XAttribute.Value 属性

定义

命名空间:

System.Xml.Linq

程序集:

System.Xml.XDocument.dll

Source:

XAttribute.cs

获取或设置此属性的值。

public:
 property System::String ^ Value { System::String ^ get(); void set(System::String ^ value); };

public string Value { get; set; }

member this.Value : string with get, set

Public Property Value As String

属性值

String

包含此属性的值的 String。

例外

ArgumentNullException

设置时，value 为 null。

示例

以下示例创建一个具有 特性的元素。 然后，它会检索属性的值，然后对其进行设置。

请注意，Visual Basic 示例使用 XML 属性。

XElement root = new XElement("Root",  
    new XAttribute("Att", "content")  
);  
XAttribute att = root.FirstAttribute;  
Console.WriteLine(att.Value);  
att.Value = "new text";  
Console.WriteLine(att.Value);  

Dim root As XElement = <Root Att="content"/>  
Console.WriteLine(root.@Att)  
root.@Att = "new text"  
Console.WriteLine(root.@Att)  

该示例产生下面的输出：

content  
new text  

以下示例演示了使用显式转换运算符获取可能不存在的属性的值的好处：

XElement root = new XElement("Root",  
    new XAttribute("Att1", "attribute 1 content"),  
    new XAttribute("Att2", "2")  
);  

// The following assignments demonstrate why it is easier to use  
// casting when the attribute might or might not exist.  

string c1 = (string)root.Attribute("Att1");  
Console.WriteLine("c1:{0}", c1 == null ? "attribute does not exist" : c1);  

int? c2 = (int?)root.Attribute("Att2");  
Console.WriteLine("c2:{0}", c2 == null ? "attribute does not exist" : c2.ToString());  

string c3 = (string)root.Attribute("Att3");  
Console.WriteLine("c3:{0}", c3 == null ? "attribute does not exist" : c3);  

int? c4 = (int?)root.Attribute("Att4");  
Console.WriteLine("c4:{0}", c4 == null ? "attribute does not exist" : c4.ToString());  

Console.WriteLine();  

// The following assignments show the necessary code when using  
// the value property when the attribute might or might not exist.  

XAttribute att1 = root.Attribute("Att1");  
string v1;  
if (att1 == null)  
    v1 = null;  
else  
    v1 = att1.Value;  
Console.WriteLine("v1:{0}", v1 == null ? "attribute does not exist" : v1);  

XAttribute att2 = root.Attribute("Att2");  
int? v2;  
if (att2 == null)  
    v2 = null;  
else  
    v2 = Int32.Parse(att2.Value);  
Console.WriteLine("v2:{0}", v2 == null ? "attribute does not exist" : v2.ToString());  

XAttribute att3 = root.Attribute("Att3");  
string v3;  
if (att3 == null)  
    v3 = null;  
else  
    v3 = att3.Value;  
Console.WriteLine("v3:{0}", v3 == null ? "attribute does not exist" : v3);  

XAttribute att4 = root.Attribute("Att4");  
int? v4;  
if (att4 == null)  
    v4 = null;  
else  
    v4 = Int32.Parse(att4.Value);  
Console.WriteLine("v4:{0}", v4 == null ? "attribute does not exist" : v4.ToString());  

Dim root As XElement = <Root Att1="attribute 1 content" Att2="2"/>  

' The following assignments demonstrate why it is easier to use  
' casting when the attribute might or might not exist.  

Dim c1 As String = CStr(root.Attribute("Att1"))  
Console.WriteLine("c1:{0}", IIf(c1 Is Nothing, "attribute does not exist", c1))  

Dim c2 As Nullable(Of Integer) = CType(root.Attribute("Att2"), Nullable(Of Integer))  
Console.WriteLine("c2:{0}", IIf(c2.HasValue, c2, "attribute does not exist"))  

Dim c3 As String = CStr(root.Attribute("Att3"))  
Console.WriteLine("c3:{0}", IIf(c3 Is Nothing, "attribute does not exist", c3))  

Dim c4 As Nullable(Of Integer) = CType(root.Attribute("Att4"), Nullable(Of Integer))  
Console.WriteLine("c4:{0}", IIf(c4.HasValue, c4, "attribute does not exist"))  

Console.WriteLine()  

' The following assignments show the necessary code when using  
' the value property when the attribute might or might not exist.  

Dim att1 As XAttribute = root.Attribute("Att1")  
Dim v1 As String  
If att1 Is Nothing Then  
    v1 = Nothing  
Else  
    v1 = att1.Value  
End If  

Console.WriteLine("v1:{0}", IIf(v1 Is Nothing, "attribute does not exist", v1))  

Dim att2 As XAttribute = root.Attribute("Att2")  
Dim v2 As Nullable(Of Integer)  
If att2 Is Nothing Then  
    v2 = Nothing  
Else  
    v2 = Int32.Parse(att2.Value)  
End If  

Console.WriteLine("v2:{0}", IIf(v2.HasValue, v2, "attribute does not exist"))  

Dim att3 As XAttribute = root.Attribute("Att3")  
Dim v3 As String  
If att3 Is Nothing Then  
    v3 = Nothing  
Else  
    v3 = att3.Value  
End If  

Console.WriteLine("v3:{0}", IIf(v3 Is Nothing, "attribute does not exist", v3))  

Dim att4 As XAttribute = root.Attribute("Att4")  
Dim v4 As Nullable(Of Integer)  
If att4 Is Nothing Then  
    v4 = Nothing  
Else  
    v4 = Int32.Parse(att4.Value)  
End If  

Console.WriteLine("v4:{0}", IIf(v4.HasValue, v4, "attribute does not exist"))  

该示例产生下面的输出：

c1:attribute 1 content  
c2:2  
c3:attribute does not exist  
c4:attribute does not exist  

v1:attribute 1 content  
v2:2  
v3:attribute does not exist  
v4:attribute does not exist  

注解

可以使用此属性获取或设置特性的值。

设置此属性将引发 Changed 和 Changing 事件。

如果获取值并且属性可能不存在，则使用显式转换运算符，并将属性分配给可为 null 的类型（如 string 或 Nullable<T> ） Int32会更方便。 如果该属性不存在，则可为 null 的类型设置为 null。 在使用此属性之前，必须确保 Attribute 方法不返回 null。

另请参阅

• SetValue(Object)
• Name
• ToString()
• LINQ to XML 概述