select xml data in c#

First step is to load the xml file or string into xmldocument object..
xml.LoadXml(str);  // suppose that str string contains ""
//alternatively you can use Load to load xml file by giving path of the file..
//Use xpath to select nodes based on the attribute value as
XmlNodeList xnList = xml.SelectNodes("/Root/Inbox[@id='1']");
this is how you generate filtered nodes...
If you wanted to show selected nodes in gridview then you can easily do that directly by using xmldatasource property and setting xpath expression...
check here for exanmple and explaination on how to do this..
http://msdn.microsoft.com/en-us/library/aa479341.aspx

Show this example you may get idea.

do first create a new XML-File and populate it with some data,show here example:

<?xml version="1.0" encoding="utf-8" ?>
<Employee>
  <em>
    <id>1</id>
    <name>Florian</name>
    <phone>123</phone>
  </em>
  <em>
    <id>2</id>
    <name>Andreas</name>
    <phone>234</phone>
  </em>
  <em>
    <id>3</id>
    <name>Martin</name>
    <phone>345</phone>
  </em>
</Employee>

bind xml file to gridview

Drag a GridView control on your web form. Next you need some code to bind the data from the XML-file to your GridView.

Imports System.Data
Partial Class _Default
    Inherits System.Web.UI.Page
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim oDs As New DataSet
        oDs.ReadXml(Request.PhysicalApplicationPath + "XMLFile.xml")
        GridView1.DataSource = oDs
        GridView1.DataBind()
    End Sub
End Class

At the Page_Load event we create a new DataSet and use the method ReadXml to get the data from the XML-file. Then we bind the DataSet to the GridView