HOW TO：擷取項目的值 (LINQ to XML)

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04/19/2011

本主題顯示如何取得項目的值。以下有兩種主要的方式可達成此目標。其中一種方式為，將 XElement 或 XAttribute 轉型為所需的型別。然後，明確的轉換運算子會將項目或屬性的內容轉換為指定的型別，並將其指派給您的變數。或者，您可以使用 XElement.Value 屬性或 XAttribute.Value 屬性。

不過，使用 C# 時，轉型 (Casting) 通常是較好的方法。如果您要將項目或屬性轉型為可為 Null 的型別 (Nullable Type)，擷取可能存在或可能不存在之項目 (或屬性) 的值時，較容易撰寫程式碼。本主題中的最後一個範例會示範這個情況。不過，您無法像透過 XElement.Value 屬性般，透過轉型設定項目的內容。

使用 Visual Basic 時，最好的方法是使用 XElement.Value 屬性。

範例

若要擷取項目的值，您只要將 XElement 物件轉型為所需的型別即可。您永遠可以將項目轉型為字串，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)

此範例會產生下列輸出：

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

您也可以將項目轉型為非字串的型別。例如，如果您有包含整數的項目，您可以將其轉型為 int，如下列程式碼所示：

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))

此範例會產生下列輸出：

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

LINQ to XML 會提供下列資料型別的明確轉換運算子：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 會提供 XAttribute 物件相同的轉換運算子。

您可以使用 Value 屬性來擷取項目的內容：

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)

此範例會產生下列輸出：

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

即使您不確定項目是否存在，您有時候還是會嘗試擷取項目的值。在這個情況下，當您將轉型的項目指派給可為 Null 的型別 (.NET Framework 中的 string 或其中一個可為 Null 的型別) 時，如果項目不存在，被指派的變數只會設定為 null (在 Visual Basic 中沒有任何值)。下列程式碼顯示，項目可能存在或可能不存在時，使用轉型比使用 Value 屬性更為容易。

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))

此程式碼會產生下列輸出：

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

一般而言，使用轉換擷取元素及屬性內容時，您可以撰寫較簡易的程式碼。

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概念

LINQ to XML 座標軸

共用方式為

HOW TO：擷取項目的值 (LINQ to XML)

範例

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概念

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