Zafar:
This has turned out to be a very interesting bug. I will be very pleased if
anyone will agree with me that the only possible cause is due to multiple
threads accessing the function simultaneously. (My supervisor thinks that
I'm jumping to this conclusion too hastily!)
In any event, the exception was raised on the final line of the following
code excerpt (!):
vReader = new XmlValidatingReader(candidate);
vReader.Schemas.Add(xsc);
vReader.ValidationType = ValidationType.Schema;
Note that this is an _exact_ excerpt. As you can see, there is _no_
possibility that "read" is called before the "ValidationType" is set, as the
reader has just been instantiated 2 lines up!
This becomes more interesting when the full class is considered:
-------------------------------------------------------------------------
public class OrderValidator
{
private static XmlValidatingReader vReader = null;
private static XmlSchemaCollection xsc = new XmlSchemaCollection();
private OrderValidator()
{
// Access XSD as an embedded resource in this assembly:
System.Reflection.Assembly myAssembly;
myAssembly = System.Reflection.Assembly.GetExecutingAssembly();
System.Resources.ResourceManager myManager = new
System.Resources.ResourceManager("xxx.xxx.xxx.xxx. Order_XSD",
myAssembly);
string xsdString =
myManager.GetString("Order_xsd",System.Globalizati on.CultureInfo.InvariantCu
lture); // this is case-sensitive
StringReader sr = new StringReader(xsdString);
XmlTextReader schemaReader = new XmlTextReader(sr);
// Add XSD to schemaCollection
xsc.Add(null, schemaReader);
}
/// <summary>
/// Singleton instance via .NET-Framework-managed singleton semantics
/// </summary>
public static readonly OrderValidator Instance = new OrderValidator();
/// <summary>
/// Validation method
/// </summary>
/// <param name="candidate">fresh XMLTextReader (unread!)</param>
/// <returns>true for valid XML or throws an exception for
failure</returns>
public Boolean Validates(XmlTextReader candidate)
{
Boolean isValid = false;
vReader = new XmlValidatingReader(candidate);
vReader.Schemas.Add(xsc);
vReader.ValidationType = ValidationType.Schema;
try
{
while (vReader.Read()){}
isValid = true;
}
catch(Exception e)
{
string message = "Validation failed: ";
message += "Element name: " + (vReader.Name != null ? vReader.Name :
String.Empty) + ". ";
message += "Element value: " + (vReader.Value != null ? vReader.Value :
String.Empty) + ". ";
throw(new ApplicationException(message, e));
}
return isValid;
}
}
-------------------------------------------------------------------------
There were numerous factors which made it logical to design this class as a
Singleton, chiefly the size and complexity of the XSD. Once instantiated,
the singleton instance of this class does service validation requests from
multiple threads.
The only explanation that I can come up with is that 2 contending threads
are in the "Validates" method simultaneously. I'm very open to any other
explanation!
In the meantime, I have already implemented the following modifications
based on my "thread contention" theory:
--------------------------------------------------------------------------
// object used for method-level locking
private static Object privateStaticObject = new Object();
public Boolean Validates(XmlTextReader candidate)
{
Boolean isValid = false;
lock (privateStaticObject)
{
vReader = new XmlValidatingReader(candidate);
vReader.Schemas.Add(xsc);
vReader.ValidationType = ValidationType.Schema;
try
{
while (vReader.Read()){}
isValid = true;
}
catch(Exception e)
{
string message = "Validation failed: ";
message += "Element name: " + (vReader.Name != null ? vReader.Name :
String.Empty) + ". ";
message += "Element value: " + (vReader.Value != null ? vReader.Value :
String.Empty) + ". ";
throw(new ApplicationException(message, e));
}
}
return isValid;
}
--------------------------------------------------------------------------
Based on a previous incident, however, our architect thinks I'm too quick to
jump to a "thread contention" conclusion. If anyone has any other
interpretations or suggestions please let me know!
