Get a value of attribute by C# LINQ to XML .

LinqToXml.cs

  XElement po = XElement.Load(path_xml);
  // <Substrates> 
  //    <Substrate SubstrateType = "Strip" SubstrateId = "@@@"> 
  var elems = po.Elements().Descendants()
                .Where(node => node.Name.LocalName == "Substrate"
                            && (string)node.Attribute("SubstrateType") == "Strip"
                            && node.Attribute("SubstrateId") != null
                            )
                .Select(elem => elem.Attribute("SubstrateId").Value);

  foreach (var el in elems)
  {
      // write out @@@ 
      Trace.WriteLine(el.ToString());
  }

sample.xml

<?xml version="1.0" encoding="utf-8"?>
<MapData xmlns="urn:semi-org:xsd.E142-1.V1005.SubstrateMap">
    <Layouts>
        <Layout LayoutId="WaferLayout_abcxyz" DefaultUnits="mm" TopLevel="true">
            <Dimension X="1" Y="4" />
            <ChildLayouts>
                <ChildLayout LayoutId="DeviceLayout_abcxyz" />
            </ChildLayouts>
        </Layout>
        <Layout LayoutId="DeviceLayout_abcxyz" DefaultUnits="mm">
            <Dimension X="1" Y="600" />
        </Layout>
    </Layouts>
    <Substrates>
        <Substrate SubstrateType="Strip" SubstrateId="MyStripID" />
    </Substrates>
    <SubstrateMaps>
        <SubstrateMap SubstrateType="Strip" SubstrateId="MyStripID" 
                      LayoutSpecifier="WaferLayout_abcxyz/DeviceLayout_abcxyz" 
                      Orientation="0" 
                      OriginLocation="LowerRight" 
                      AxisDirection="UpLeft">
        </SubstrateMap>
    </SubstrateMaps>
</MapData>