Esta es la definicion de esta función que viene en visual studio .net. Es de visual basic, pero se puede utilizar como código trasero (codbehind) en asp.net
Visual Basic Scripting Edition
Returns a Boolean value that indicates whether an expression contains no valid data (Null).
IsNull(expression)
The expression argument can be any expression.
Remarks
IsNull returns True if expression is Null, that is, it contains no valid data; otherwise, IsNull returns False. If expression consists of more than one variable, Null in any constituent variable causes True to be returned for the entire expression.
The Null value indicates that the variable contains no valid data. Null is not the same as Empty, which indicates that a variable has not yet been initialized. It is also not the same as a zero-length string (""), which is sometimes referred to as a null string.
Caution Use the IsNull function to determine whether an expression contains a Null value. Expressions that you might expect to evaluate to True under some circumstances, such as If Var = Null and If Var <> Null, are always False. This is because any expression containing a Null is itself Null, and therefore, False.
The following example uses the IsNull function to determine whether a variable contains a Null:
Dim MyVar, MyCheck
MyCheck = IsNull(MyVar) ' Returns False.
MyVar = Null ' Assign Null.
MyCheck = IsNull(MyVar) ' Returns True.
MyVar = Empty ' Assign Empty.
MyCheck = IsNull(MyVar) ' Returns False.
