Question

How can I optimize this query & convert into lambda ?

XElement xde = new XElement("Elements", (from a in rootElement.Elements("Class")
                   where a.Attribute("Location").Value.Equals("FirstFloor") 
                   select (from b in a.Elements("Subject")
                           where b.Attribute("Notify").Value.Equals("001")
                           select b.Elements())));

Answer 1

In terms of performance, I think your query can’t be optimized.

Hoewever you can make it more readable repacing Equals by == and removing an unecessary select

XElement xde =
        new XElement("Elements", (from a in rootElement.Elements("Class")
                                  where a.Attribute("Location").Value == "FirstFloor"
                                  from b in a.Elements("Subject")
                                  where b.Attribute("Notify").Value == "001"
                                  select b.Elements()));