[WebMethod]
[WebScript]
public static string refAssignments(getVal rf)
{
    string value = rf.valueFromAJAX;
    MyClass obj = new MyClass();

    string result = obj.analyse(value);
}

if (rf != null)
{
    string value = rf.valueFromAJAX;
    MyClass obj = new MyClass();
    string result = obj.analyse(value);
}
else
{
   //Do something
}