XPathNavigator.Evaluate Method

Definition

Namespace:

System.Xml.XPath

Assembly:

System.Xml.ReaderWriter.dll

Evaluates the specified XPath expression and returns the typed result.

Overloads

Evaluate(String)	Evaluates the specified XPath expression and returns the typed result.
Evaluate(XPathExpression)	Evaluates the XPathExpression and returns the typed result.
Evaluate(String, IXmlNamespaceResolver)	Evaluates the specified XPath expression and returns the typed result, using the IXmlNamespaceResolver object specified to resolve namespace prefixes in the XPath expression.
Evaluate(XPathExpression, XPathNodeIterator)	Uses the supplied context to evaluate the XPathExpression, and returns the typed result.

Evaluate(String)

Source:

XPathNavigator.cs

Evaluates the specified XPath expression and returns the typed result.

public:
 virtual System::Object ^ Evaluate(System::String ^ xpath);

public virtual object Evaluate (string xpath);

abstract member Evaluate : string -> obj
override this.Evaluate : string -> obj

Public Overridable Function Evaluate (xpath As String) As Object

Parameters

xpath

String

A string representing an XPath expression that can be evaluated.

Returns

Object

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

Exceptions

ArgumentException

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

XPathException

The XPath expression is not valid.

Examples

The following example evaluates an XPath expression and returns a Double.

XPathDocument^ document = gcnew XPathDocument("books.xml");
XPathNavigator^ navigator = document->CreateNavigator();

Double total = (double)navigator->Evaluate("sum(descendant::book/price)");
Console::WriteLine("Total price for all books: {0}", total.ToString());

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

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

Dim document As XPathDocument = New XPathDocument("books.xml")
Dim navigator As XPathNavigator = document.CreateNavigator()

Dim total As Double = CType(navigator.Evaluate("sum(descendant::book/price)"), Double)
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>  

Remarks

The following C# code converts the Price/text() node to a number, multiplies it by 10, and returns the resulting value.

nav.Evaluate("Price/text()*10");  

Note

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.

See also

• Compile(String)
• XPathExpression
• ReturnType

Evaluate(XPathExpression)

Source:

XPathNavigator.cs

Evaluates the XPathExpression and returns the typed result.

public:
 virtual System::Object ^ Evaluate(System::Xml::XPath::XPathExpression ^ expr);

public virtual object Evaluate (System.Xml.XPath.XPathExpression expr);

abstract member Evaluate : System.Xml.XPath.XPathExpression -> obj
override this.Evaluate : System.Xml.XPath.XPathExpression -> obj

Public Overridable Function Evaluate (expr As XPathExpression) As Object

Parameters

expr

XPathExpression

An XPathExpression that can be evaluated.

Returns

Object

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

Exceptions

ArgumentException

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

XPathException

The XPath expression is not valid.

Examples

The following example evaluates an XPathExpression and returns a Double.

XPathDocument^ document = gcnew 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());

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());

Dim document As XPathDocument = New XPathDocument("books.xml")
Dim navigator As XPathNavigator = document.CreateNavigator()

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

Dim total As Double = CType(navigator.Evaluate(query), Double)
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>  

Remarks

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);  

Note

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.

See also

• Compile(String)
• ReturnType

Evaluate(String, IXmlNamespaceResolver)

Source:

XPathNavigator.cs

Evaluates the specified XPath expression and returns the typed result, using the IXmlNamespaceResolver object specified to resolve namespace prefixes in the XPath expression.

public:
 virtual System::Object ^ Evaluate(System::String ^ xpath, System::Xml::IXmlNamespaceResolver ^ resolver);

public virtual object Evaluate (string xpath, System.Xml.IXmlNamespaceResolver? resolver);

abstract member Evaluate : string * System.Xml.IXmlNamespaceResolver -> obj
override this.Evaluate : string * System.Xml.IXmlNamespaceResolver -> obj

Public Overridable Function Evaluate (xpath As String, resolver As IXmlNamespaceResolver) As Object

Parameters

xpath

String

A string representing an XPath expression that can be evaluated.

resolver

IXmlNamespaceResolver

The IXmlNamespaceResolver object used to resolve namespace prefixes in the XPath expression.

Returns

Object

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

Exceptions

ArgumentException

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

XPathException

The XPath expression is not valid.

Examples

The following example evaluates an XPath expression and returns a Double using the XmlNamespaceManager object specified to resolve namespace prefixes in the XPath expression.

XPathDocument^ document = gcnew XPathDocument("contosoBooks.xml");
XPathNavigator^ navigator = document->CreateNavigator();

XmlNamespaceManager^ manager = gcnew XmlNamespaceManager(navigator->NameTable);
manager->AddNamespace("bk", "http://www.contoso.com/books");

Double total = (double)navigator->Evaluate("sum(descendant::bk:book/bk:price)", manager);
Console::WriteLine("Total price for all books: {0}", total.ToString());

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

XmlNamespaceManager manager = new XmlNamespaceManager(navigator.NameTable);
manager.AddNamespace("bk", "http://www.contoso.com/books");

Double total = (double)navigator.Evaluate("sum(descendant::bk:book/bk:price)", manager);
Console.WriteLine("Total price for all books: {0}", total.ToString());

Dim document As XPathDocument = New XPathDocument("contosoBooks.xml")
Dim navigator As XPathNavigator = document.CreateNavigator()

Dim manager As XmlNamespaceManager = New XmlNamespaceManager(navigator.NameTable)
manager.AddNamespace("bk", "http://www.contoso.com/books")

Dim total As Double = CType(navigator.Evaluate("sum(descendant::bk:book/bk:price)", manager), Double)
Console.WriteLine("Total price for all books: {0}", total.ToString())

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

<?xml version="1.0" encoding="utf-8" ?>  
<bookstore xmlns="http://www.contoso.com/books">  
    <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>  

Remarks

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);  

Note

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.

Evaluate(XPathExpression, XPathNodeIterator)

Source:

XPathNavigator.cs

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

public:
 virtual System::Object ^ Evaluate(System::Xml::XPath::XPathExpression ^ expr, System::Xml::XPath::XPathNodeIterator ^ context);

public virtual object Evaluate (System.Xml.XPath.XPathExpression expr, System.Xml.XPath.XPathNodeIterator? context);

abstract member Evaluate : System.Xml.XPath.XPathExpression * System.Xml.XPath.XPathNodeIterator -> obj
override this.Evaluate : System.Xml.XPath.XPathExpression * System.Xml.XPath.XPathNodeIterator -> obj

Public Overridable Function Evaluate (expr As XPathExpression, context As XPathNodeIterator) As Object

Parameters

expr

XPathExpression

An XPathExpression that can be evaluated.

context

XPathNodeIterator

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

Returns

Object

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

Exceptions

ArgumentException

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

XPathException

The XPath expression is not valid.

Examples

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

XPathDocument^ document = gcnew 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());

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());

Dim document As XPathDocument = New XPathDocument("books.xml")
Dim navigator As XPathNavigator = document.CreateNavigator()

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

Dim total As Double = CType(navigator.Evaluate(query, nodes), Double)
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>  

Remarks

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:

• The context argument is null.

• MoveNext has not been called on the XPathNodeIterator.

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.