XPathNavigator.Evaluate Method (XPathExpression, XPathNodeIterator)


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Uses the supplied context to evaluate the XPathExpression, and returns the typed result.

Namespace:   System.Xml.XPath
Assembly:  System.Xml (in System.Xml.dll)

public virtual object Evaluate(
	XPathExpression expr,
	XPathNodeIterator context


Type: System.Xml.XPath.XPathExpression

An XPathExpression that can be evaluated.

Type: System.Xml.XPath.XPathNodeIterator

An XPathNodeIterator that points to the selected node set that the evaluation is to be performed on.

Return Value

Type: System.Object

The result of the expression (Boolean, number, string, or node set). This maps to Boolean, Double, String, or XPathNodeIterator objects respectively.

Exception Condition

The return type of the XPath expression is a node set.


The XPath expression is not valid.

The expression is evaluated using the Current node of the XPathNodeIterator as the context node. If context is null, the node on which the XPathNavigator is currently positioned is used as the context node.

The position() and last() functions, unless used as a predicate in a location step, always return 0 under the following conditions:

Because the position() and last() functions work on the current node, you should not use the Current property to move away from the selected node set. This could invalidate the state of the XPathNavigator.

This method has no effect on the state of the XPathNavigator.

The following example evaluates an XPathExpression and returns a Double using the Current node of the XPathNodeIterator as the context node.

XPathDocument document = new XPathDocument("books.xml");
XPathNavigator navigator = document.CreateNavigator();

XPathNodeIterator nodes = navigator.Select("//book");
XPathExpression query = nodes.Current.Compile("sum(descendant::price)");

Double total = (double)navigator.Evaluate(query, nodes);
Console.WriteLine("Total price for all books: {0}", total.ToString());

The example takes the books.xml file as an input.

<?xml version="1.0" encoding="utf-8" ?> 
    <book genre="autobiography" publicationdate="1981-03-22" ISBN="1-861003-11-0">
        <title>The Autobiography of Benjamin Franklin</title>
    <book genre="novel" publicationdate="1967-11-17" ISBN="0-201-63361-2">
        <title>The Confidence Man</title>
    <book genre="philosophy" publicationdate="1991-02-15" ISBN="1-861001-57-6">
        <title>The Gorgias</title>

.NET Framework
Available since 1.1
Available since 4.0
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