Framework 2.0 版中是新增的。
指示指定的 String 對(duì)象是空引用（在 Visual Basic 中為 Nothing）還是 Empty 字符串。
這個(gè)方法在VB，VB.Net, C#,C++,JScript,J#中都有，很好很強(qiáng)大的一個(gè)方法。

String.Empty 字段
表示空字符串。此字段為只讀。
命名空間:System
程序集:mscorlib（在 mscorlib.dll 中）
語(yǔ)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
備注
此字段的值為零長(zhǎng)度字符串 ""。
示例
下面的代碼示例演示如何使用 Empty 字段。
在第一個(gè)示例中，如果另一個(gè)字段的值為空引用（在 Visual Basic 中為 Nothing），則 Empty 字符串作為默認(rèn)值返回。
Visual Basic 復(fù)制代碼

復(fù)制代碼代碼如下:

Dim myBinding As DataBinding = DataBindings("Text") 
If Not (myBinding Is Nothing) Then 
Return myBinding.Expression 
End If 
Return [String].Empty 
End Get 

C# 復(fù)制代碼

復(fù)制代碼代碼如下:

DataBinding myBinding = DataBindings["Text"]; 
if (myBinding != null) 
{ 
return myBinding.Expression; 
} 
return String.Empty; 

C++ 復(fù)制代碼

復(fù)制代碼代碼如下:

DataBinding^ myBinding = DataBindings[ "Text" ]; 
if ( myBinding != nullptr ) 
{ 
return myBinding->Expression; 
} 
return String::Empty; 

J# 復(fù)制代碼

復(fù)制代碼代碼如下:

DataBinding myBinding = get_DataBindings().get_Item("Text"); 
if (myBinding != null) { 
return myBinding.get_Expression(); 
} 
return(""); 

在第二個(gè)示例中，Compare 中使用了 Empty 字符串來(lái)測(cè)試子字符串。
Visual Basic 復(fù)制代碼

復(fù)制代碼代碼如下:

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# 復(fù)制代碼

復(fù)制代碼代碼如下:

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++ 復(fù)制代碼

復(fù)制代碼代碼如下:

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# 復(fù)制代碼

復(fù)制代碼代碼如下:

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 復(fù)制代碼

復(fù)制代碼代碼如下:

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(gè)示例中，XPathNavigator 對(duì)象的決策塊中使用了 Empty 字符串來(lái)作出有關(guān) XML 分析的決策。
Visual Basic 復(fù)制代碼

復(fù)制代碼代碼如下:

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# 復(fù)制代碼

復(fù)制代碼代碼如下:

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++ 復(fù)制代碼

復(fù)制代碼代碼如下:

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# 復(fù)制代碼

復(fù)制代碼代碼如下:

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 

平臺(tái)
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 并不是對(duì)每個(gè)平臺(tái)的所有版本都提供支持。有關(guān)受支持版本的列表，請(qǐng)參見系統(tǒng)要求。
版本信息
.NET Framework
受以下版本支持：2.0、1.1、1.0
.NET Compact Framework
受以下版本支持：2.0、1.0

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