XSLT conditional sorting

I have following XML:

<Users>
  <User Id="1">
    <Name>abc</Name>
    <LastName>d</LastName>
  </User>
  <User Id="2">
    <Name></Name>
    <LastName>ab</LastName>
  </User>
  <User Id="3">
    <Name>a</Name>
    <LastName>efg</LastName>
  </User>
</Users>

Now I sort users using following template:

<xsl:template match="Users"&开发者_Go百科gt;
  <Users>
    <xsl:for-each select="User">
      <xsl:sort select="Name"/>
      <xsl:sort select="LastName"/>

      <User>
        <xsl:attribute name="Id">
          <xsl:value-of select="attribute::Id"/>
        </xsl:attribute>
        <Name>
          <xsl:value-of select="Name"/>
        </Name>
        <LastName>
          <xsl:value-of select="LastName"/>
        </LastName>
      </User>
    </xsl:for-each>
  </Users>
</xsl:template>

But I need sorting, which satisfies following condition: Sort by Name. If Name is empty or null, I need to sort by LastName. So in produced XML I need following ordering: User3, User2, User1.

Any help is appreciated.

P.S.: I use ASP.NET 3.5

I would use first the identity transformation, and then apply the sorting to the union of the elements (excluding those whose Name is empty)

 <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

    <xsl:template match="node()|@*">
        <xsl:copy>
            <xsl:apply-templates select="node()|@*"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="Users">
        <xsl:copy>
            <xsl:apply-templates select="User">
                <xsl:sort select="Name[.!='']|LastName"/>
            </xsl:apply-templates>
        </xsl:copy>
    </xsl:template>

</xsl:stylesheet>

When applied on the input shown in the question, we obtain:

<Users>
   <User Id="3">
      <Name>a</Name>
      <LastName>efg</LastName>
   </User>
   <User Id="2">
      <Name/>
      <LastName>ab</LastName>
   </User>
   <User Id="1">
      <Name>abc</Name>
      <LastName>d</LastName>
   </User>
</Users>

You can select the Name and LastName elements with predicate filters that use normalize-space() to filter out the empty ones, use the union operator | to combine them and group them with the perenthesis(what would create a sequence in XSLT/XPath 2.0). Then, select the first one in the grouping to use for the sort.

Also, rather than re-construct the <Users> element, you can just use <xsl:copy-of>.

<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output indent="yes" />

<xsl:template match="Users">
  <Users>
    <xsl:for-each select="User">
      <xsl:sort select="(Name[normalize-space()]|LastName[normalize-space()])[1]"/>
        <xsl:copy-of select="."/>
    </xsl:for-each>
  </Users>
</xsl:template>

</xsl:stylesheet>