在C＃中使用XPATH修改XML属性

Question

我有以下相当长的XML文件。我正在尝试选择一个节点并操作ist属性（@Name）。我不知道为什么，但是它以NULL异常结束。

<?xml version="1.0" encoding="utf-8"?>
<Document>
  <Engineering version="V14 SP1" />
  <DocumentInfo>
    <Created>2018-08-16T07:12:25.7973125Z</Created>
    <ExportSetting>WithDefaults, WithReadOnly</ExportSetting>
    <InstalledProducts>
      <Product>
        <DisplayName>Totally Integrated Automation Portal</DisplayName>
        <DisplayVersion>V14 SP1 Update 6</DisplayVersion>
      </Product>
      <OptionPackage>
        <DisplayName>TIA Portal Openness</DisplayName>
        <DisplayVersion>V14 SP1 Update 6</DisplayVersion>
      </OptionPackage>
      <Product>
        <DisplayName>SINAMICS Startdrive</DisplayName>
        <DisplayVersion>V14 SP1 Update 6</DisplayVersion>
      </Product>
      <Product>
        <DisplayName>STEP 7 Professional</DisplayName>
        <DisplayVersion>V14 SP1 Update 6</DisplayVersion>
      </Product>
      <OptionPackage>
        <DisplayName>STEP 7 Safety</DisplayName>
        <DisplayVersion>V14 SP1 Update 4</DisplayVersion>
      </OptionPackage>
      <Product>
        <DisplayName>WinCC Professional</DisplayName>
        <DisplayVersion>V14 SP1 Update 6</DisplayVersion>
      </Product>
      <OptionPackage>
        <DisplayName>SIMATIC Visualization Architect</DisplayName>
        <DisplayVersion>V14 SP1</DisplayVersion>
      </OptionPackage>
    </InstalledProducts>
  </DocumentInfo>
  <SW.Blocks.FB ID="0">
    <AttributeList>
      <AutoNumber>true</AutoNumber>
      <CodeModifiedDate ReadOnly="true">2018-08-16T07:11:59.336375Z</CodeModifiedDate>
      <CompileDate ReadOnly="true">2018-08-16T07:12:12.3617656Z</CompileDate>
      <CreationDate ReadOnly="true">2018-08-16T07:10:53.345164Z</CreationDate>
      <DownloadWithoutReinit ReadOnly="true">false</DownloadWithoutReinit>
      <HandleErrorsWithinBlock ReadOnly="true">false</HandleErrorsWithinBlock>
      <HeaderAuthor />
      <HeaderFamily />
      <HeaderName />
      <HeaderVersion>0.1</HeaderVersion>
      <Interface><Sections xmlns="http://www.siemens.com/automation/Openness/SW/Interface/v2">
  <Section Name="Input" />
  <Section Name="Output" />
  <Section Name="InOut" />
  <Section Name="Static" />
  <Section Name="Temp" />
  <Section Name="Constant" />
</Sections></Interface>
      <InterfaceModifiedDate ReadOnly="true">2018-08-16T07:10:53.345164Z</InterfaceModifiedDate>
      <IsConsistent ReadOnly="true">true</IsConsistent>
      <IsIECCheckEnabled>false</IsIECCheckEnabled>
      <IsKnowHowProtected ReadOnly="true">false</IsKnowHowProtected>
      <ISMultiInstanceCapable ReadOnly="true">true</ISMultiInstanceCapable>
      <IsRetainMemResEnabled>false</IsRetainMemResEnabled>
      <IsWriteProtected ReadOnly="true">false</IsWriteProtected>
      <LibraryConformanceStatus ReadOnly="true">Fehler: Der Baustein enthält Aufrufe von Einzelinstanzen.
Warnung: Der Baustein enthält direkte Zugriffe auf Eingänge, Ausgänge, Merker, indirekte Zugriffe auf Register oder indirekte Zugriffe auf externe, zum Übersetzungszeitpunkt unbekannte Speicherbereiche.
</LibraryConformanceStatus>
      <MemoryLayout>Optimized</MemoryLayout>
      <MemoryReserve>100</MemoryReserve>
      <ModifiedDate ReadOnly="true">2018-08-16T07:11:59.336375Z</ModifiedDate>
      <Name>Baustein_29</Name>
      <Number>36</Number>
      <ParameterModified ReadOnly="true">2018-08-16T07:10:53.345164Z</ParameterModified>
      <PLCSimAdvancedSupport ReadOnly="true">true</PLCSimAdvancedSupport>
      <ProgrammingLanguage>FBD</ProgrammingLanguage>
      <StructureModified ReadOnly="true">2018-08-16T07:10:53.345164Z</StructureModified>
      <UDABlockProperties />
      <UDAEnableTagReadback>false</UDAEnableTagReadback>
    </AttributeList>
    <ObjectList>
      <MultilingualText ID="1" CompositionName="Comment">
        <ObjectList>
          <MultilingualTextItem ID="2" CompositionName="Items">
            <AttributeList>
              <Culture>de-DE</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
          <MultilingualTextItem ID="3" CompositionName="Items">
            <AttributeList>
              <Culture>en-GB</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
          <MultilingualTextItem ID="4" CompositionName="Items">
            <AttributeList>
              <Culture>fr-CH</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
        </ObjectList>
      </MultilingualText>
      <SW.Blocks.CompileUnit ID="5" CompositionName="CompileUnits">
        <AttributeList>
          <NetworkSource><FlgNet xmlns="http://www.siemens.com/automation/Openness/SW/NetworkSource/FlgNet/v1">
  <Parts>
    <Access Scope="GlobalVariable" UId="21">
      <Symbol>
        <Component Name="INPUT_BOOL" />   //this has to modified
