How to access a file in the current url or _layouts folder?

Question

My code behind code from the aspx webpage uses a transform.xsl to do some custom xslt transformation.

Scenario 1: Try to get the file from the _layouts folder.

SPWeb web = SPContext.Current.Web;
SPFile file = web.GetFile(@"/layouts/MyFeature/transform.xsl");

Note : The file is accesible on this url http://mysite:8080/_layouts/MyFeature/transform.xsl when using the browser.

Scenario 2: Try to get the file from the current (http://mysite:8080/MyFeature/MyPage.aspx) url.

SPWeb web = SPContext.Current.Web;
SPFile file = web.GetFile(@"transform.xsl");

or

SPFile file = web.GetFile(@"MyFeature/transform.xsl");

or

SPFile file = web.GetFile(@"/MyFeature/transform.xsl");

All scenarios fail, how to solve this?

Solution
The 'transform.xsl' is now located at 'MyFeature/transform.xsl' and I've followed the tip from Anders Rask, and my code looks like this now:

SPWeb web = SPContext.Current.Web;
XslCompiledTransform transformer = new XslCompiledTransform();

XsltSettings settings = new XsltSettings(true, true);
string xslLocation = web.Url + "/FormWrapper/transform.xsl";
transformer.Load(xslLocation, settings, GetResolverWithDefaultCredentials());

// Do XSLT transform...

Answer 1

I'm pretty sure SPWeb.GetFile() returns your site pages, like default.aspx. Also your XSLT file isnt an SPFile if its located in the _LAYOUTS folder.

Instead try something similar to the below code (note boilerplate code so error handling etc should be added)

// fetch xsl processor
XslCompiledTransform processor = GetXslt(@"/layouts/MyFeature/transform.xsl");
StringWriter writer = new StringWriter(CultureInfo.CurrentCulture);

// Transform XSLT code here


private static XslCompiledTransform GetXslt(string xsltUrl)
{
    XslCompiledTransform processor = new XslCompiledTransform(false);
    bool enableDocumentFunctions = true;
    bool enableScript = true;
    XsltSettings settings = new XsltSettings(enableDocumentFunctions, enableScript);

    // add error handling here
    processor.Load(xsltUrl, settings, GetResolverWithDefaultCredentials());
    return processor;
}

// use resolver to avoid getting security exceptions
private static XmlUrlResolver GetResolverWithDefaultCredentials()
{
    XmlUrlResolver resolver = new XmlUrlResolver();
    resolver.Credentials = System.Net.CredentialCache.DefaultCredentials;
    return resolver;
}

Answer 2

You could write a Module Feature that "uploads" your transform.xsl into a document library (Style Library for example) which allows you to access the file as an SPFile.