如何取消SaveChanges

我做第一个SaveChanges和苍蝇异常(UpdateException)。我做了第二次SaveChanges，再次犯了第一次错误。该怎么办

bool isUpdate = false;
var resource = new Resource() { url = tbUrl.Text };
//block1
try
{
    context.Resource.AddObject(resource);
    context.SaveChanges();
    isUpdate = true;
}
catch (UpdateException ex)
{
}
//block2
if (!isUpdate)
{
    resource = (from res in context.Resource where res.url == tbUrl.Text select res).First();
    context.NameToResourcer.AddObject(new NameToResourcer()
                    {
                        id_resource = resource.id,
                        name = tag
                    });
    context.SaveChanges();//error!
}