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="22">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="23">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="24">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="25">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="26">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="27">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="28">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="29">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="30">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="31">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="32">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="33">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="34">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="35">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="36">
      <Symbol>
        <Component Name="INPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="824" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="37">
      <Symbol>
        <Component Name="INPUT_TIME" />
        <Address Area="Memory" Type="Time" BitOffset="200" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="38">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="39">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="40">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="41">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="42">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="43">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="44">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Access Scope="GlobalVariable" UId="45">
      <Symbol>
        <Component Name="OUTPUT_BOOL" />
        <Address Area="Memory" Type="Bool" BitOffset="880" Informative="true" />
      </Symbol>
    </Access>
    <Call UId="46">
      <CallInfo Name="VAL" BlockType="FB">
        <IntegerAttribute Name="BlockNumber" Informative="true">610</IntegerAttribute>
        <DateAttribute Name="ParameterModifiedTS" Informative="true">2017-05-10T09:20:20</DateAttribute>
        <Instance UId="47" Scope="GlobalVariable">
          <Component Name="VAL_DB" />
          <Address Area="DB" BlockNumber="131" BitOffset="0" Informative="true" />
        </Instance>
        <Parameter Name="LOCK_1" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="LOCK_2" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="LOCK_3" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="LOCK_4" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="EXT_FLT" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="LOCAL" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="LOCAL_ON" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="MAN" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="AUT" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="AUT_ON" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="FB_OP" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="FB_CL" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="ACK" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="MODE" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="FUNCTION" Section="Input" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="FB_TIME" Section="Input" Type="Time">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QCONTROL" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QFLT" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QAUTO" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QMAIN" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QSIM" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QFB_OP" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QFB_CL" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
        <Parameter Name="QLOCK" Section="Output" Type="Bool">
          <StringAttribute Name="InterfaceFlags" Informative="true">S7_Visible</StringAttribute>
        </Parameter>
      </CallInfo>
    </Call>
  </Parts>
  <Wires>
    <Wire UId="48">
      <IdentCon UId="21" />
      <NameCon UId="46" Name="en" />
    </Wire>
    <Wire UId="49">
      <IdentCon UId="22" />
      <NameCon UId="46" Name="LOCK_1" />
    </Wire>
    <Wire UId="50">
      <IdentCon UId="23" />
      <NameCon UId="46" Name="LOCK_2" />
    </Wire>
    <Wire UId="51">
      <IdentCon UId="24" />
      <NameCon UId="46" Name="LOCK_3" />
    </Wire>
    <Wire UId="52">
      <IdentCon UId="25" />
      <NameCon UId="46" Name="LOCK_4" />
    </Wire>
    <Wire UId="53">
      <IdentCon UId="26" />
      <NameCon UId="46" Name="EXT_FLT" />
    </Wire>
    <Wire UId="54">
      <IdentCon UId="27" />
      <NameCon UId="46" Name="LOCAL" />
    </Wire>
    <Wire UId="55">
      <IdentCon UId="28" />
      <NameCon UId="46" Name="LOCAL_ON" />
    </Wire>
    <Wire UId="56">
      <IdentCon UId="29" />
      <NameCon UId="46" Name="MAN" />
    </Wire>
    <Wire UId="57">
      <IdentCon UId="30" />
      <NameCon UId="46" Name="AUT" />
    </Wire>
    <Wire UId="58">
      <IdentCon UId="31" />
      <NameCon UId="46" Name="AUT_ON" />
    </Wire>
    <Wire UId="59">
      <IdentCon UId="32" />
      <NameCon UId="46" Name="FB_OP" />
    </Wire>
    <Wire UId="60">
      <IdentCon UId="33" />
      <NameCon UId="46" Name="FB_CL" />
    </Wire>
