December 21, 2014

Weak References with Compiled Transforms

There is a good description of this generic WeakReference<> class here.
Here is an example of using WeakReferences for caching XslCompiledTransform's:
private static Dictionary<string, WeakReference<XslCompiledTransform>> xsltLookupTable = 
  new Dictionary<string, WeakReference<XslCompiledTransform>>();

public XslCompiledTransform GetCompiledTransform(string xslFileName)
    XslCompiledTransform xct = null;
    bool found = xsltLookupTable.ContainsKey(xslFileName);
    if (found) // IF the transform is already cached
    {   // Try and get it
        WeakReference<XslCompiledTransform> xctWr = xsltLookupTable[xslFileName] 
           as WeakReference<XslCompiledTransform>;
        xctWr.TryGetTarget(out xct); // Try and get it from the WeakReference
        m_logger.WriteTrace("Found XslCompiledTransform entry for \'" + 
           xslFileName + "\' in the cache");
        // Note the entry maybe null (if the weak reference expired)
    if (xct == null) // IF the compiled transform was not already cached
        // Create it
        xct = new XslCompiledTransform();
          new XsltSettings { EnableDocumentFunction = true }, 
          new XmlUrlResolver());
        // Insert it into a WeakReference
        WeakReference<XslCompiledTransform> wr = new 
        if (found)
          m_logger.WriteTrace("Removing XslCompiledTransform entry for \'" + 
              xslFileName + "\' as it was null in the cache");
        xsltLookupTable.Add(xslFileName, wr); // Add the WeakReference to the cache
        m_logger.WriteTrace("Adding XslCompiledTransform entry for \'" + 
          xslFileName + "\'");
    return xct;
It uses the new generic WeakReference<> class (available in .NET 4.5?). This class could be further refactored into a generic caching class if it was required.

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