XPathNavigator.Evaluate Method (XPathExpression)

Evaluates the XPathExpression and returns the typed result.

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

public virtual Object Evaluate(
	XPathExpression expr
)

Parameters

expr
Type: System.Xml.XPath.XPathExpression

An XPathExpression that can be evaluated.

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.

ExceptionCondition
ArgumentException

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

XPathException

The XPath expression is not valid.

The following C# code returns a number after converting the Price/text() node to a number and multiplying the value by 10.

XPathExpression expr = nav.Compile("Price/text()*10");
nav.Evaluate(expr);
NoteNote

The XPath position() and last() functions, unless used as a predicate in a location step, require a reference to a node set in order to be evaluated. In this case, you must use the overload which takes an XPathNodeIterator as an argument; otherwise, position() and last() return 0.

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

The following example evaluates an XPathExpression and returns a Double.

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

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

Double total = (double)navigator.Evaluate(query);
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" ?> 
<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>

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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