Steven Engelhardt

What’s wrong with the following code?

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I recently wrote a piece of code that looked something like the following:

What’s wrong with this code?

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Let’s say you have the following unmanaged code:

Note that StreamWriter’s destructor uses m_pStream (perhaps by flushing the stream). This means that the order of destruction is important — StreamWriter must be destroyed before its underlying Stream is.

Now let’s try to write and use some simple managed C++ wrappers for these classes:

See the problem?

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Subtle differences in variable scoping in XSLTs between MSXML 3.0 and 4.0 can result in XSLT files breaking if you upgrade your version of MSXML. Consider the following XSLT:

<?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"
                version="1.0"
                encoding="UTF-8"
                indent="yes" />

    <xsl:template match="/">
        <root>
            <elem>
                <xsl:variable name="foo">Value</xsl:variable>
                <xsl:value-of select="$foo" />
            </elem>
            <elem>
                <!-- This refers to the variable defined in
                     the previous sibling elem node -->
                <xsl:value-of select="$foo" />
            </elem>
        </root>
    </xsl:template>
</xsl:stylesheet>

This stylesheet (which does not depend on the input XML) works on MSXML 3.0 but fails on MSXML 4.0 with the error message

A reference to variable or parameter ‘foo’ cannot be resolved. The variable or parameter may not be defined, or it may not be in scope.

Clearly, MSXML 4.0 limits the scope of the foo variable to the first elem node, whereas MSXML 3.0 does not. I suspect MSXML 3.0 scopes a variable to its enclosing template.

These scoping differences cut both ways. Consider this attempt to fix the XSLT:

<?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"
                version="1.0"
                encoding="UTF-8"
                indent="yes" />

    <xsl:template match="/">
        <root>
            <elem>
                <xsl:variable name="foo">Value</xsl:variable>
                <xsl:value-of select="$foo" />
            </elem>
            <elem>
                <xsl:variable name="foo">Value</xsl:variable>
                <xsl:value-of select="$foo" />
            </elem>
        </root>
    </xsl:template>
</xsl:stylesheet>

This stylesheet works on MSXML 4.0 but fails on MSXML 3.0 with the error message

Variable or parameter ‘foo’ cannot be defined twice within the same template.

If you want the stylesheet to work on both processors, you must push up the variable declaration as follows:

<?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"
                version="1.0"
                encoding="UTF-8"
                indent="yes" />

    <xsl:template match="/">
        <root>
            <xsl:variable name="foo">Value</xsl:variable>
            <elem>
                <xsl:value-of select="$foo" />
            </elem>
            <elem>
                <xsl:value-of select="$foo" />
            </elem>
        </root>
    </xsl:template>
</xsl:stylesheet>

Be careful. Even the smallest of changes can break your software.

I was looking for a free, open source software library for generating PDFs, and Sean O’Connor pointed me to HARU. I’ll have to check it out.

What’s wrong with this code? (It is adapted from something I wrote yesterday)

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In my posts Implementing IXmlWriter Series, I wrote a streaming XML writing class whose interface is based on .NET’s XmlWriter. I recently discovered that MSXML provides its own method to write streaming XML through the class MXXMLWriter.

MXXMLWriter supports a large set of functionality including encoding, indentation, disabling output escaping, and writing XML fragments. The generated XML can be written to an IStream, a BSTR, or a DOMDocument object. However, it’s interface leaves much to be desired. Usage looks like this:

The rough IXmlWriter equivalent is:

However, there might be a case to change IXmlWriter to use MXXMLWriter internally.

The Microsoft XML team recently posted in their blog a set of recommendations on which version of MSXML to use. In short, they recommend using MSXML 6.0 but falling back to 3.0 if it isn’t available.

My product currently tries MSXML 4.0 first — but only SP2 and above. We ran into bugs with previous versions of MSXML 4.0. I will have to evaluate moving to MSXML 6.0.

Long-time readers know that I have a bit of a penchant for error handling, especially with respect to exceptions. I just noticed that I have never, to my knowledge, posted about the classic article “Exception Handling: A False Sense of Security” by Tom Cargill.

Read it and weep.

Google has released a new application called Google Code Search that allows you to search for arbitrary strings in public source code. Here is a search for the phrase “Steven Engelhardt”.

This makes me want to write more public source code.