xpath表達式count（）失敗，出現異常，Expression必須求值爲節點集

Question

我收到一個異常：System.Xml.XPath.XPathException：表達式必須求值爲節點集。

當我運行這個代碼，我期待計算節點的數量，我做錯了什麼？

var doc = new XPathDocument("contosoBooks.xml"); 
var nav = doc.CreateNavigator(); 

var expr = nav.Compile("count(//bookstore/book)"); 

var iterator = nav.Select(expr); 
while (iterator.MoveNext()) 
{ 
    if (iterator.Current != null && !iterator.Current.HasChildren) continue; 
    if (iterator.Current != null) 
    { 
     var childIter = iterator.Current.SelectChildren(XPathNodeType.Element); 
     while (childIter.MoveNext()) 
     { 
      if (childIter.Current != null) Console.WriteLine(childIter.Current.Value); 
     } 
    } 
} 

我的XML文件看起來像這樣

<?xml version="1.0" encoding="utf-8" ?> 
<bookstore> 
    <book genre="autobiography" publicationdate="1981-03-22" ISBN="1-861003-11-0"> 
    <title>The Autobiography of Benjamin Franklin</title> 
    <author> 
     <first-name>Benjamin</first-name> 
     <last-name>Franklin</last-name> 
    </author> 
    <price>8.99</price> 
    </book> 
    <book genre="novel" publicationdate="1967-11-17" ISBN="0-201-63361-2"> 
    <title>The Confidence Man</title> 
    <author> 
     <first-name>Herman</first-name> 
     <last-name>Melville</last-name> 
    </author> 
    <price>11.99</price> 
    </book> 
    <book genre="philosophy" publicationdate="1991-02-15" ISBN="1-861001-57-6"> 
    <title>The Gorgias</title> 
    <author> 
     <name>Plato</name> 
    </author> 
    <price>9.99</price> 
    </book> 
</bookstore> 

注：謝謝你的effords來幫助我。我想出了這個解決方案：

var doc = new XPathDocument("contosoBooks.xml"); 
var nav = doc.CreateNavigator(); 

const string query = "count(//bookstore/book)";    
var expr = nav.Compile(query); 

switch (expr.ReturnType) 
{   
    case XPathResultType.Number: 
     Console.WriteLine(nav.Evaluate(query)); 
     break; 

    case XPathResultType.String: 
    // case XPathResultType.Navigator: 
     Console.WriteLine(nav.Evaluate(query)); 
     break; 

    case XPathResultType.Boolean: 
     Console.WriteLine(nav.Evaluate(query)); 
     break; 

    case XPathResultType.NodeSet: 
     var nodes = nav.Select(expr); 
     while (nodes.MoveNext()) 
     {      
      if (nodes.Current != null && !nodes.Current.HasChildren) continue; 
      if (nodes.Current == null) continue; 
      var childIter = nodes.Current.SelectChildren(XPathNodeType.Element); 
      while (childIter.MoveNext()) 
      { 
       if (childIter.Current != null) Console.WriteLine(childIter.Current.Value); 
      } 
     } 
     break; 

    case XPathResultType.Any: 
     Console.WriteLine(nav.Evaluate(query)); 
     break;    

    default: 
     Console.WriteLine(nav.Evaluate(query));      
     break; 
}    

Answer 1

使用Evaluate，不選擇：

雙數=（雙）nav.Evaluate（表達式）;

Answer 2

在這一行：

var iterator = nav.Select(expr); 

表達式必須是XPath表達式返回的節點集。您的XPath表達式返回一個標量值，即書籍節點的計數。

Answer 3

簡單地更改爲：

var expr = nav.Compile("//bookstore/book"); 

Answer 4

var expr = nav.Compile("count(//bookstore/book)"); 
Console.WriteLine(nav.Evaluate(expr));