如何：检索元素的值 (LINQ to XML)

项目
04/19/2011

本主题演示如何获取元素的值。有两种主要方法可以完成此操作。一种方法是将 XElement 或 XAttribute 强制转换为所需的类型。然后，显式转换运算符将元素或属性的内容转换为指定的类型，并将其分配给变量。此外，还可以使用 XElement.Value 属性或 XAttribute.Value 属性。

但是，对于 C#，强制转换通常是更好的方法。在检索可能存在也可能不存在的元素（或属性）的值时，如果将元素或属性强制转换为可以为 null 的类型，则代码会更易于编写。本主题最后一个示例对此进行了演示。但是，无法通过强制转换设置元素的内容，而通过 XElement.Value 属性可以做到这一点。

对于 Visual Basic，最好的方法是使用 XElement.Value 属性。

示例

若要检索元素的值，只需将 XElement 对象强制转换为所需的类型即可。任何时候都可以将元素强制转换为字符串，如下所示：

XElement e = new XElement("StringElement", "abcde");
Console.WriteLine(e);
Console.WriteLine("Value of e:" + (string)e);

Dim e As XElement = <StringElement>abcde</StringElement>
Console.WriteLine(e)
Console.WriteLine("Value of e:" & e.Value)

此示例产生以下输出：

<StringElement>abcde</StringElement>
Value of e:abcde

此外，还可以将元素强制转换为字符串以外的其他类型。例如，如果有一个包含一个整数的元素，可以将它强制转换为 int，如下面的代码所示：

XElement e = new XElement("Age", "44");
Console.WriteLine(e);
Console.WriteLine("Value of e:" + (int)e);

Dim e As XElement = <Age>44</Age>
Console.WriteLine(e)
Console.WriteLine("Value of e:" & CInt(e))

此示例产生以下输出：

<Age>44</Age>
Value of e:44

LINQ to XML 提供了以下数据类型的显式强制转换运算符：string、bool、bool?、int、int?、uint、uint?、long、long?、ulong、ulong?、float、float?、double、double?、decimal、decimal?、DateTime、DateTime?、TimeSpan、TimeSpan?、GUID 和 GUID?。

LINQ to XML 为 XAttribute 对象提供了相同的强制转换运算符。

可以使用 Value 属性来检索元素的内容：

XElement e = new XElement("StringElement", "abcde"); 
Console.WriteLine(e);
Console.WriteLine("Value of e:" + e.Value);

Dim e As XElement = <StringElement>abcde</StringElement>
Console.WriteLine(e)
Console.WriteLine("Value of e:" & e.Value)

此示例产生以下输出：

<StringElement>abcde</StringElement>
Value of e:abcde

有时，尽管不能确定某个元素是否存在，还是会尝试检索该元素的值。在这种情况下，将强制转换后的元素分配给可以为 null 的类型（string 或 .NET Framework 中可以为 null 的类型之一）时，如果该元素不存在，则将分配的变量设置为 null（Visual Basic 中的 Nothing）。下面的代码演示当元素可能存在也可能不存在时，使用强制转换比使用 Value 属性更加简单。

XElement root = new XElement("Root",
    new XElement("Child1", "child 1 content"),
    new XElement("Child2", "2")
);

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

string c1 = (string)root.Element("Child1");
Console.WriteLine("c1:{0}", c1 == null ? "element does not exist" : c1);

int? c2 = (int?)root.Element("Child2");
Console.WriteLine("c2:{0}", c2 == null ? "element does not exist" : c2.ToString());

string c3 = (string)root.Element("Child3");
Console.WriteLine("c3:{0}", c3 == null ? "element does not exist" : c3);

int? c4 = (int?)root.Element("Child4");
Console.WriteLine("c4:{0}", c4 == null ? "element does not exist" : c4.ToString());

Console.WriteLine();

// The following assignments show the required code when using
// the Value property when the element might or might not exist.
// Notice that this is more difficult than the casting approach.

XElement e1 = root.Element("Child1");
string v1;
if (e1 == null)
    v1 = null;
else
    v1 = e1.Value;
Console.WriteLine("v1:{0}", v1 == null ? "element does not exist" : v1);

XElement e2 = root.Element("Child2");
int? v2;
if (e2 == null)
    v2 = null;
else
    v2 = Int32.Parse(e2.Value);
Console.WriteLine("v2:{0}", v2 == null ? "element does not exist" : v2.ToString());

XElement e3 = root.Element("Child3");
string v3;
if (e3 == null)
    v3 = null;
else
    v3 = e3.Value;
Console.WriteLine("v3:{0}", v3 == null ? "element does not exist" : v3);

XElement e4 = root.Element("Child4");
int? v4;
if (e4 == null)
    v4 = null;
else
    v4 = Int32.Parse(e4.Value);
Console.WriteLine("v4:{0}", v4 == null ? "element does not exist" : v4.ToString());

Dim root As XElement = <Root>
                           <Child1>child 1 content</Child1>
                           <Child2>2</Child2>
                       </Root>

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

Dim c1 As String = CStr(root.Element("Child1"))
Console.WriteLine("c1:{0}", IIf(c1 Is Nothing, "element does not exist", c1))

Dim c2 As Nullable(Of Integer) = CType(root.Element("Child2"), Nullable(Of Integer))
Console.WriteLine("c2:{0}", IIf(Not (c2.HasValue), "element does not exist", c2.ToString()))

Dim c3 As String = CStr(root.Element("Child3"))
Console.WriteLine("c3:{0}", IIf(c3 Is Nothing, "element does not exist", c3))

Dim c4 As Nullable(Of Integer) = CType(root.Element("Child4"), Nullable(Of Integer))
Console.WriteLine("c4:{0}", IIf(Not (c4.HasValue), "element does not exist", c4.ToString()))

Console.WriteLine()

' The following assignments show the required code when using
' the Value property when the attribute might or might not exist.
' Notice that this is more difficult than the casting approach.

Dim e1 As XElement = root.Element("Child1")
Dim v1 As String
If (e1 Is Nothing) Then
    v1 = Nothing
Else
    v1 = e1.Value
End If
Console.WriteLine("v1:{0}", IIf(v1 Is Nothing, "element does not exist", v1))

Dim e2 As XElement = root.Element("Child2")
Dim v2 As Nullable(Of Integer)
If (e2 Is Nothing) Then
    v2 = Nothing
Else
    v2 = e2.Value
End If
Console.WriteLine("v2:{0}", IIf(Not (v2.HasValue), "element does not exist", v2))

Dim e3 As XElement = root.Element("Child3")
Dim v3 As String
If (e3 Is Nothing) Then
    v3 = Nothing
Else
    v3 = e3.Value
End If
Console.WriteLine("v3:{0}", IIf(v3 Is Nothing, "element does not exist", v3))

Dim e4 As XElement = root.Element("Child4")
Dim v4 As Nullable(Of Integer)
If (e4 Is Nothing) Then
    v4 = Nothing
Else
    v4 = e4.Value
End If
Console.WriteLine("v4:{0}", IIf(Not (v4.HasValue), "element does not exist", v4))

这段代码产生以下输出：

c1:child 1 content
c2:2
c3:element does not exist
c4:element does not exist

v1:child 1 content
v2:2
v3:element does not exist
v4:element does not exist

通常情况下，当使用强制转换来检索元素和属性的内容时，可以编写更简易的代码。

请参见

概念

LINQ to XML 轴

通过

如何：检索元素的值 (LINQ to XML)

示例

请参见

概念

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