IIf not working properly?

[klaudyuxxx]

I need to exec a stored procedure. Some of the parameters are strings (but can also be null) and need to have this format: < 'IntVal1,IntVal2,IntVal3' >.
IntVals are basically city IDs.
I managed to format them with Join function:  Join(",",Parameters.Cities.Value).
The problem I'm facing is when the parameter City is null (I sometimes need it to be null). The Join function outputs an error in that case. To avoid calling the Join function when the param City is null I tried the following:
@queryParamCity   =   IIf  (Parameters.Cities.Value,  Join(",",Parameters.Cities.Value),  null)
or
@queryParamCity   =   IIf  (Parameters.Cities.Value IS NOT null,  Join(",",Parameters.Cities.Value),  null)
or 
@queryParamCity   =   IIf(Parameters.Cities.Value is null,null,Join(",",Parameters.Cities.Value))
.....but the Join function is still being called even if the City parameter is null, so it generates an error and the execution is stopped.

So, no matter what is the value of the expression, the other two member of the IIf function are called.

[klaudyuxxx]

The reporting engine first evaluates the function parameters and then passes the result values to the function. This means that you need to make sure the parameters of the function are valid statements prior to executing the function.

Found my answer...But not a solution

[klaudyuxxx]

and the solution
= IIf(Parameters.Cities.Value is null,null,Join(",", IsNull(Parameters.Cities.Value, Parameters.fake.Value)))

Where the 'fake' param is a string with Value < ="" >

[erwin]

Another option would be to use COALESCE in the SQL Query.

Update: sorry didn't notice at first, you are using Parameters, so COALESCE is not an option.

[Gaurav]

What will happen if the logic is like : -
IIF(A=0,0,B/A)
As stated
The reporting engine first evaluates the function parameters and then passes the result values to the function. This means that you need to make sure the parameters of the function are valid statements prior to executing the function
Now as per my understanding by the statement :-
Function evaluate B/A first ?  Am i wrong ?  Than how can i handle Device by zero conditions ?

[Alexandru]

null

[klaudyuxxx]

try something like this
=IIf (A=0, 0, B / IIf(A=0,1,A))

[Gaurav]

 @Alexandru

Null ??