How to check for NULL value in ASP.NET?

Im using “IsNull()” function to check for NULL value,
but I get the following error

Compiler Error Message: BC30451: The name 'IsNull' is not declared.

Source Error:

Line 124:	w = 800
Line 125:	h = 400
Line 126:	if IsNull(ctL) then
Line 127:		If not IsNull(rs("wS")) then w = rs("wS")
Line 128:		If not IsNull(rs("hS")) then h = rs("hS")

Source File: C:\\WebServers\\Gazeta\\default.aspx    Line: 126

Which function should I use?


w = 800
h = 400
if NOT ctL then
   If rs("wS") then w = rs("wS")
      If rs("hS") then h = rs("hS")