Unable to overrides XMLSiteMapProvider for roles.

Hi guys,
I need your help, i don't know why but i'm unable to overrides my XmlSiteMapProvider and use a custom IsAccessibleToUser
My XML are this
<?xml version="1.0" encoding="utf-8" ?> <siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" > <siteMapNode roles="*"> <siteMapNode url="0" title="temp0"  description="temp0" roles="*"/> <siteMapNode url="1" title="temp1"  description="temp1" roles="*" /> <siteMapNode url="2" title="temp2"  description="temp2" roles="*" /> <siteMapNode url="3" title="temp3"  description="temp3" ...