    <Wire UId="61">
      <IdentCon UId="34" />
      <NameCon UId="46" Name="ACK" />
    </Wire>
    <Wire UId="62">
      <IdentCon UId="35" />
      <NameCon UId="46" Name="MODE" />
    </Wire>
    <Wire UId="63">
      <IdentCon UId="36" />
      <NameCon UId="46" Name="FUNCTION" />
    </Wire>
    <Wire UId="64">
      <IdentCon UId="37" />
      <NameCon UId="46" Name="FB_TIME" />
    </Wire>
    <Wire UId="65">
      <NameCon UId="46" Name="QCONTROL" />
      <IdentCon UId="38" />
    </Wire>
    <Wire UId="66">
      <NameCon UId="46" Name="QFLT" />
      <IdentCon UId="39" />
    </Wire>
    <Wire UId="67">
      <NameCon UId="46" Name="QAUTO" />
      <IdentCon UId="40" />
    </Wire>
    <Wire UId="68">
      <NameCon UId="46" Name="QMAIN" />
      <IdentCon UId="41" />
    </Wire>
    <Wire UId="69">
      <NameCon UId="46" Name="QSIM" />
      <IdentCon UId="42" />
    </Wire>
    <Wire UId="70">
      <NameCon UId="46" Name="QFB_OP" />
      <IdentCon UId="43" />
    </Wire>
    <Wire UId="71">
      <NameCon UId="46" Name="QFB_CL" />
      <IdentCon UId="44" />
    </Wire>
    <Wire UId="72">
      <NameCon UId="46" Name="QLOCK" />
      <IdentCon UId="45" />
    </Wire>
  </Wires>
</FlgNet></NetworkSource>
          <ProgrammingLanguage>FBD</ProgrammingLanguage>
        </AttributeList>
        <ObjectList>
          <MultilingualText ID="6" CompositionName="Comment">
            <ObjectList>
              <MultilingualTextItem ID="7" CompositionName="Items">
                <AttributeList>
                  <Culture>de-DE</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="8" CompositionName="Items">
                <AttributeList>
                  <Culture>en-GB</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="9" CompositionName="Items">
                <AttributeList>
                  <Culture>fr-CH</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
            </ObjectList>
          </MultilingualText>
          <MultilingualText ID="A" CompositionName="Title">
            <ObjectList>
              <MultilingualTextItem ID="B" CompositionName="Items">
                <AttributeList>
                  <Culture>de-DE</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="C" CompositionName="Items">
                <AttributeList>
                  <Culture>en-GB</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="D" CompositionName="Items">
                <AttributeList>
                  <Culture>fr-CH</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
            </ObjectList>
          </MultilingualText>
        </ObjectList>
      </SW.Blocks.CompileUnit>
      <SW.Blocks.CompileUnit ID="E" CompositionName="CompileUnits">
        <AttributeList>
          <NetworkSource />
          <ProgrammingLanguage>FBD</ProgrammingLanguage>
        </AttributeList>
        <ObjectList>
          <MultilingualText ID="F" CompositionName="Comment">
            <ObjectList>
              <MultilingualTextItem ID="10" CompositionName="Items">
                <AttributeList>
                  <Culture>de-DE</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="11" CompositionName="Items">
                <AttributeList>
                  <Culture>en-GB</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="12" CompositionName="Items">
                <AttributeList>
                  <Culture>fr-CH</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
            </ObjectList>
          </MultilingualText>
          <MultilingualText ID="13" CompositionName="Title">
            <ObjectList>
              <MultilingualTextItem ID="14" CompositionName="Items">
                <AttributeList>
                  <Culture>de-DE</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="15" CompositionName="Items">
                <AttributeList>
                  <Culture>en-GB</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
              <MultilingualTextItem ID="16" CompositionName="Items">
                <AttributeList>
                  <Culture>fr-CH</Culture>
                  <Text />
                </AttributeList>
              </MultilingualTextItem>
            </ObjectList>
          </MultilingualText>
        </ObjectList>
      </SW.Blocks.CompileUnit>
      <MultilingualText ID="17" CompositionName="Title">
        <ObjectList>
          <MultilingualTextItem ID="18" CompositionName="Items">
            <AttributeList>
              <Culture>de-DE</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
          <MultilingualTextItem ID="19" CompositionName="Items">
            <AttributeList>
              <Culture>en-GB</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
          <MultilingualTextItem ID="1A" CompositionName="Items">
            <AttributeList>
              <Culture>fr-CH</Culture>
              <Text />
            </AttributeList>
          </MultilingualTextItem>
        </ObjectList>
      </MultilingualText>
    </ObjectList>
  </SW.Blocks.FB>
</Document>

