how can i use a variable in this SelectSingleNode statement

Question

how can i use a variable in this SelectSingleNode statement

   oldCd = root.SelectSingleNode("/students/student[id={0}]",id);

Answer 1

if id is an attribute of student element

root.SelectSingleNode(String.Format("//students/student[@id='{0}']",id))

if it is sub element

root.SelectSingleNode(String.Format("//students/student[id[text()='{0}']]",id))