Who failed to validate XML?
Note: this entry has moved.
Today, you validate XML in .NET v1.x by creating an XmlValidatingReader,
setting the schema, and reading:
<![CDATA[// Configure the validating reader
XmlValidatingReader vr = new XmlValidatingReader(theinput);
// Add the schema to the reader (usually the schema is preloaded only once).
vr.Schemas.Add(theschema);
while (vr.Read())
{
// Do your stuff.
}]]>
You have two options for handling invalid content in the input document (with
regards to the schema/s):
-
Catch the exception thrown at the first error, halting processing:
<![CDATA[try
{
while (vr.Read())
{
// Do your stuff.
}
}
catch (XmlException ex)
{
// Report the *parse* exception/rethrow.
}
catch (XmlSchemaException ex)
{
// Report the *validation* exception.
}
]]>
-
Attach to the
ValidationEventHandler (according to .NET naming conventions this
would have been named ValidationError or something like that):
<![CDATA[vr.ValidationEventHandler += new ValidationEventHandler(OnValidationError);
while (vr.Read())
{
// Do your stuff.
}
if (_haserrors)
{
// Report the errors/throw.
}]]>
Here you get a chance of sort of recovering from errors, as you can keep
reading and working with data. The _haserrors flag is set by your
OnValidationError event handler, as well as the accumulation of
error messages.
So far so good. All this is clearly explained in the
MSDN documentation. The validation handler signature looks just like
what you would expect:
<![CDATA[void ValidationCallback(object sender, ValidationEventArgs e)
{
}]]>
In case 2, what happens to the invalid XML item in the input? Well, it's read
anyways, as well as its content. Now, suppose that the element just found
doesn't even exist in your schema, and most probably its inner content either.
Your validation error messages will be filled with errors about each and every
single item inside the erroneous element. What's more, I may want my application
to work in a "forgiveness" mode and so do something useful with what IS valid so far.
Easy enough, I though. I have a sender in my validation callback. I
bet it's the reader. I just have to cast it back, call the
Skip method, accumulate just one error for the current
validation failure, and move on:
<![CDATA[private void OnValidationError(object sender, ValidationEventArgs e)
{
if (e.Severity = XmlSeverityType.Error)
{
// Accumulate error, set flag.
((XmlReader)sender).Skip();
}
}]]>
Unfortunately, the sender is null in v1.x, so no luck.
The good news is that this has been fixed in the PDC bits. Maybe we can hope a
service pack/hotfix for v1.1...