为什么找不到xml中的节点？解决方案

为什么找不到xml中的节点？
我的ＸＭＬ文件是.txt文件，里面格式是ＸＭＬ的，用以下方法找不到节点SuccessfulValidations．

                    XmlDocument doc = new XmlDocument();

                    doc.Load(txtFile);

                    XmlNode node = doc.SelectSingleNode("SuccessfulValidations");

请高手帮帮忙，我想取节点SuccessfulValidations下的所有子节点，从而取所有子点点下的属性．

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扩展名改成.xml试过吗
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参考：


http://www.cnblogs.com/insus/p/4308740.html
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有namespace，下面是用System.Xml.Linq来获取的

string xml = @"

<ComputerBuildReportAckResponse xmlns=""http://schemas.ms.it.oem/digitaldistribution/2010/10"">

  <ComputerBuildReportAcks>

    <ComputerBuildReportAck>

      <MSReportUniqueID>f5de0a5c-3810-4128-b1c1-acfaa470b87d</MSReportUniqueID>

      <CustomerReportUniqueID>c7b1274f-feaf-4e42-bee2-4575ccd5f288</CustomerReportUniqueID>

      <MSReceivedDateUTC>2014-06-23T00:00:00+0000</MSReceivedDateUTC>

      <SoldToCustomerID>200001208</SoldToCustomerID>

      <ReceivedFromCustomerID>200001208</ReceivedFromCustomerID>

      <CBRAckFileTotal>1</CBRAckFileTotal>

      <CBRAckFileNumber>1</CBRAckFileNumber>

 

      <SuccessfulValidations>

        <SuccessfulValidationResult>

          <ProductKeyID>2621320037148</ProductKeyID>

          <HardwareHash>JgAAAAAAAAABAAAAAQAAAAAAAgABAAEAJJSYSHpenIh+VY/74KU=</HardwareHash>

        </SuccessfulValidationResult>

        <SuccessfulValidationResult>

          <ProductKeyID>2621320037152</ProductKeyID>

          <HardwareHash>JgAAAAAAAAABAAAAAQAAAAAAAgABAAEAJJSCHNjbCgtQDhPT4KU=</HardwareHash>

        </SuccessfulValidationResult>

        <SuccessfulValidationResult>

          <ProductKeyID>2621320037156</ProductKeyID>

          <HardwareHash>JgAAAAAAAAABAAAAAQAAAAAAAgABAAEAJJSMD8bXnIj7fC7C4KU=</HardwareHash>

        </SuccessfulValidationResult>

        <SuccessfulValidationResult>

          <ProductKeyID>2621320037157</ProductKeyID>

          <HardwareHash>JgAAAAAAAAABAAAAAQAAAAAAAgABAAEAJJSGtmLhpBWtcf6E4KU=</HardwareHash>

        </SuccessfulValidationResult>

      </SuccessfulValidations>

 

    </ComputerBuildReportAck>

  </ComputerBuildReportAcks>

</ComputerBuildReportAckResponse>";

            XElement root = XElement.Parse(xml);

            var nsp = root.GetDefaultNamespace().NamespaceName;

            var ele = root.Element(XName.Get("ComputerBuildReportAcks", nsp))

                .Element(XName.Get("ComputerBuildReportAck", nsp))

            .Element(XName.Get("SuccessfulValidations", nsp));

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要加上路径啊
doc.SelectSingleNode("ComputerBuildReportAckResponse\ComputerBuildReportAcks\ComputerBuildReportAck\SuccessfulValidations");