using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

using System.Diagnostics;

​

using static System.Console;

public class Program

{

​

  public static void Main() {

   string input = @"<ExtensionInfo>

  <ObjectExtension />

  <AttributeExtension>

    <Attribute Name=""PV"" ExtensionType=""inputoutputextension"" InheritedFromTagName=""$CT_STQ_V2"" />

    <Attribute Name=""PV"" ExtensionType=""logdatachangeeventextension"" InheritedFromTagName=""$CT_STQ_V2"" />

    <Attribute Name=""STS"" ExtensionType=""inputoutputextension"" InheritedFromTagName=""$CT_STQ_V2"" />

    <Attribute Name=""STS"" ExtensionType=""booleanextension"" InheritedFromTagName=""$CT_STQ_V2"" />

    <Attribute Name=""STS"" ExtensionType=""alarmextension"" InheritedFromTagName=""$CT_STQ_V2"" />

    <Attribute Name=""STS"" ExtensionType=""logdatachangeeventextension"" InheritedFromTagName=""$CT_STQ_V2"" />

  </AttributeExtension>

</ExtensionInfo>";

      XDocument doc = XDocument.Parse(input);

      var element = doc.Descendants("AttributeExtension")

          .Elements("Attribute")

          .FirstOrDefault(x=>(string)x.Attribute("Name") == "STS" && (string)x.Attribute("ExtensionType") == "alarmextension");

      if(element!= null)

      {

          element.Attribute("InheritedFromTagName").Value.Dump();

      }

  }

​

}