然后我尝试使用if周期修改访问对象的每个名称-属性： 我正在尝试使用以下代码：

XDocument xdoc = XDocument.Load("c:/export/" + NAME + ".xml");
                    xdoc.XPathSelectElement("/Document/SW.Blocks.FB/AttributeList/Name").Value = NAME;
                    xdoc.XPathSelectElement("/Document/SW.Blocks.FB/ObjectList/SW.Blocks.CompileUnit[1]/ObjectList/MultilingualText[1]/ObjectList/MultilingualTextItem[1]/AttributeList/Text").Value = KOMMENTAR;
                    //((IEnumerable<object>)xdoc.XPathEvaluate("/Document/SW.Blocks.FB/ObjectList/SW.Blocks.CompileUnit[1]/AttributeList/NetworkSource/FlgNet/Parts/Call/CallInfo/Instance/Component/@Name")).OfType<XAttribute>().Single().Value = DBNAME;

                    for (int i = 0; i < AccVariablen.Length; i++)
                    {
                        if (AccVariablen[i] != String.Empty)
                        {
                            int j = i + 21;
                            //xdoc.XPathSelectElement ("//Access["+j+"]/Symbol/Component/@Name").SetValue(AccVariablen[i]);
                            XElement modMethodXElement = xdoc.XPathSelectElement(" / Document / SW.Blocks.FB / ObjectList / SW.Blocks.CompileUnit [1]/ AttributeList / NetworkSource / FlgNet / Parts / Access[@UId = '" + j + "'] / Symbol / Component");

                            modMethodXElement.SetAttributeValue("Name", AccVariablen[i]);
                        }
                    }
                    xdoc.Save("c:/export/" + NAME + ".xml");

最后以NULL异常结束。 我该如何解决这个问题？