Webservice dynamic attribute creation

Hi,

I have created a xml webservice in .NET

where i have 3 classes and one web method

A is base class and b and c are subclasses of the A

In a webmethod named action() return response type as A

inside webmethod 

if true then
return B
else
return c
end if

in case of True  while returning the xml structure must include A as root element and B as it attribute 
In case of False  while returning the xml structure must include A as root element and c as it attribute