子节点出现空命名空间问题，固定命名空间下输出正常求解析

Hey there, let's dig into why you're running into empty namespace problems and how to fix them—your current fixed namespace setup works, but we need to understand the "why" first to address the root issue.

Why Your Fixed Setup Works (And Empty Namespaces Cause Chaos) #

First off, your current XSLT adds a namespace attribute to the <Foo> element—this is just a regular XML attribute, not an actual XML namespace declaration. That's why it works fine in that specific case: you're just adding a piece of metadata, not altering how XML namespaces are handled for child elements.

The trouble starts when your input XML has elements in an empty namespace (either explicitly marked with xmlns="" or implicitly because they don't belong to any declared namespace). When you output those elements (or create new ones in your XSLT without specifying a namespace), XML's namespace rules kick in: to maintain correctness, the output will add xmlns="" to those elements to make it clear they aren't part of any parent namespace.

On the flip side, if you declare a default namespace on <Foo> (using xmlns="http://MyLink/FixedVersion" instead of a namespace attribute), all child elements like <Bar> and <SomeData> inherit that namespace automatically. No empty namespace declaration is needed, so your output stays clean.

Fixing the Empty Namespace Problem #

Let's cover the most common solutions based on what you're actually trying to achieve:

1. Assign Output Elements to a Proper XML Namespace #

If your goal is to have <Foo> and its children belong to the http://MyLink/FixedVersion namespace (not just add an attribute), adjust your XSLT to declare the default namespace on the root output element:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" indent="yes"/>
    
    <xsl:template match="/NewDataSet">
        <!-- Declare default namespace for all child elements -->
        <Foo xmlns="http://MyLink/FixedVersion">
            <Bar>
                <SomeData/>
            </Bar>
        </Foo>
    </xsl:template>
</xsl:stylesheet>

Now every element under <Foo> inherits the namespace, so no unexpected xmlns="" will pop up in your output.

2. Handle Input Namespaces Correctly #

If your input XML uses namespaces (including the empty one), you need to reference those elements properly in your XSLT. For example, if your input's <NewDataSet> is in a specific namespace, declare it in your stylesheet and use a prefix to match:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:input="http://your-input-namespace-here">
    <xsl:output method="xml" indent="yes"/>
    
    <!-- Match the input element using its namespace prefix -->
    <xsl:template match="/input:NewDataSet">
        <Foo xmlns="http://MyLink/FixedVersion">
            <!-- Apply templates to input elements while respecting their namespace -->
            <xsl:apply-templates select="input:Bar"/>
        </Foo>
    </xsl:template>
    
    <xsl:template match="input:Bar">
        <Bar>
            <xsl:apply-templates select="input:SomeData"/>
        </Bar>
    </xsl:template>
</xsl:stylesheet>

This ensures you're not accidentally pulling elements from the empty namespace, which eliminates those unwanted declarations.

3. Explicitly Override Empty Namespaces for Specific Elements #

If you just need to get rid of xmlns="" on particular elements, explicitly set their namespace when creating them in your XSLT:

<!-- Instead of plain <Bar>, define its namespace -->
<Bar xmlns="http://MyLink/FixedVersion">
    <SomeData/>
</Bar>

This overrides any inherited empty namespace context and forces the element into your desired namespace.

内容的提问来源于stack exchange，提问作者Westerlund.io